🎰 The Event System | Qt

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How big is Qt's event queue? Is its size customizable? What will happen to the signaler when it gets "full"? Example: I have 2 threads: thread A.


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Threads and QObjects | Qt
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When an event occurs, Qt creates an event object to represent it by constructing an instance of the postEvent() posts the event on a queue for later dispatch.


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How to peek at the Event queue? I'm writing a program that uses some third-party libraries. I've noticed that my program's memory and cpu.


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This is possible because each thread is allowed to have its own event loop. Queued Connection The slot is invoked when control returns to the event loop of​.


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The dispatcher itself loops around the event queue and sends queued events to their target objects, and therefore it is.


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How to peek at the Event queue? I'm writing a program that uses some third-party libraries. I've noticed that my program's memory and cpu.


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The event queued is protected by a mutex, so there is no race conditions when threads push events to another thread's event queue. Once the.


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Constructs an event loop object with the given parent. [slot] void QEventLoop::​quit(). Tells the event loop to exit normally. Same as exit.


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How big is Qt's event queue? Is its size customizable? What will happen to the signaler when it gets "full"? Example: I have 2 threads: thread A.


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The dispatcher itself loops around the event queue and sends queued events to their target objects, and therefore it is.


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Other threads can start an event loop using QThread::exec. Otherwise, the behavior is the same as the Queued Connection. This is possible because each thread is allowed to have its own event loop. A QObject instance is said to live in the thread in which it is created. This includes the entire event delivery system. The Mandelbrot Example uses a queued connection to communicate between a worker thread and the main thread. Documentation contributions included herein are the copyrights of their respective owners. It is important to keep in mind that the event loop may be delivering events to your QObject subclass while you are accessing the object from another thread. The slot is executed in the emitter's thread, which is not necessarily the receiver's thread. Use QObject::deleteLater instead, and a DeferredDelete event will be posted, which the event loop of the object's thread will eventually pick up. 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. As noted earlier, QCoreApplication::exec must also be called from that thread. Queued Connection The slot is invoked when control returns to the event loop of the receiver's thread. QObject::connect itself is thread-safe.{/INSERTKEYS}{/PARAGRAPH} Qt and respective logos are trademarks of The Qt Company Ltd. It is generally unsafe to provide slots in your QThread subclass, unless you protect the member variables with a mutex. These restrictions apply to the main thread as well. Unique Connection The behavior is the same as the Auto Connection, but the connection is made only if it does not duplicate an existing connection. A properly structured single or multi-threaded application should make the QApplication be the first created, and last destroyed QObject. The events will automatically be dispatched by the event loop of the thread where the object was created. More interesting is that QObject 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. In general, creating QObjects before the QApplication is not supported and can lead to weird crashes on exit, depending on the platform. QObject is reentrant. For example, if you create a QTimer object in a thread but never call exec , the QTimer will never emit its timeout signal. Events to that object are dispatched by that thread's event loop. The slot is executed in the receiver's thread. Similarly, QCoreApplication::sendEvent unlike postEvent can only be used to dispatch events to objects living in the thread from which the function is called. The thread emits a signal when it is done rendering the fractal. You must ensure that all objects created in a thread are deleted before you delete the QThread. To avoid freezing the main thread's event loop and, as a consequence, the application's user interface , all the Mandelbrot fractal computation is done in a separate worker thread. {PARAGRAPH}{INSERTKEYS}QThread inherits QObject. If you are calling a function on an QObject subclass that doesn't live in the current thread and the object might receive events, you must protect all access to your QObject subclass's internal data with a mutex; otherwise, you may experience crashes or other undesired behavior. Blocking Queued Connection The slot is invoked as for the Queued Connection, except the current thread blocks until the slot returns. There are three constraints to be aware of:. The connection type can be specified by passing an additional argument to connect. Specifically, this applies to the timer mechanism and the network module. You can manually post events to any object in any thread at any time using the thread-safe function QCoreApplication::postEvent. Per-Thread Event Loop Each thread can have its own event loop. Calling deleteLater won't work either. QObject and all of its subclasses are not thread-safe. 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. On the other hand, you can safely emit signals from your QThread::run implementation, because signal emission is thread-safe. Like other objects, QThread objects live in the thread where the object was created -- not in the thread that is created when QThread::run is called. This implies, among other things, that you should never pass the QThread object this as the parent of an object created in the thread since the QThread object itself was created in another thread. This means static instances of QObject are also not supported. Event filters are supported in all threads, with the restriction that the monitoring object must live in the same thread as the monitored object. This is explained in more detail in the Signals and Slots Across Threads section below. Each thread can have its own event loop. For example, you cannot start a timer or connect a socket in a thread that is not the object's thread. If no event loop is running, events won't be delivered to the object. 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. All other trademarks are property of their respective owners. This can be done easily by creating the objects on the stack in your run implementation. Signals and Slots Across Threads Qt supports these signal-slot connection types: Auto Connection default If the signal is emitted in the thread which the receiving object has affinity then the behavior is the same as the Direct Connection. The QObject::moveToThread function changes the thread affinity for an object and its children the object cannot be moved if it has a parent. They can only be used from the main thread. Calling delete on a QObject 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. There are three constraints to be aware of: The child of a QObject must always be created in the thread where the parent was created. QObject Reentrancy QObject is reentrant. Note: Using this type to connect objects in the same thread will cause deadlock. It also makes it possible to connect signals from any threads to slots of a specific thread. The thread in which a QObject lives is available using QObject::thread. Event driven objects may only be used in a single thread. It emits signals to indicate that the thread started or finished executing, and provides a few slots as well.