🖐 c++ - Is it safe to emit signal from another thread? - Stack Overflow

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in another thread? Qt has a solution: cross-thread signals and slots. You can emit a signal from one thread, and have the slot invoked by another thread. Not just.


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Can an object living in one thread emit a signal from another thread? | Qt Forum
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qt emit signal from another thread

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How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections the queue, and if the receiver is living in another thread, we notify the child thread might have already emitted, or is about to emit the signal.


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qt emit signal from another thread

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How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections the queue, and if the receiver is living in another thread, we notify the child thread might have already emitted, or is about to emit the signal.


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qt emit signal from another thread

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The QThread is the central class for of the Qt threading system. When run() is completed, thread will emit the finished() signal. The Thread::doWork() function lives in another thread than Object::started(), which has been.


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More interesting is that QObjects can be used in multiple threads, emit signals that invoke slots in other.


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qt emit signal from another thread

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in another thread? Qt has a solution: cross-thread signals and slots. You can emit a signal from one thread, and have the slot invoked by another thread. Not just.


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in another thread? Qt has a solution: cross-thread signals and slots. You can emit a signal from one thread, and have the slot invoked by another thread. Not just.


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How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections the queue, and if the receiver is living in another thread, we notify the child thread might have already emitted, or is about to emit the signal.


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qt emit signal from another thread

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