Qt Signal Slot Between Threads

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Qt Signal Slot Between Threads

QThread inherits QObject. It emits signals to indicate that the thread started or finished executing, and provides a few slots as well.
All passing (such as updateCount) happens using signals and slots. When passing data between threads using signals and slots Qt handles thread synchronization for you. The stopWork function is called via a signal so the function runs on the thread the work is running on between iterations of the while loop.
The Qt signals/slots and property system are based on the ability to introspect the objects at runtime. Introspection means being able to list the methods and properties of an object and have all kinds of information about them such as the type of their arguments. QtScript and QML would have hardly been possible without that ability.
When one QObject instance is the receiver of a signal, its thread affinity is checked, and by default the signal is delivered directly as a function call when is the same thread affinity of the sender. But if the thread affinity differs, it will be delivered posting an event to the object. 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. Otherwise, the behavior is the same as the Queued Connection.' The connection mechanism uses a vector indexed by signals. But all the slots waste space in the vector and there are usually more slots than signals in an object. So from Qt 4.6, a new internal signal index which only includes the signal index is used. While developing with Qt, you only need to know about the absolute method index.
More interesting is that QObjects 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

QObject is reentrant. Most of its non-GUI subclasses, such as QTimer, QTcpSocket, QUdpSocket and QProcess, are 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. 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. 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).
Event driven objects may only be used in a single thread. Specifically, this applies to the timer mechanism and the network module. For example, you cannot start a timer or connect a socket in a thread that is not the object's 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.
Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread. As noted earlier, QCoreApplication::exec() must also be called from that thread. Neosurf casino bonus codes 2019.
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 Mandelbrot Example and the Blocking Fortune Client Example.
In general, creating QObjects before the QApplication is not supported and can lead to weird crashes on exit, depending on the platform. This means static instances of QObject are also not supported. A properly structured single or multi-threaded application should make the QApplication be the first created, and last destroyed QObject.
Per-Thread Event LoopEach thread can have its own event loop. The initial thread starts its event loop using QCoreApplication::exec(), or for single-dialog GUI applications, sometimes QDialog::exec(). Other threads can start an event loop using QThread::exec(). Like QCoreApplication, QThread provides an exit(int) function and a quit() 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 QTimer, QTcpSocket, and QProcess). 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 Signals and Slots Across Threads section below.
A QObject 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 QObject lives is available using QObject::thread().
The QObject::moveToThread() 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 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. Use QObject::deleteLater() instead, and a DeferredDelete event will be posted, which the event loop of the object's thread will eventually pick up. By default, the thread that owns a QObject is the thread that creates the QObject, but not after QObject::moveToThread() has been called.
If no event loop is running, events won't be delivered to the object. For example, if you create a QTimer object in a thread but never call exec(), the QTimer will never emit its timeout() signal. Calling deleteLater() 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 time using the thread-safe function QCoreApplication::postEvent(). 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, QCoreApplication::sendEvent() (unlike postEvent()) can only be used to dispatch events to objects living in the thread from which the function is called.
Accessing QObject Subclasses from Other ThreadsQObject 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 QObject subclass while you are accessing the object from another thread.
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.
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. It is generally unsafe to provide slots in your QThread subclass, unless you protect the member variables with a mutex.
On the other hand, you can safely emit signals from your QThread::run() implementation, because signal emission is thread-safe.
Signals and Slots Across ThreadsQt 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. Otherwise, the behavior is the same as the Queued Connection.'
Direct Connection The slot is invoked immediately, when the signal is emitted. The slot is executed in the emitter's thread, which is not necessarily the receiver's thread.
Queued Connection The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.
Blocking Queued Connection The slot is invoked as for the Queued Connection, except the current thread blocks until the slot returns.Note:  Casino near alligator alley. Using this type to connect objects in the same thread will cause deadlock.
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. i.e., if the same signal is already connected to the same slot for the same pair of objects, then the connection is not made and connect() returns false.
The connection type can be specified by passing an additional argument to connect(). 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.
QObject::connect() itself is thread-safe.
The Mandelbrot Example 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 interface), all the Mandelbrot fractal computation is done in a separate worker thread. Tattoo poker chips. The thread emits a signal when it is done rendering the fractal.
Similarly, the Blocking Fortune Client Example uses a separate thread for communicating with a TCP server asynchronously.
Qt Send Signal© 2020 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 GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners. 
Example

Some times you see a signal is emitted in sender thread but connected slot doesn't called (in other words it doesn't receive signal), you have asked about it and finaly got that the connection type Qt::DirectConnection would fix it, so the problem found and everything is ok.
But generaly this is bad idea to use Qt:DirectConnection until you really know what is this and there is no other way. Lets explain it more, Each thread created by Qt (including main thread and new threads created by QThread) have Event loop, the event loop is responsible for receiving signals and call aproporiate slots in its thread. Generaly executing a blocking operation inside an slot is bad practice, because it blocks the event loop of that threads so no other slots would be called.
If you block an event loop (by making very time consuming or blocking operation) you will not receive events on that thread until the event loop will be unblocked. If the blocking operation, blocks the event loop forever (such as busy while), the slots could never be called.
In this situation you may set the connection type in connect to Qt::DirectConnection, now the slots will be called even the event loop is blocked. so how this could make broke everything? In Qt::DirectConnection Slots will be called in emiter threads, and not receiver threads and it can broke data synchronizations and ran into other problems. So never use Qt::DirectConnection unless you know what are you doing. If your problem will be solved by using Qt::DirectConnection, you have to carefull and look at your code and finding out why your event loop is blocked. Its not a good idea to block the event loop and its not recomended in Qt.
Here is small example which shows the problem, as you can see the nonBlockingSlot would be called even the blockingSlot blocked event loop with while(1) which indicates bad coding

Qt Signals Slots Threads Example
Qt Signal Slot Between Threads Bolt

QObject is reentrant. Most of its non-GUI subclasses, such as QTimer, QTcpSocket, QUdpSocket and QProcess, are 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. 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. 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).
Event driven objects may only be used in a single thread. Specifically, this applies to the timer mechanism and the network module. For example, you cannot start a timer or connect a socket in a thread that is not the object's 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.
Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread. As noted earlier, QCoreApplication::exec() must also be called from that thread. Neosurf casino bonus codes 2019.
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 Mandelbrot Example and the Blocking Fortune Client Example.
In general, creating QObjects before the QApplication is not supported and can lead to weird crashes on exit, depending on the platform. This means static instances of QObject are also not supported. A properly structured single or multi-threaded application should make the QApplication be the first created, and last destroyed QObject.
Per-Thread Event LoopEach thread can have its own event loop. The initial thread starts its event loop using QCoreApplication::exec(), or for single-dialog GUI applications, sometimes QDialog::exec(). Other threads can start an event loop using QThread::exec(). Like QCoreApplication, QThread provides an exit(int) function and a quit() 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 QTimer, QTcpSocket, and QProcess). 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 Signals and Slots Across Threads section below.
A QObject 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 QObject lives is available using QObject::thread().
The QObject::moveToThread() 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 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. Use QObject::deleteLater() instead, and a DeferredDelete event will be posted, which the event loop of the object's thread will eventually pick up. By default, the thread that owns a QObject is the thread that creates the QObject, but not after QObject::moveToThread() has been called.
If no event loop is running, events won't be delivered to the object. For example, if you create a QTimer object in a thread but never call exec(), the QTimer will never emit its timeout() signal. Calling deleteLater() 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 time using the thread-safe function QCoreApplication::postEvent(). 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, QCoreApplication::sendEvent() (unlike postEvent()) can only be used to dispatch events to objects living in the thread from which the function is called.
Accessing QObject Subclasses from Other ThreadsQObject 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 QObject subclass while you are accessing the object from another thread.
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.
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. It is generally unsafe to provide slots in your QThread subclass, unless you protect the member variables with a mutex.
On the other hand, you can safely emit signals from your QThread::run() implementation, because signal emission is thread-safe.
Signals and Slots Across ThreadsQt 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. Otherwise, the behavior is the same as the Queued Connection.'
Direct Connection The slot is invoked immediately, when the signal is emitted. The slot is executed in the emitter's thread, which is not necessarily the receiver's thread.
Queued Connection The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.
Blocking Queued Connection The slot is invoked as for the Queued Connection, except the current thread blocks until the slot returns.Note:  Casino near alligator alley. Using this type to connect objects in the same thread will cause deadlock.
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. i.e., if the same signal is already connected to the same slot for the same pair of objects, then the connection is not made and connect() returns false.
The connection type can be specified by passing an additional argument to connect(). 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.
QObject::connect() itself is thread-safe.
The Mandelbrot Example 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 interface), all the Mandelbrot fractal computation is done in a separate worker thread. Tattoo poker chips. The thread emits a signal when it is done rendering the fractal.
Similarly, the Blocking Fortune Client Example uses a separate thread for communicating with a TCP server asynchronously.
Qt Send Signal© 2020 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 GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners. 
ExampleSome times you see a signal is emitted in sender thread but connected slot doesn't called (in other words it doesn't receive signal), you have asked about it and finaly got that the connection type Qt::DirectConnection would fix it, so the problem found and everything is ok.
But generaly this is bad idea to use Qt:DirectConnection until you really know what is this and there is no other way. Lets explain it more, Each thread created by Qt (including main thread and new threads created by QThread) have Event loop, the event loop is responsible for receiving signals and call aproporiate slots in its thread. Generaly executing a blocking operation inside an slot is bad practice, because it blocks the event loop of that threads so no other slots would be called.
If you block an event loop (by making very time consuming or blocking operation) you will not receive events on that thread until the event loop will be unblocked. If the blocking operation, blocks the event loop forever (such as busy while), the slots could never be called.
In this situation you may set the connection type in connect to Qt::DirectConnection, now the slots will be called even the event loop is blocked. so how this could make broke everything? In Qt::DirectConnection Slots will be called in emiter threads, and not receiver threads and it can broke data synchronizations and ran into other problems. So never use Qt::DirectConnection unless you know what are you doing. If your problem will be solved by using Qt::DirectConnection, you have to carefull and look at your code and finding out why your event loop is blocked. Its not a good idea to block the event loop and its not recomended in Qt.
Here is small example which shows the problem, as you can see the nonBlockingSlot would be called even the blockingSlot blocked event loop with while(1) which indicates bad coding

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