🤑 Threads Events QObjects - Qt Wiki

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We derive a class from QThread and reimplement the run() method. If the thread where the QObject lives doesn't have an event loop, the event will never be.


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A QThread instance manages one thread of execution within the program. Each QThread comes with its own event loop that can process events outside the​.


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What the heck is the event loop anyway? - Philip Roberts - JSConf EU

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QThread usage without an event loop. QThread p Subclass QThread and override QThread::run(). Create an instance and start the new thread via.


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Since Button didn't override event , the base class implementation QWidget is called. While the worker is busy working, what's the event loop doing? 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. QThread is the central, low-level class for thread support in Qt. We enter a local QEventLoop, and when the reply has finished, the local event loop quits. We can change a QObject's affinity by calling QObject::moveToThread ; this will change the affinity of the object and of its children. 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. QCoreApplication::processEvents will process all the events in the event queue and return to the caller. Qt has had thread support for many years Qt 2. Be very careful when reentering the event loop "by other paths": it can lead to unwanted recursions! In main we started the event loop, as usual, by calling QApplication::exec line 2. We must look at the thread that created them: it's the thread that ran the MyThread constructor. We also have the option to manually force the event loop to run, by repeatedly calling QCoreApplication::processEvents inside our blocking task. 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. You should've guessed it: nothing! 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. 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. So, what do we do if we have a long task to run and don't want to block the event loop? 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 event , while signals are emitted "in the wild". Conceptually, this is how an event loop looks see the Qt Quarterly article linked above :. With the event delivery stuck, widgets won't update themselves QPaintEvent objects will sit in the queue , no further interaction with widgets is possible for the same reason , timers won't fire and networking communications will slow down and stop. Luckily, the same thing does not apply to deletion events the ones posted in the event queue by QObject::deleteLater. This implies that:. Signals and slots across threads work in a similar way. 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. Before discussing why you should never ever block the event loop , let's try to figure out what this "blocking" means. 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. From a code point of view, all events are instances of some subclass of QEvent , and all QObject-derived classes can override the QObject::event virtual method in order to handle events targeted to their instances. It dispatched the mouse press event and it's blocked waiting for the event handler to return. Another available option we can use to forcibly reenter the event loop is the QEventLoop class. 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. Qt now offers several classes for dealing with threads; let's start with an overview. QRunnable is a lightweight abstract class that can be used to start a task in another thread in a "run and forget" fashion. 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. This is not the case: QThread objects can start thread-local event loops running in the threads they represent. When "obj" emits its aSignal signal, which kind of connection will be used? For the same reason, you can't delete a QObject from another thread, but you must use QObject::deleteLater , which will post an event that will ultimately cause its deletion by the thread the object is living in. We also say that the thread affinity of a QObject is a certain thread, meaning that the object is living in that thread. Given these premises, how do we call methods on QObjects living in other threads? As we mentioned before, since Qt 4. Auto Connection default The behavior is the same as the Direct Connection, if the emitter and receiver are in the same thread. The same thing applies to local event loops started with QEventLoop. The QFutureWatcher 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. Therefore, the event loop can be woken up by:. 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. We overrode this method to emit the Button::clicked signal line 6 , which invokes the Worker::doWork slot of our worker object line 7. The behavior is the same as the Queued Connection, if the emitter and receiver are in different threads. By calling QEventLoop::exec we reenter the event loop, and we can connect signals to the QEventLoop::quit slot to make it quit. A quick and easy workaround for this is passing QEventLoop::ExcludeUserInputEvents to QCoreApplication::processEvents , which tells the event loop to not dispatch any user input event the events will simply stay in the queue. Let's go back to the Button example. After you click the button, the stack trace will look like this the stack grows downwards :. 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. The only notable exception I've found to this rule as of Qt 4. All Qt applications have a global thread pool available by calling QThreadPool::globalInstance , but one can always create a private QThreadPool instance and manage it explicitely. If we decide to implement cross-thread event posting with an internal socketpair 2 , all that is left is being woken up by activity on:. In order to use a QThread to run some code in a thread, we can subclass it and override the QThread::run method:. 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. In a UNIX-like system, window manager activity i. 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. 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. 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. This applies to objects which are built in the constructor of a QThread object:. For instance:. In every case, keep in mind the thread the emitting object is living in has no importance at all! So far we've always talked about " the event loop", taking somehow per granted that there's only one event loop in a Qt application. Being an event-driven toolkit, events and event delivery play a central role in Qt architecture. 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. Note that since Qt 4. Not being a general-purpose introduction to threads programming, we expect you to have some previous knowledge about:. 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. We can use the thread-safe QCoreApplication::postEvent method for posting an event for a certain object. In order to do so, all we have to do is subclass QRunnable and implement its run pure virtual method:. In this article we'll not give a comprehensive coverage about this topic; we'll instead focus on some thread-related key concepts see here and here for more information about the Qt event system. 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. In this document we'll follow the Qt naming conventions , which are:. 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. A QThread object represents one thread of execution. If we call QCoreApplication::processEvents inside the doWork slot, and the user clicks again on the button, the doWork slot will be invoked again :. QWidget::event detects the event is actually a mouse click and calls the specialized event handler, that is, Button::mousePressEvent line 5. That's why is so important to quickly react to events and return to the event loop as soon as possible! We can query anytime the thread affinity of a QObject by calling QObject::thread. Notice that since the arguments need to be copied in the event which is built behind the scenes, their types need to provide a public constructor, a public destructor and a public copy constructor, and must be registered within Qt type system by using the qRegisterMetaType function. One possible answer is to move the task into another thread: in the next sections we'll see how to do that. 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. 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.

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.

In particular, the current Qt documentation is simply wrong when it states:. Moreover, Qt requires that the child of a Qthread event loop must live in the same thread where the parent is living.

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, qthread event loop a look to the recommended reading list on this page. This is because the QThread object is living in another threadnamely, the one in which it was created.

We managed to block the event loopwhich means that no event is sent any more, until we return from the doWork slot, up qthread event loop stack, to the event loop, and let it process pending events. You should've guessed it: a direct connection.

The dispatcher itself loops around the event queue and sends queued events to their target objects, and therefore it is called the event loop. 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. 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! The signal aSignal will be emitted by the new thread represented by the Thread object ; since it is not the thread the Object object is living in which, by the way, is the same thread the Thread object is living in , just to stress that the sender's thread affinity doesn't matter , a queued connection will be used. A QThread local event loop can be started instead by calling QThread::exec inside its run method :. Therefore, we say that the main event loop is the one created by the thread which invoked main , and started with QCoreApplication::exec which must be called from that thread. When we connect a signal to a slot, the fifth argument of QObject::connect is used to specify the connection type:.