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Qt - Passing custom objects among threads .. Communication between threads in a qt program is essentially done by using signals/slots. This is by far one of .. Debao's Blog..Reference; 23 Jul 2013 .. Introduction. For any C++ developer who's used Qt, we've grown to love the Signals/Slots idiom it presents for creating clean Observer code.


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Signals and slots. Instead of having observable objects and observers, and registering them, Qt provides two high level concepts: signals and slots. A signal is a message that an object can send, most of the time to inform of a status change. A slot is a function that is used to accept and respond to a signal.


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It emits signals to indicate that the thread started or finished executing, and provides a few slots as well.
More interesting is that s can be used in multiple threads, emit signals that invoke slots in other threads, and post events to objects that "live" in other threads.
This is possible because each thread is allowed to have its own event loop.
QObject Reentrancy is reentrant.
Most of its non-GUI subclasses, such as, andare also reentrant, making it possible to use these classes from multiple threads simultaneously.
Note that these classes are designed to be created and used from within a single thread; creating an object in one thread and calling its functions from another thread is not guaranteed to work.
This implies, among other things, that you should never pass the object this as the parent of an object created in the thread since the object itself was created in another thread.
Specifically, this applies to the and the.
For example, you cannot start a timer or connect a socket in a thread that is not the.
This can be done easily by creating the objects on the stack in your implementation.
Although is reentrant, the GUI classes, notably and all its subclasses, are not reentrant.
They can only be used from the main thread.
As noted earlier, must also be called from that thread.
In practice, the impossibility of using GUI classes in other threads than the main thread can easily be worked around by putting time-consuming operations in a separate worker thread and displaying the results on screen in the main thread when the worker thread is finished.
This is the approach used for implementing the and the.
In general, creating QObjects before the is not supported and can lead to weird crashes on exit, depending on the platform.
This means static instances of are also not supported.
A properly structured single or multi-threaded application should make the be the first created, and last destroyed.
Per-Thread Event Loop Each thread can have its own event loop.
The initial thread starts its event loop usingor for single-dialog GUI applications, sometimes.
Other threads can start an event loop using.
Likeprovides an int function and a slot.
An event loop in a thread makes it possible for the thread to use certain non-GUI Qt classes that require the presence of an event loop such as, qt signals and slots threads />It also makes it possible to connect signals from any threads to slots of a specific thread.
This is explained in more detail in the section below.
A instance is said to live in the thread in which it is created.
Events to that object are dispatched by that thread's qt signals and slots threads loop.
The thread in which a lives is available using.
The function changes the thread affinity for an object and its children the object cannot be moved if it has a parent.
Calling delete on a from a thread other than the one that owns the object or accessing the object in other ways is unsafe, unless you guarantee that the object isn't processing events at that moment.
Use instead, and a event will be posted, which the event loop of the object's thread will eventually pick up.
By default, the thread that owns a is the thread that creates thebut not qt signals and slots threads has been called.
If no event loop is running, events won't be delivered to the object.
For example, if you create a object in a thread but never callthe will never emit its signal.
Calling won't work either.
These restrictions apply to the main thread as well.
You can qt signals and slots threads post events to any object in any thread at any time using the thread-safe function.
The events will automatically be dispatched by the event loop of the thread where the object was created.
Event filters are supported in all threads, with the restriction that the monitoring object must live in the same thread as the monitored object.
Similarly, unlike can only be used to dispatch qt signals and slots threads to objects living in the thread from which the function is called.
Accessing QObject Subclasses from Other Threads and all of its subclasses are not thread-safe.
This includes the entire event delivery system.
It is important to keep in mind that the event loop may be delivering events to your subclass while you are accessing the object from another thread.
If you are calling a function on an subclass that doesn't live in the current thread and the object might receive events, you must protect all access to your subclass's internal data with a mutex; otherwise, you may experience crashes or other undesired behavior.
Like other objects, objects live click the following article the thread where the object was created -- not in the thread that is created when is qt signals and slots threads />It is generally unsafe to provide slots in your subclass, unless you protect the member variables with a mutex.
On the other hand, you can safely emit signals from your implementation, because signal emission is thread-safe.
Otherwise, the behavior is the same as the Queued Connection.
The slot continue reading executed in the emitter's thread, which is not necessarily the receiver's thread.
The slot is executed in the receiver's thread.
Note: Using this type to connect objects in the same thread will cause deadlock.
The connection type can be specified by passing an additional argument to.
Be aware that using direct connections when the sender and receiver live in different threads is unsafe if an event loop is running in the receiver's thread, for the same reason that calling any function on an object living in another thread is unsafe.
The uses a queued connection to communicate between a worker thread and the main thread.
To avoid freezing the main thread's event loop and, as a consequence, the application's user interfaceall the Mandelbrot fractal computation is done in a separate worker thread.
The thread emits a signal when it is done rendering the fractal.
Similarly, the uses a separate thread for communicating with a TCP server asynchronously.
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So signals and slots between them will not work as you think.. Problem with signal-slot connection across threads [SOLVED]. Problem with signal-slot connection.


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Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Since signals and slots are type-safe, type errors are reported as warnings and do not cause crashes to occur.


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Qt 4.1: Thread Support in QtMain thread - worker thread communication. [Archive] - Qt Centre Forum pyside signals and slots across threads. Signals/slots accross threadsc++ - How to emit cross-thread signal in Qt? - Stack OverflowQT signal pyside signals and slots across threads to change the GUI out side the main thread - DaniWeb


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Support for Signals and Slots¶ One of the key features of Qt is its use of signals and slots to communicate between objects. Their use encourages the development of reusable components. A signal is emitted when something of potential interest happens. A slot is a Python callable.


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Data is transferred between threads via signals. See the WorkerScript documentation for details on how to implement the script, and for a list of data types that can be passed between threads. Choosing an Appropriate Approach. As demonstrated above, Qt provides different solutions for developing threaded applications.


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The code inside the Worker's slot would then execute in a separate thread. However, you are free to connect the Worker's slots to any signal, from any object, in any thread. It is safe to connect signals and slots across different threads, thanks to a mechanism called queued connections.


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Below are some suggestions for troubleshooting signals and slots in the Qt C++ library. 1. Check for compiler warnings about non-existent signals and/or slots. 2. Use break points or qDebug to check that signal and slot code is definitely reached: - the connect statement - code where the signal is fired - the slot code. 3. Check…


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Qt 4.1: Thread Support in QtMain thread - worker thread communication. [Archive] - Qt Centre Forum pyside signals and slots across threads. Signals/slots accross threadsc++ - How to emit cross-thread signal in Qt? - Stack OverflowQT signal pyside signals and slots across threads to change the GUI out side the main thread - DaniWeb


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Since QRect and QImage objects can be serialized for transmission via the signals and slots mechanism, they can be sent between threads in this way, making it convenient to use threads in a wide range of situations where built-in types are used. Running the Example. We only need one more piece of code to complete the example:


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Signals and Slots. In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal.


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Qt offers many classes and functions for working with threads.
Below are four different approaches that Qt programmers can use to implement multithreaded applications.
QThread: Low-Level API with Https://tossy.info/and-slots/slot-drill-speeds-and-feeds.html Event Loops is the foundation of all thread control click here Qt.
Each instance represents and controls one thread.
Instantiating a provides a parallel event loop, allowing slots to be invoked in a secondary thread.
Subclassing a allows the application to initialize the new thread before starting its event loop, or to run parallel code without an event loop.
See the and the for demonstrations on how to use.
QThreadPool and QRunnable: Reusing Threads Creating and destroying threads frequently can be expensive.
To reduce this overhead, existing threads can be reused for new tasks.
To run code qt signals and slots threads one of a 's threads, reimplement and instantiate the subclassed.
Use to put the in the 's run queue.
When a thread becomes available, the code within will execute in that thread.
Each Qt application has a global thread pool, which is accessible through.
This global thread pool automatically maintains an optimal number of threads based on the number of cores in the CPU.
However, a separate can be created and managed explicitly.
Qt Concurrent: Using a High-level API The module provides high-level functions that deal with some common parallel computation patterns: qt signals and slots threads, filter, and reduce.
Unlike using andthese functions never require the use of such as mutexes or semaphores.
Instead, they return a object which can be used to retrieve the functions' results when they are ready.
For convenience, enables interactions with s via signals and slots.
This module also provides the function, which can run any function in another thread.
However, only supports a subset of features available to the map, filter and reduce functions.
The can be used qt slots and signals parameters retrieve the function's return value and to check if the thread is running.
See the module documentation for details on the individual functions.
WorkerScript: Threading in QML The QML type lets JavaScript code run in parallel with the GUI thread.
Each instance can have one.
When is called, the script will run in a separate thread and a separate.
When the script finishes running, it can send a reply back to the GUI thread which will invoke the signal handler.
Using a is similar to using a worker that has been moved to another thread.
Data is transferred between threads via signals.
See the documentation for more info on how to implement the script, and for a list of data types that can be passed between threads.
Choosing an Appropriate Approach As demonstrated above, Qt provides different solutions for developing threaded applications.
Below is a comparison of Qt's threading technologies, followed by recommended solutions for some example use cases.
Emit signals to update progress.
Write qt signals and slots threads a to update progress.
Write to a to update progress.
One call Run an existing function within another thread and get its return value.
Run the function using.
Have a emit the signal when the function has returned, and call to get the function's return value.
One call Perform an operation on all items of a container, using all available cores.
For example, producing thumbnails from a list of images.
Use Qt Concurrent's function to select container elements, and the function to apply an operation to each element.
To fold the output into a single result, use and instead.
Place the computation code in a.
Call to start the computation in a new thread.
Let the script call sendMessage too, to pass the result back to the GUI thread.
Handle the result in onMessage and update the GUI there.
Subclass a to create a worker.
Instantiate this worker object and a.
Move the worker to the new thread.
Send commands or data to the worker object over queued signal-slot connections.
Permanent Repeatedly perform an expensive operation in another thread, where the thread does not need to receive any signals or events.
Write the infinite loop directly within a reimplementation of.
Start https://tossy.info/and-slots/which-slots-to-play-and-how-to-win.html thread without an event loop.
Let the thread emit signals to send data back to the GUI thread.
© 2019 The Qt Company Ltd.
Documentation contributions included herein are the copyrights of their respective owners.
The documentation provided herein is licensed under the terms of the as published by the Free Software Foundation.
Qt and respective logos are trademarks of The Qt Company Ltd.
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I generally use Qt:: QueuedConnection explicitly if I know that the QObjects are in different threads. Qt::DirectConnection. A Qt:: DirectConnection is the connection with the most minimal overhead you can get with Signals & Slots. You can visualize it that way: If you call the Signal the method generated by Qt for you calls all Slots in place.


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QtWS15- Creating multithreaded applications with Qt, Bo Thorsen, Viking Software

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Qt documentation states that signals and slots can be direct, queued and auto.. It also stated that if object that owns slot 'lives' in a thread different from object that owns signal, emitting such signal will be like posting message - signal emit will return instantly and slot method will be called in target thread's event loop.


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The article is almost done, but it needs a bit of polishing and some good examples.
Any review or contribution is welcome!
Discussion about this article happens.
Hughes One of the most popular topics on the " qt IRC channel": is threading: many people join the channel and ask how they should solve their problem with some code running in a different thread.
Nine times out of ten, a quick inspection of their code shows that click to see more biggest problem is the very fact they're using threads in the first place, and they're falling in one of the endless pitfalls of parallel programming.
Moreover, threading support in Qt is a double-edged sword: while it makes it very simple for you to do multithread programming, it adds a certain number of features especially when it comes to interaction with QObjects you must be aware of.
The purpose of this document is not to teach you how to use threads, do proper locking, exploit parallelism, nor write scalable programs; there are many good books about these topics; for instance, take a look to the recommended reading list.
Instead, this small article is meant to be a guide to introduce users to threading in Qt 4, in order to avoid the most common pitfalls and help them to develop code that is at the same time more robust and with a better structure.
Prerequisites Think of it this way: threads are like salt, not like pasta.
You like salt, I like salt, we all like salt.
But we eat more pasta.
A function is reentrant if it's safe to invoke it from more than one thread at the same, provided that each invocation references unique data.
A function is thread-safe if it's safe to invoke it from more than one thread at the same time even if the invocations reference shared data.
Events and the event loop Being an event-driven toolkit, events and event delivery play a central role in Qt architecture.
In this article we'll not give a comprehensive coverage about this topic; we'll instead focus on some thread-related key concepts see and for more information click here the Qt event system.
An event in Qt is an object which represents something interesting that happened; the main difference between an event and a signal is that events are targeted to a specific object in our application which decides what to do with that eventwhile signals are emitted "in the wild".
From a code point of view, all events are instances of some subclass ofand all QObject-derived classes can override the QObject::event virtual method in order to handle events targeted to their instances.
The important thing about events is that they're not delivered as soon as they're generated; they're instead queued up in an event queue and sent sometime later.
The dispatcher itself loops around the event queue and sends queued events to their target objects, and therefore it is called the event loop.
If we think about it, all that can generate events at that point is some external source dispatching for all internal events is now complete and there were no more pending events in the event queue to delivery.
In a UNIX-like system, window manager activity i.
All Qt needs to do is converting what select returns into an object of the right QEvent subclass and queue it up in the event queue.
Now you know what's inside an event loop : What requires a running event loop?
This isn't an exhaustive list, but if you have the overall picture, you should be able to guess which classes require a running event loop.
The same thing holds for all kinds of interaction keyboard, mouse, etc.
When you call readthey just return already available data; when you call writethey schedule the writing for later.
High-level classes, like QNetworkAccessManager, simply do not offer any synchronous API and require an event loop.
Blocking the event loop Before discussing why you should never ever block the event loop, let's try to figure out what this qt signals and slots threads means.
Suppose you have a Button widget which emits a signal when clicked; connected to this signal there's a slot of our Worker object, which does a lot of work.
The window manager sent us the mouse click, which was picked up by the Qt kernel, converted in a QMouseEvent and sent to our widget's event method line 4 by QApplication::notify not shown here.
Since Button didn't override eventthe base class implementation QWidget is called.
QWidget::event detects the event is actually a mouse click and calls the specialized event handler, that is, Button::mousePressEvent line 5.
We overrode this method to emit the Button::clicked signal line 6which invokes the Worker::doWork slot of our worker object line 7.
While the worker is busy working, what's the event loop doing?
You should've guessed it: nothing!
It dispatched the mouse press event and it's blocked waiting for the event handler to return.
We managed to block the event loop, which means that no event is sent any more, until we return from the doWork slot, up the stack, to the event loop, and let it process pending events.
With the event delivery stuck, widgets won't update themselves QPaintEvent objects will sit in the queueno further interaction with widgets is possible for the same reasontimers won't fire and networking communications will slow down and stop.
Moreover, many window managers will detect that your application is not qt signals and slots threads events any more and tell the user that your application isn't responding.
That's why is so important to quickly react to events and return to the event loop as soon as possible!
Forcing event dispatching So, what do we do if we have a long task to run and don't want to block the event loop?
One possible answer is to move the task into another thread: in the next sections we'll see how to do that.
We also have the option to manually force the event loop to run, by repeatedly calling QCoreApplication::processEvents inside our blocking task.
QCoreApplication::processEvents will process how play slot machines the events in the event queue and return to the caller.
Another available option we can use to forcibly reenter the event loop is the class.
By calling QEventLoop::exec we reenter the event loop, and we can connect signals to the QEventLoop::quit slot to make it quit.
We enter a local QEventLoop, and when the reply has finished, the local event loop quits.
Be very careful when reentering the event loop "by other paths": it can lead to unwanted recursions!
Let's go back to the Button example.
A quick and easy workaround for this is passing QEventLoop::ExcludeUserInputEvents to QCoreApplication::processEventswhich tells the event loop to not dispatch any user input event the events will simply stay in the queue.
Luckily, the same thing does not apply to deletion events the ones click in the event queue by QObject::deleteLater.
In fact, they are handled in a special way by Qt, and are processed only if the running event loop has a smaller degree of "nesting" w.
The same thing applies to local event loops started with QEventLoop.
The only notable exception I've found to this rule as of Qt 4.
This is pretty much reasonable, since Qt does not know about any hoyle slots and video poker download loop that will eventually perform the deletion, and therefore deletes the object immediately.
Qt thread classes A computer is a state machine.
Threads are for people who can't program state machines.
Qt now offers several classes for dealing with threads; let's start with an overview.
QThread is the central, low-level class for thread support in Qt.
A QThread object represents one thread of execution.
Due to the cross-platform nature of Qt, QThread manages to hide all the platform-specific code that is needed to use threads on different operating systems.
Note that since Qt 4.
QRunnable and QThreadPool is a lightweight abstract class that can be used to start a task in another thread in a "run and forget" fashion.
By calling QThreadPool::start runnable we put a QRunnable in a QThreadPool's runqueue; as soon as a thread becomes available, the QRunnable will be picked up and run into that thread.
All Qt applications have a global thread pool available by calling QThreadPool::globalInstancebut one can always create a private QThreadPool instance and manage it explicitely.
Notice that, not being a QObject, QRunnable has no built-in means of explicitely communicating something to other components; you have to code that by hand, using low-level threading primitives like a mutex-guarded queue for collecting results, etc.
QtConcurrent is a higher-level API, built on top of QThreadPool, useful to deal with the most common parallel computation patterns:, and ; it also offers a QtConcurrent::run method that can be used to easily run a function in another thread.
The class can be used to monitor a QFuture progress and interact with it by means of signals and slots notice that QFuture, being a value-based class, doesn't inherit QObject.
This is not the case: QThread objects can start thread-local event loops running in the threads they represent.
Therefore, we say that the main event loop is the one created by the thread which invoked mainand started with QCoreApplication::exec which must be called from that thread.
This is also called the GUI thread, because it's the only thread in which GUI-related operations are allowed.
Exactly like QCoreApplication, QThread has also the QThread::quit and QThread::exit methods to stop the event loop.
A thread event loop delivers events for all QObjects that are living in that thread; this includes, by default, all objects that are created into that thread, or that were moved to that thread more info about this later.
We also say that the thread affinity of a QObject is a certain thread, meaning that the object is living in that thread.
We must look at the thread that created them: it's the thread that ran the MyThread constructor.
Therefore, all three objects are not living in the MyThread thread, but in the thread that created the MyThread instance which, by the way, is where the instance is living as well.
We can query anytime the thread affinity of a QObject by calling QObject::thread.
Notice that QObjects created before a QCoreApplication object have no thread affinity, and therefore no event dispatching will be done for them in other words, QCoreApplication builds up the QThread object that represents the main thread.
We can use the thread-safe QCoreApplication::postEvent method for posting an event for a certain object.
This will enqueue the event in the event loop of the thread the object is living in; therefore, the event will not be dispatched unless that thread qt signals and slots threads a tulalip and slots event loop.
It is very important to understand that QObject and all of its subclasses are not thread-safe although they can be reentrant ; therefore, you can viva and slots friendzy access a QObject from more than one thread at the same time, unless you serialize all accesses to the object's internal data for instance, by protecting it with a mutex.
Remember that the object may be handling events dispatched by the event loop of the thread it is living in while you're accessing it from another thread!
For the same reason, you can't delete a QObject from another thread, but you must use QObject::deleteLaterwhich will post an event that will ultimately cause its deletion by the thread the object is living in.
Moreover, QWidget and all of its subclasses, along with other GUI-related classes even not QObject-based, like QPixmap are not reentrant either: they can be used exclusively from the GUI thread.
We can change a QObject's affinity by calling QObject::moveToThread ; this will change the affinity of the object and of its children.
Since QObject is not thread-safe, we must use it from the thread the object is living in; that is, you can only push objects from the thread they're living in to other threads, and not pull them or move them around from other threads.
Moreover, Qt requires that the child of a QObject must live in the same thread where the parent is living.
} }; This is because the QThread object is living in another thread, namely, the free slots with nudges and holds in which it was created.
Qt also requires that all objects living in a thread are deleted before the QThread object that represents qt signals and slots threads thread is destroyed; this can be easily done qt signals and slots threads creating all the objects living in that thread on the QThread::run method's stack.
Signals and slots across threads Given these premises, how do we call methods on QObjects living in other threads?
Qt offers a very nice and clean solution: we post an event in that thread's event queue, and the handling of that event will consist in invoking the method we're interested in this of course requires that the thread has a running event loop.
Signals and slots across threads work in a qt signals and slots threads way.
In every case, keep in mind the thread the emitting object is living in has no importance at all!
In case of an automatic connection, Qt looks at the thread that invoked the signal and compares it with the thread the receiver is living in to determine which connection type it has to use.
In particular, the is simply wrong when it states: Auto Connection default The behavior is the same as the Direct Connection, if the emitter and receiver are in the same thread.
The behavior is the same as the Queued Connection, if the emitter and receiver are in different threads.
You should've guessed it: a direct connection.
That's because the Thread object is living in the thread that emits the signal.
In the aSlot slot we could then access some Thread's member variable while they're being accessed by the run method, which is running concurrently: this is the perfect recipe for disaster.
A solution you'll often found in forums, blog posts etc.
What's wrong here is that we're misunderstanding the purpose of a thread object the QThread subclass : QThread objects are not threads; they're control objects around a thread, therefore meant to be used from another thread usually, the one they're living in.
It's perfectly safe and they'll do the "right thing" see above; the sender's thread affinity does not matter.
Use QThread::wait to wait for their termination.
If you want some kind of "self-destruction", you can connect the finished signal with the deleteLater slot.
When should I use threads?
When you have to use a blocking API If you need to use a library or other code that doesn't offer a non-blocking API by means of signals and slots, or events, or callbacks, etc.
Since creating a new worker process, having it doing the job and communicating back the results is definetely harder and more expensive than just starting a thread, the latter is the most common choice.
A good example of such an API is address resolution just to show you that we're not talking about 3rd-party crappy API.
This is something included in every C library out therewhich is the process of taking an host name and converting it into an address.
This process involves a query to a usually remote system — the Domain Name System, or DNS.
While, usually, the response is almost instantaneous, the remote servers might fail, some packet might get lost, the network connection might break, and so on; in short, it might take dozens of seconds before we get a qt signals and slots threads from our query.
The only standard API available on UNIX systems is blocking not only the old-fashioned gethostbyname 3but also the newer and better getservbyname 3 and getaddrinfo 3.
Other simple examples are image loading and scaling.
If you're dealing with very large images, these processes can take up to tens of seconds.
When you want to scale with the number of CPUs Threads allow your program to take advantage from multiprocessor systems.
Since each thread is scheduled charlie and the chocolate factory online slot by the operating system, if your application is running on such a machine the scheduler is likely to run each thread on a different processor at the same time.
For instance, consider an application that generates thumbnails from a set of images.
A thread farm of n threads that is, a thread pool with a fixed number of threadsone per each CPU available in the system see also QThread::idealThreadCountcan spread the work of scaling down the images into thumbnails on all the threads, effectively gaining an almost linear speedup with the number of the processors for simplicity's sake, we consider the CPU being the bottleneck.
When you don't want to be possibly blocked by others MEH.
BETTER START WITH AN EXAMPLE.
This is quite an advanced topic, so feel free to skip it for now.
A nice example of this use case comes from QNetworkAccessManager usage inside WebKit.
WebKit is a modern browser engine, that is, a set of classes to lay out and display web pages.
The Qt widget that uses WebKit is QWebView.
QNetworkAccessManager is a Qt class that deals with HTTP requests and responses for all purposes, we can consider it to be the networking engine of a web browser.
While not using threads for networking is a very good idea, it has also a major drawback: if you don't read data from the socket as soon as possible, the kernel buffers will fill up, packets will begin to be dropped, and the transfer speed will decrease considerably.
Blocking the event loop will therefore lead to a loss of transfer performance, because nobody will be notified that there are data to read and thus nobody will read them.
But what could block the event loop?
The sad answer is: WebKit itself!
As soon as some data are received, WebKit uses here to start laying out the web page.
Unfortunately, the layout process is quite complicated and expensive, therefore it blocks the event loop for a short while, enough to impact on ongoing transfers broadband connections play their role here, filling up kernel buffers in a small fraction of second.
The overall page loading time is therefore worsened by this self-induced transfer slowness.
Notice that since QNetworkAccessManagers and QNetworkReplys are Simulcast and midway slots, they're not thread-safe, therefore you can't just move them to another thread and continue using them from your thread, because they may be accessed at the same time by two threads: yours and the one they're living in, due to events that will be dispatched to them by the latter thread's event loop.
As of Qt 4.
When shouldn't I use threads?
If you think you need threads then your processes are too fat.
Notice that the above snippet does not take into account any error handling, otherwise it would have been even more cumbersome.
What is very wrong in this design is that we're forgetting that networking is asynchronous by designand if we build a synchronous processing around we're shooting ourselves in the foot.
To solve this problem, many people simple move this code into a different thread.
Let's take a step back and consider from an higher point of view what we're building here: we want to create a state machine that reacts on inputs of some sort and acts consequently.
Now, there are several ways to build a state machine and Qt even offers a class for that:the simplest one being an enum i.
Exactly what we want them to be: for instance, in the networking example, we might want to connect the socket's QAbstractSocket::connected and the QIODevice::readyRead signals to our slot.
Of course, we can also easily add more slots if that suits better in our case like a slot to manage error situations, which are notified by the QAbstractSocket::error signal.
This is a true asynchronous, signal-driven design!
Jobs splittable in chunks Suppose that we have a long computation which can't be easily moved to another thread or that it can't be moved at all, because for instance it must run in the GUI thread.
If we can split the computation in small chunks, we can return to the event loop, let it dispatch events, and make it invoke the method that processes the next chunk.
This can be easily done if we remember how queued connections are implemented: an event is posted in the event loop of the thread the receiver object is living in; when the event is delivered, the corresponding slot is invoked.
If we also want to pass parameters to the method, they need to be registered within the Qt metatype system using qRegisterMetaType.

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Qt uses signals and slots normally in a single thread, so calling a signal will call a slot in the same thread signal called. Is it any way to use a signal-slot mechanism to pass a message to qt th...


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Data is transferred between threads via signals. See the WorkerScript documentation for details on how to implement the script, and for a list of data types that can be passed between threads. Choosing an Appropriate Approach. As demonstrated above, Qt provides different solutions for developing threaded applications.


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Signals and slots are used for communication between objects.
The signals and slots see more is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.
Signals and slots are made possible by Qt's.
Introduction In GUI programming, when we change one widget, we often want another widget to be notified.
More generally, we want objects of any kind to be able to communicate with one another.
For example, qt signals and slots threads a user clicks a Close button, we probably want the window's function to be called.
Other toolkits achieve this kind of communication using callbacks.
A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function the callback to the processing function.
The processing function then calls the callback when appropriate.
While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.
Signals and Slots In Qt, we have an alternative to the callback technique: We use signals and slots.
A signal is idea drilled and slotted rotors brake kit final when a particular event occurs.
Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them.
A slot is a function that is called in response to a particular signal.
Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.
The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot.
In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.
Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax.
The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime.
Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal.
Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time.
Signals and slots can take any number of arguments of any type.
They are completely type safe.
All classes that inherit from or one of its subclasses e.
Signals are emitted by objects when they change their state in a way that may be interesting to other objects.
This is all the object does to communicate.
It does not know or care whether anything is receiving the signals it emits.
This is true information encapsulation, and ensures that the object can be used as a software component.
Slots can be used for receiving signals, but they are also normal member functions.
Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it.
This ensures that truly independent components can be created with Qt.
You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need.
It is even possible to connect a signal directly to another signal.
This will emit the second signal immediately whenever the first is emitted.
Together, signals and slots make up a powerful component programming mechanism.
Signals Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner.
Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.
When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call.
When this happens, the signals and slots qt signals and slots threads is totally independent of any GUI event loop.
Execution of the code following the emit statement will occur once all slots have returned.
The situation is slightly different when using ; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.
If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.
Signals are automatically generated by the and must not be implemented in the.
They can never have return types i.
A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types.
If were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for.
Connecting different input widgets together would be impossible.
Slots A slot is called when a signal connected to it is emitted.
Slots are normal C++ functions and can be called normally; their only special feature is that viva bingo and slots friendzy can be connected to them.
Since slots are qt signals and slots threads member functions, they follow the normal C++ rules when called directly.
However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection.
This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.
You can also define slots to be virtual, which we have found quite useful in practice.
Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant.
In click here, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls.
This is the overhead required to locate the connection object, to safely iterate over all connections i.
While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example.
As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs.
The same is true whenever you do a system call in a slot; or indirectly call more than ten functions.
The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.
Note that other libraries that define slot and pin gear called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application.
To solve this problem, undef the offending preprocessor symbol.
This class can tell the outside world that its state has changed by emitting a signal, valueChangedand it has a slot which other objects can send signals to.
All classes that contain signals or slots must mention at the top of their declaration.
They must also derive directly or indirectly from.
Slots are implemented by the application programmer.
Here is a possible implementation of the Counter::setValue slot: void Counter ::setValue int value { if value!
Then b emits the same valueChanged signal, but since no slot has been connected to b's valueChanged signal, the signal is ignored.
Note that the setValue function sets the value and emits the signal only if value!
This prevents infinite looping in the case of cyclic connections e.
By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections.
You can break all of these connections with a single call.
If you pass the type, the connection will only be made if it is not a duplicate.
If there is already a duplicate exact same signal to the exact same slot on the same objectsthe connection will fail and connect will return false.
This example illustrates that objects can work together without needing to know any information about each other.
To enable this, the objects only need to be connected together, and this can be achieved with some simple function calls, or with 's feature.
A Real Example The following is an example of the header of a simple widget class without member functions.
The purpose is to show how you can utilize signals and slots in your own applications.
It is somewhat similar to the built-in widget.
The macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of "undefined reference to vtable for LcdNumber", you have probably forgotten to or to include the moc output in the link command.
The LcdNumber class emits a signal, overflowwhen it is asked to show an impossible value.
If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow signal, i.
If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots.
Qt will call both in the order they were connected.
LcdNumber uses it, as the code above indicates, to set the displayed number.
Since display is part of the class's interface with the rest of the program, the slot is public.
Several of the example programs connect the signal of a to the display slot, so the LCD number continuously shows the value of the scroll bar.
Note that display is overloaded; Qt will select the appropriate version when you connect a signal to the slot.
With callbacks, you'd have to find five different names and keep track of the types yourself.
Signals And Slots With Default Arguments The signatures of signals and slots may contain arguments, and the arguments can have default values.
We want to catch this signal, wherever we might have a dangling reference to the deletedso we can clean https://tossy.info/and-slots/blade-and-soul-warlock-free-slot.html up.
There are several ways to connect signal and slots.
First, it allows the compiler to check that the signal's arguments are compatible with the slot's arguments.
Arguments can also be implicitly converted by the compiler, if needed.
The context object provides information about in which thread the receiver should be executed.
This is important, as providing the context ensures that the receiver is executed in the context thread.
The lambda will be disconnected when the sender or context is destroyed.
You should take care that any objects used inside the functor are still alive when the signal is emitted.
The other way to connect a signal expansions slots and cards a slot is to use and the SIGNAL and SLOT macros.
The rule about whether to include arguments or not in the SIGNAL and SLOT macros, if the arguments have default values, is that the signature passed to the SIGNAL macro must not have fewer arguments than the signature passed to the SLOT macro.
This connection will report a runtime error.
Note that signal and slot arguments are not checked by the compiler when using this overload.
Advanced Signals and Slots Usage For cases where you may require information on the just click for source of the signal, Qt provides the function, which returns a pointer to the object that sent the signal.
You can even use both mechanisms in the same project.
Just add the following line to your qmake project.
© 2019 The Qt Company Qt signals and slots threads />Documentation contributions included herein are the copyrights of their respective owners.
The documentation provided herein is licensed under the terms of the as published by the Free Software Foundation.
Qt and respective logos are trademarks of The Qt Company Ltd.
All check this out trademarks are property of their respective owners.

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How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections This blog is part of a series of blogs explaining the internals of signals and slots. Part 1 - How Qt Signals and Slots Work


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It emits signals to indicate that the thread started or finished executing, and provides a few slots as well.
More interesting is that qt signals and slots threads can be used in multiple threads, emit signals that invoke slots in other threads, and post events to objects that "live" in other threads.
This is possible because each thread is allowed to have its own event loop.
QObject Reentrancy is reentrant.
Most of its non-GUI subclasses, such as, andare also reentrant, making it possible to use these classes from multiple threads simultaneously.
Note that these classes are designed to be created and used from within a single thread; creating an object in one thread and calling its functions from another thread is not see more to work.
This implies, among other things, that you should never pass the object this as the parent of an object created in the thread since the object itself was created in another thread.
Specifically, this applies to the and the.
For example, you cannot start a timer or connect a socket in a thread that is not the.
This can be done easily by creating the objects on the stack in your implementation.
Although is reentrant, the GUI classes, notably and all its subclasses, are not reentrant.
They can only be used from the main thread.
As noted earlier, must also be called from that thread.
In practice, the impossibility of using GUI classes in other threads than the main thread can easily be worked around by putting time-consuming operations in a separate worker thread and displaying the results on screen in the main thread when the worker thread is finished.
This is the approach used for implementing the and the.
In general, creating QObjects before the is not supported and can lead to weird crashes on exit, depending on the platform.
and the charm slot means static instances of are also not supported.
A properly structured single or multi-threaded application should make the be the first created, and last destroyed.
Per-Thread Event Loop Each thread can have its own event loop.
The initial thread starts its event loop usingor for single-dialog GUI applications, sometimes.
Other threads can start an event loop using.
Likeprovides an int function and a slot.
An event loop in a thread makes it visit web page for the thread to qt signals and slots threads certain non-GUI Qt classes that require the presence of an event loop such as, and.
It also makes it possible to connect signals from any threads to slots of a specific thread.
This is explained in more detail in the section below.
A instance is said to live in the thread in which it is created.
Events to that object are dispatched by that thread's event loop.
The thread in which a lives is available using.
The function changes the thread affinity for an object qt signals and slots threads its children the object cannot be moved if it has a parent.
Calling delete on a from a thread other than the one that owns the object or accessing the object in other ways is unsafe, unless you guarantee that the object isn't processing events at that moment.
Use instead, and a event will be posted, which the event loop of the object's thread will eventually pick up.
By default, the thread that owns a is the thread that creates thebut qt signals and slots threads after has been called.
If no event loop is running, events won't be delivered to the object.
For example, if you create a object in a thread but never callthe will never emit its signal.
Calling won't work either.
These restrictions apply to the main thread as well.
You can manually post events to any object in any thread at any qt signals and slots threads using the thread-safe function.
The events will automatically be dispatched by the event loop of the thread where the object was created.
Event filters are supported in all threads, with the restriction that qt signals and slots threads monitoring object must live in the same thread as the monitored object.
Similarly, unlike can only be used to dispatch events to objects living in the thread from which the function is called.
Accessing QObject Subclasses from Other Threads and all of its subclasses are not thread-safe.
This includes the entire event delivery system.
It is important to keep in mind that the event loop may be delivering events to your subclass while you are accessing the object from another thread.
If you are calling a function on an subclass that doesn't live in the current thread and the object might receive events, you must protect all access to your subclass's internal data with a mutex; otherwise, you may experience crashes or other undesired behavior.
Like other objects, objects live in the thread where the object was created -- not in the thread that is created when is called.
It is generally unsafe to provide slots in your subclass, unless you protect the member variables with a mutex.
On the other hand, you can safely emit signals from your implementation, because signal emission is thread-safe.
Otherwise, the behavior is the same as the Queued Connection.
The slot is executed in the emitter's thread, which is not necessarily the qt signals and slots threads thread.
The slot is executed in the receiver's thread.
Note: Using this type to connect objects in the same thread will cause deadlock.
The connection type can be specified by passing an additional argument to.
Be aware that using direct connections when the sender and receiver live in different threads is unsafe if an event loop is running source the receiver's thread, for the same reason that calling any function on an object living in another thread is unsafe.
The uses a queued connection to communicate between a worker thread and the main thread.
To avoid freezing the main thread's event loop and, as a consequence, the application's user interfaceall the Mandelbrot fractal computation is done in a separate worker thread.
The thread emits a signal when it is done rendering the fractal.
Similarly, the uses a separate thread for communicating with a TCP server asynchronously.
© 2019 The Qt Company Ltd.
Documentation contributions included herein are the copyrights of their see more owners.
The documentation provided herein is licensed under the terms of the as published by the Free Software Foundation.
Qt and respective logos are trademarks of The Qt Company Ltd.
All other trademarks are property of their respective owners.

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Support for Signals and Slots¶ One of the key features of Qt is its use of signals and slots to communicate between objects. Their use encourages the development of reusable components. A signal is emitted when something of potential interest happens. A slot is a Python callable.


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The article is almost done, but it needs a bit of polishing and some good examples.
Any review or contribution is welcome!
Discussion about this article happens.
Hughes One of the most popular topics on the " qt IRC channel": is threading: many people join the channel and ask how they should solve their problem with some code running in a different thread.
Nine times out of ten, a quick inspection of their code shows that the biggest problem is the very fact they're using threads in the first place, and they're falling in one of the endless pitfalls of parallel programming.
Moreover, threading support in Qt is a double-edged sword: while it makes it very simple for you to do multithread programming, it adds a certain number of features especially when it comes to interaction with QObjects you must be aware of.
The purpose of this document is not to teach you how to use threads, do proper locking, exploit parallelism, nor write scalable programs; there Alibaba and the 40 Thieves slot machine many good books about these topics; for instance, take a look to the recommended reading list.
Instead, this small article is meant to be a guide to introduce users to threading in Qt 4, in order to avoid the most common pitfalls and help them to develop code that is at the same time more robust and with a better qt signals and slots threads />Prerequisites Think of it this way: threads are like salt, not like pasta.
You like salt, I like salt, we all like salt.
But qt signals and slots threads eat more pasta.
A function is reentrant if it's safe to invoke it from more than one thread at the same, provided that each invocation references unique data.
A function is thread-safe if it's safe to invoke it from more than one thread at the same time even if the invocations reference shared data.
In this article we'll not give a comprehensive coverage about this topic; we'll instead focus on some thread-related key concepts see and for more information about the Qt event system.
An event in Qt is an object which represents something interesting that happened; the main difference between an event and a signal is that events are targeted to a specific object in our application which decides what to do with that eventwhile signals are emitted "in the wild".
From a code point of view, all events are instances of some subclass ofand all QObject-derived classes can override the QObject::event virtual method in order to handle events targeted to their instances.
The important thing about events is that they're not delivered as soon as they're generated; they're instead queued up in an event queue and sent sometime later.
The dispatcher itself loops around the event queue and sends queued events to their target objects, and therefore it is called the event loop.
If we think about it, all that can generate events at that point is some external source dispatching for all internal events is now complete and there were no more pending events in the event queue to delivery.
In a UNIX-like system, window manager activity i.
All Qt needs to do is converting what select returns into an object of the right QEvent subclass and queue it up in the event queue.
Now you know what's inside an event loop : What requires a running event loop?
This isn't an exhaustive list, but if you have the overall picture, you should be able to guess which classes require a running event loop.
The same thing holds for all kinds of interaction keyboard, mouse, etc.
When you call readthey just return already available data; when you call writethey schedule the writing for later.
High-level classes, like QNetworkAccessManager, simply do not offer any synchronous API and require an event loop.
Blocking the event loop Before discussing why you should never ever block the event loop, let's try to figure out what this "blocking" means.
Suppose you have a Button widget which emits a signal when clicked; connected to this signal there's a slot of our Worker object, which does a lot of work.
The window manager sent us the mouse click, which was picked up by the Qt kernel, converted in a QMouseEvent and sent to our widget's event method line 4 by QApplication::notify not shown here.
Since Button didn't override eventthe base class implementation QWidget is called.
QWidget::event detects the event is actually a mouse click and calls the specialized event handler, that is, Button::mousePressEvent line 5.
We overrode more info method to emit the Button::clicked signal line 6which invokes the Worker::doWork slot of our worker object line 7.
While the worker is busy working, what's the event loop doing?
You should've guessed it: nothing!
It dispatched the mouse press event and it's blocked waiting for the event handler to return.
We managed to block the event loop, which means that no event is sent any more, until we return from the doWork slot, up the stack, to the event loop, and let it process pending events.
With the event delivery stuck, widgets won't update themselves QPaintEvent objects will sit in the queueno further interaction with widgets is possible for the same reasontimers won't fire and networking communications will slow down and stop.
Moreover, many window managers will detect that your application is not handling events any more and tell the user that your application isn't responding.
That's why is so important to quickly react to events and return to the event loop as soon as possible!
Forcing event dispatching So, what do we do if we have a long task to run and don't want to block the event loop?
One possible answer is to move the task into another thread: in the next sections we'll see how to do that.
We also have the option to manually force the event loop to run, by repeatedly calling QCoreApplication::processEvents inside our blocking task.
QCoreApplication::processEvents will process all the events in the event queue and return to the caller.
Another available option we can use to forcibly reenter the event loop is the class.
By calling QEventLoop::exec we reenter the event loop, and we can connect signals to the QEventLoop::quit slot to make it quit.
We enter a local QEventLoop, and when the reply has finished, the local event loop quits.
Be very careful when reentering the event loop "by other paths": it can lead to unwanted recursions!
Let's go back to the Button example.
A quick and easy workaround for this is passing QEventLoop::ExcludeUserInputEvents to QCoreApplication::processEventswhich tells the event loop to not dispatch any user input event the events will simply stay in the queue.
Luckily, the same thing does not apply to deletion events the ones posted in the event queue by QObject::deleteLater.
In fact, they are handled in a special way by Qt, and are processed only if the running event loop has a smaller degree of "nesting" w.
The same thing applies to local event loops started with QEventLoop.
The only notable exception I've found to this rule as of Qt 4.
This is pretty much reasonable, since Qt does not know about any "outer" loop that will eventually perform the deletion, and therefore deletes the object immediately.
Qt thread classes A computer is a state machine.
Threads are for people who can't program state machines.
Qt now offers several classes for dealing with threads; let's start with an overview.
QThread is the central, low-level class for thread support in Qt.
A QThread object represents one thread of execution.
Due to the cross-platform nature of Qt, QThread manages to hide all the platform-specific code that is needed to use threads on different operating systems.
Note that since Qt 4.
QRunnable and QThreadPool is a lightweight abstract class that can be used to start a task in another thread in a "run and forget" fashion.
By calling QThreadPool::start runnable we put a QRunnable in a QThreadPool's runqueue; as soon as a thread becomes available, the QRunnable will be picked up and run into that thread.
All Qt applications have a global thread pool available by calling QThreadPool::globalInstancebut one can always create a private QThreadPool instance and manage it explicitely.
Notice that, not being a QObject, QRunnable has no built-in means of explicitely communicating something to other components; you have to code that by hand, using low-level threading primitives like a mutex-guarded queue for collecting results, etc.
QtConcurrent is a higher-level API, built on top of QThreadPool, useful to deal with the most common parallel computation patterns:, and ; it also offers a QtConcurrent::run method that can be used to easily run a function in another thread.
The class can be used to monitor a QFuture progress and interact with it by means of signals and slots notice that QFuture, being a value-based class, doesn't inherit QObject.
This is not the case: QThread objects can start qt signals and slots threads event loops running in the threads they represent.
Therefore, we say that the main event loop is the one created by the thread which invoked mainand started with QCoreApplication::exec which must be called from that thread.
This is also called the GUI thread, because it's the only thread in which GUI-related operations are allowed.
Exactly like QCoreApplication, QThread has also the QThread::quit and QThread::exit methods to stop the event loop.
A thread event loop delivers events for all QObjects that are living in that thread; this includes, by default, all objects that are created into that thread, or that were moved to that thread more info about this later.
We also say that the thread affinity of a QObject is a certain thread, meaning that the object is living in that thread.
We must look at the thread that created them: it's the thread that ran the MyThread constructor.
Therefore, all three objects are not living in the MyThread thread, but in the thread that created the MyThread instance which, by the way, is where the instance is living as well.
We can query anytime the thread affinity of a QObject by calling QObject::thread.
Notice that QObjects created before a QCoreApplication motherboard ports and slots have no thread affinity, and therefore no event dispatching will be done for them in other words, QCoreApplication builds up the QThread object that represents the main thread.
We can use the thread-safe QCoreApplication::postEvent method for posting an event for a certain object.
This will enqueue the event in the event loop of the thread the object is living in; therefore, the event will not be dispatched unless that thread has a running event loop.
It is very important to understand that QObject and all of its subclasses are not thread-safe although they can be reentrant ; therefore, you can not access a QObject from more than one thread at the same time, unless you serialize all accesses to the object's internal data for instance, by protecting it with a mutex.
Remember that the object may be handling events dispatched by the event loop of the thread it is living in while you're accessing it from another thread!
For the same reason, you can't delete a QObject from another thread, but you must use QObject::deleteLaterwhich will post an event that will ultimately cause its deletion by the thread the object is living in.
Moreover, QWidget and all of its subclasses, along with other Qt signals and slots threads classes even not QObject-based, like QPixmap are not reentrant either: they can be used exclusively from the GUI thread.
We can change a QObject's affinity by calling QObject::moveToThread ; this will change the affinity of the object and of its children.
Since QObject is not thread-safe, we must use it from the thread the object is living in; that is, you can only push objects from the thread they're living in to other threads, and not pull them or move them around from other threads.
Moreover, Qt requires that the child of a QObject must live in the same thread where the parent is living.
} }; This is because the QThread object is living in another thread, namely, the one in which it was created.
Qt also requires that all objects living in a thread are deleted before the QThread object that represents the thread is destroyed; this can be easily done by creating all the objects living in that thread on the QThread::run method's stack.
Signals and slots across threads Given these premises, how do we call methods on QObjects living in other threads?
Qt offers a very nice and clean solution: we post an event in that thread's event queue, and the handling of that event will consist in invoking the method we're interested in this of course requires that the thread has a running event loop.
Signals and slots across threads work in a similar way.
In every case, keep in mind the thread the emitting object is living in has no importance at all!
In case of an automatic connection, Qt looks at the thread that invoked qt signals and slots threads signal and compares it with the thread the receiver is living in to determine which connection type it has to use.
In particular, the is simply wrong when it states: Auto Connection default The behavior is the same as the Direct Connection, if the emitter and receiver are in the same thread.
The behavior is tab and slot welding tables same as the Queued Connection, if the emitter and receiver are in different threads.
You should've guessed it: a direct connection.
That's because the Thread object is living in the thread that emits the signal.
In the aSlot slot we could then access some Thread's member variable while they're being accessed by the run method, which is running concurrently: this is the perfect recipe for disaster.
A solution you'll often found in forums, blog posts etc.
What's wrong here is that we're misunderstanding the purpose of a thread object the QThread subclass : QThread objects are not threads; they're control objects around a thread, therefore meant to be used from another thread usually, the one they're living in.
It's perfectly safe and they'll do the "right thing" see above; the sender's thread affinity does not matter.
Use QThread::wait to wait for their termination.
If you want some kind of "self-destruction", you can connect the finished signal with the deleteLater slot.
When should I use threads?
When you have to use a blocking API If you need to use a library or other code that doesn't offer a non-blocking API by means of signals and slots, or events, or callbacks, etc.
Since creating a new worker process, having it doing the job and communicating back the results is definetely harder and more expensive than just starting a thread, the latter is the most common choice.
A good example of such an API is address resolution just to show you that we're not talking about 3rd-party crappy API.
This is something included in every C library out therewhich is the process of taking an host name and converting it into an address.
This process involves a query to a usually remote system — the Domain Name System, or DNS.
While, usually, the response is almost instantaneous, the remote servers might fail, some packet might get lost, the network connection might break, and so on; in short, it might take dozens of seconds before we get a reply from our query.
The only standard API available on UNIX systems is blocking not only the old-fashioned gethostbyname 3but also the newer and better getservbyname 3 and getaddrinfo 3.
Other simple examples are image loading and scaling.
If you're dealing with very large images, these processes can take up to tens of seconds.
When you want to scale with the number of CPUs Threads allow your program to take advantage from multiprocessor systems.
Since each thread is scheduled independently by the operating system, if your application is running on such a machine the scheduler is likely to run each thread on a different processor at the same time.
For instance, consider an application that generates thumbnails from a set of images.
A thread farm of n threads that is, a thread pool with a fixed number of threadsone per each CPU available in the system see also QThread::idealThreadCountclick spread the work of scaling down the images into thumbnails on all the threads, effectively gaining an almost linear speedup with the number of the processors for simplicity's sake, we consider the CPU being the bottleneck.
When you don't want to be possibly blocked by others MEH.
BETTER START WITH AN EXAMPLE.
This is quite an advanced topic, so feel free to skip it for now.
A nice example of this use case comes from QNetworkAccessManager usage inside WebKit.
WebKit is a modern browser engine, that is, a set of classes to lay out and display web pages.
The Qt widget that uses WebKit is QWebView.
QNetworkAccessManager is a Qt class that deals with HTTP requests and responses for all purposes, we can consider it to be the networking engine of a web browser.
While not using threads for networking is a very good idea, it has also a major drawback: if you don't read data from the socket as soon as possible, the kernel buffers will fill up, packets will begin to be dropped, and the transfer speed will decrease considerably.
Blocking the event loop will therefore lead to a loss of transfer performance, because nobody will be notified that there are data to read and thus nobody will read them.
But what could block the event loop?
The sad answer is: WebKit itself!
As soon as some data are received, WebKit uses them to start laying out the web page.
Unfortunately, the layout process is quite complicated and expensive, therefore it blocks the event loop for a short while, enough to impact on ongoing transfers broadband connections play their role here, filling up kernel buffers in a small fraction of second.
The overall page loading time is therefore worsened by this self-induced transfer slowness.
Notice that since QNetworkAccessManagers and QNetworkReplys are QObjects, they're not thread-safe, therefore you can't just move them to another thread and continue using them from your thread, because they may be accessed at the same time by two threads: yours and the one they're living in, due to events that will be dispatched to them by the latter thread's event loop.
As of Qt 4.
When shouldn't I use threads?
If you think you need threads then your processes are too fat.
Notice that the above snippet does not take into account any error handling, otherwise it would have qt signals and slots threads even more cumbersome.
What is very wrong in this design is that we're forgetting that networking is asynchronous by designand if we build a synchronous processing around we're shooting ourselves in the foot.
To solve this problem, many people simple move this code into a different thread.
Let's take a step back and consider from an higher point of view what we're building here: we want to create a state machine that reacts on inputs of some sort and acts consequently.
Now, there are several ways to build a state machine and Qt even offers a class for that:the simplest one being an enum i.
Exactly what we want them to be: for instance, in the networking example, we might want to connect the socket's QAbstractSocket::connected and the QIODevice::readyRead signals to our slot.
Of course, we can also easily add more slots if that suits better in our case like a slot to manage error situations, which are notified by the QAbstractSocket::error signal.
This is a true asynchronous, signal-driven design!
Jobs splittable in chunks Suppose that we have a long computation which can't be easily moved to another thread or that it can't be moved at the chocolate and slot charlie factory online, because for instance it must run in the GUI thread.
If we can split the computation in small chunks, we can return to the event loop, let it dispatch events, and make it invoke the method that processes the next chunk.
This can be easily done if we remember how queued connections are implemented: an event is posted in the event loop of the thread the receiver object is living in; when the event is delivered, the corresponding slot is invoked.
If we also want to pass parameters to the method, they need to be registered qt slots signals parameters the Qt metatype system using qRegisterMetaType.