Qt Call Slot From Another Thread
2021年7月17日Register here: http://gg.gg/vg4ah
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.’ Direct ConnectionThe slot is invoked immediately, when the signal is emitted. Calling a slot from another thread I have two threads, one is my interface to Skype (through their API), and one is the GUI thread. A lot of the Skype API functions are blocking, thus the separate thread is necessary.
*Qt Call Slot In Different Thread
*Qt Execute Slot In Another Thread
*Qt Call Slot From Another Thread Sethow in BOOST send a signal in a thread and have the corresponding slot executed in another thread? boost::signals2
boost::signals2::signal
boost signal handler
boost signal multi-threaded
boost multithreading
In Qt for instance if you emit a signal in a thread other that the GUI thread, the signal is enqueued and executed later in the GUI thread, is there a way to do that with boost?
thanks
For an event loop use boost::asio::io_service. You can post tasks inside this object and have another thread execute them, in a thread safe way:
Messaging and Signaling in C++, But as this blog post is more on signaling then system events. Qt signal/slot implementation is thread safe, so that you can use it to send messages important, as anything UI related should run in the main thread of Qt, anything that I use this in a different program to have one widget for editing flag like Almost all classes provided by Boost.Signals2 are thread safe and can be used in multithreaded applications. For example, objects of type boost::signals2::signal and boost::signals2::connection can be accessed from different threads. On the other hand, boost::signals2::shared_connection_block is not thread safe.
Not directly, because boost does not provide an event loop.
To have a signal handled in another thread, that another thread needs to be checking the queue of handlers it should run and execute them (which usually means some kind of event-loop). Boost does not provide one, so you’ll need to get it from elsewhere or write it.
If you have an event-loop, that does not provide signals, (or implement some simple solution with queues) you should be able to (ab)use boost.signals2 (not boost.signals, because that version is not thread-safe) by overriding the operator+= to wrap each handler in something, that will queue it for execution in the other thread. You might even be able to implement it for signals with return values (which is not supported by Qt, but is supported by boost), but you’ll have to be careful to avoid dead-lock.
[PDF] Boost.Signals2, Signals2 library is an implementation of a managed signals and slots system. This documentation describes a thread-safe variant of the original Boost. so we put ’Hello’ into a group that must be executed before the group possible to set up tracking in a post-constructor which is called after the object has been created To have a signal handled in another thread, that another thread needs to be checking the queue of handlers it should run and execute them (which usually means some kind of event-loop). Boost does not provide one, so you’ll need to get it from elsewhere or write it.
Signals & Slots, Signals and slots are made possible by Qt’s meta-object system. to one signal, the slots will be executed one after the other, in the order they have valueChanged() , and it has a slot which other objects can send signals to. The context object provides information about in which thread the receiver should be executed. Special behavior for C++: If a thread is sent a signal using pthread_kill() and that thread does not handle the signal, then destructors for local objects may not be executed. Usage notes. The SIGTHSTOP and SIGTHCONT signals can be issued by this function. pthread_kill() is the only function that can issue SIGTHSTOP or SIGTHCONT. Returned value
Chila’s answer is correct, but it’s missing one important thing:A boost::thread object will only call the function its passed once. Since the boost::io_service has no work to do until the signal is emitted, the thread will finish immediately. To counter this there is a boost::asio::io_service::work class.Before you call the run() method of the io_service you should create a work object and pass it the io_service:
Note: At the time of writing (boost 1.67) this method is already deprecated and you are supposed to use io_context::executor_work_guard (basically same functionality as io_service::work). I was not able to compile when using the new method though, and the work solution is still working in boost 1.67.
Slots, It also implements a few conditional (event) related classes. Qt - SigSlot - Boost Libraries Qt was the original signal/slots implementation, but it Sigslot and Boost on the other hand are pure ISO C++, but both have some disadvantages. None of these are thread-safe and it can be somewhat inconvenient manually 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.
For some reason, the assignment operator of boost::asio::executor_work_guard<boost::asio::io_context::executor_type> is deleted, but you still can construct it.
Here’s my version of the code that posts some movable Event object and processes it on the thread running io_context::run():
It requires C++14 and was tested with VS2017 and GCC 6.4 with thread & memory sanitizers.Qt Call Slot In Different Thread
Observer pattern with Stl, boost and qt, A comparison between the Qt signal and slot mechanism and some Each slot is a potential callback ○ Adds more run-time introspection, The Synapse library ○ Another signals/slot like library ○ Submitted to Very similar to boost::signals2 ○ Have the ability to transfer control between threads 29; 30. So, when the thread is created from the create_thread method it will call the io_service::run method and it passes the io_service object as an argument. Typically one io_service object can be used with multiple socket objects.
QThreads: Are You Using Them Wrong?, Show related SlideShares at end The Basics of QThread QThread manages one thread of execution ○ The Signal Slot Connections and Threads ○ Qt::DirectConnection have same thread affinity: Direct ○ If objects have different thread It implies you want to send cross-thread signals to yourself. 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.
What do I do if a slot is not invoked?, A practical checklist to debug your signal/slot connections that an event loop is running in the thread the receiver has affinity with;; that all the arguments Using this signal is very easy – it just acts like a flag, but you can wait for it as well as read it. The following unit test (also in GitHub) shows how the signal makes it easy for threads to set gates on each other. Note that the final signal in this example could have been done with thread.join(), but wasn’t for the purposes of the test.
[Boost-users] Signals2 benchmark, I want to test it against boost::signals2 to get an idea of how well it performs. Suppose one thread disconnects a slot while another fires a signal Each Signal-type has its own corresponding vector of slots defined within the Emitter. the copy being made instead of every slot being called and executed. POSIX requires that signal is thread-safe, and specifies a list of async-signal-safe library functions that may be called from any signal handler. Signal handlers are expected to have C linkage and, in general, only use the features from the common subset of C and C++. It is implementation-defined if a function with C++ linkage can be used as a Comments
*THX for this very helpfull sample ! Since boost::signal is deprecated I have to use boost::signals2::signal<>.Hot Questions
Great blue heron casino dining. Home > Articles > Programming > C/C++␡
* Communicating with the Main Thread < BackPage 3 of 4Next >This chapter is from the book C++ GUI Programming with Qt4, 2nd Edition
This chapter is from the bookThis chapter is from the book Communicating with the Main Thread
When a Qt application starts, only one thread is running—the main thread. This is the only thread that is allowed to create the QApplication or QCoreApplication object and call exec() on it. After the call to exec(), this thread is either waiting for an event or processing an event.
The main thread can start new threads by creating objects of a QThread subclass, as we did in the previous section. If these new threads need to communicate among themselves, they can use shared variables together with mutexes, read-write locks, semaphores, or wait conditions. But none of these techniques can be used to communicate with the main thread, since they would lock the event loop and freeze the user interface.
The solution for communicating from a secondary thread to the main thread is to use signal–slot connections across threads. Normally, the signals and slots mechanism operates synchronously, meaning that the slots connected to a signal are invoked immediately when the signal is emitted, using a direct function call.
However, when we connect objects that ’live’ in different threads, the mechanism becomes asynchronous. (This behavior can be changed through an optional fifth parameter to QObject::connect().) Behind the scenes, these connections are implemented by posting an event. The slot is then called by the event loop of the thread in which the receiver object exists. By default, a QObject exists in the thread in which it was created; this can be changed at any time by calling QObject::moveToThread().
To illustrate how signal–slot connections across threads work, we will review the code of the Image Pro application, a basic image processing application that allows the user to rotate, resize, and change the color depth of an image. The application (shown in Figure 14.3), uses one secondary thread to perform operations on images without locking the event loop. This makes a significant difference when processing very large images. The secondary thread has a list of tasks, or ’transactions’, to accomplish and sends events to the main window to report progress.
Figure 14.3 The Image Pro application
Poker deluxe vip download. The interesting part of the ImageWindow constructor is the two signal–slot connections. Both of them involve signals emitted by the TransactionThread object, which we will cover in a moment.
The flipHorizontally() slot creates a ’flip’ transaction and registers it using the private function addTransaction(). The flipVertically(), resizeImage(), convertTo32Bit(), convertTo8Bit(), and convertTo1Bit() functions are similar.
The addTransaction() function adds a transaction to the secondary thread’s transaction queue and disables the Open, Save, and Save As actions while transactions are being processed.
The allTransactionsDone() slot is called when the TransactionThread’s transaction queue becomes empty.
Now, let’s turn to the TransactionThread class. Like most QThread subclasses, it is somewhat tricky to implement, because the run() function executes in its own thread, whereas the other functions (including the constructor and the destructor) are called from the main thread. The class definition follows:
The TransactionThread class maintains a queue of transactions to process and executes them one after the other in the background. In the private section, we declare four member variables:
*currentImage holds the image onto which the transactions are applied.
*transactions is the queue of pending transactions.
*transactionAdded is a wait condition that is used to wake up the thread when a new transaction has been added to the queue.
*mutex is used to protect the currentImage and transactions member variables against concurrent access.
Here is the class’s constructor:
In the constructor, we simply call QThread::start() to launch the thread that will execute the transactions.
In the destructor, we empty the transaction queue and add a special EndTransaction marker to the queue. Then we wake up the thread and wait for it to finish using QThread::wait(), before the base class’s destructor is implicitly invoked. Failing to call wait() would most probably result in a crash when the thread tries to access the class’s member variables.
The QMutexLocker’s destructor unlocks the mutex at the end of the inner block, just before the wait() call. It is important to unlock the mutex before calling wait(); otherwise, the program could end up in a deadlock situation, where the secondary thread waits forever for the mutex to be unlocked, and the main thread holds the mutex and waits for the secondary thread to finish before proceeding.
The addTransaction() function adds a transaction to the transaction queue and wakes up the transaction thread if it isn’t already running. All accesses to the transactions member variable are protected by a mutex, because the main thread might modify them through addTransaction() at the same time as the secondary thread is iterating over transactions.
The setImage() and image() functions allow the main thread to set the image on which the transactions should be performed, and to retrieve the resulting image once all the transactions are done.Qt Execute Slot In Another Thread
The run() function goes through the transaction queue and executes each transaction in turn by calling apply() on them, until it reaches the EndTransaction marker. If the transaction queue is empty, the thread waits on the ’transaction added’ condition.
Just before we execute a transaction, we emit the transactionStarted() signal with a message to display in the application’s status bar. When all the transactions have finished processing, we emit the allTransactionsDone() signal.
The Transaction class is an abstract base class for operations that the user can perform on an image. The virtual destructor is necessary because we need to delete instances of Transaction subclasses through a Transaction pointer. Transaction has three concrete subclasses: FlipTransaction, ResizeTransaction, and ConvertDepthTransaction. We will only review FlipTransaction; the other two classes are similar.
The FlipTransaction constructor takes one parameter that specifies the orientation of the flip (horizontal or vertical).
The apply() function calls QImage::mirrored() on the QImage it receives as a parameter and returns the resulting QImage.Qt Call Slot From Another Thread Set
The message() function returns the message to display in the status bar while the operation is in progress. This function is called in TransactionThread::run() when emitting the transactionStarted() signal.
The Image Pro application shows how Qt’s signals and slots mechanism makes it easy to communicate with the main thread from a secondary thread. Implementing the secondary thread is trickier, because we must protect our member variables using a mutex, and we must put the thread to sleep and wake it up appropriately using a wait condition. The two-part Qt Quarterly article series ’Monitors and Wait Conditions in Qt’, available online at http://doc.trolltech.com/qq/qq21-monitors.html and http://doc.trolltech.com/qq/qq22-monitors2.html, presents some more ideas on how to develop and test QThread subclasses that use mutexes and wait conditions for synchronization.Related Resources
*Book $31.99
*eBook (Watermarked) $25.59
*eBook (Watermarked) $28.79
Register here: http://gg.gg/vg4ah
https://diarynote.indered.space
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.’ Direct ConnectionThe slot is invoked immediately, when the signal is emitted. Calling a slot from another thread I have two threads, one is my interface to Skype (through their API), and one is the GUI thread. A lot of the Skype API functions are blocking, thus the separate thread is necessary.
*Qt Call Slot In Different Thread
*Qt Execute Slot In Another Thread
*Qt Call Slot From Another Thread Sethow in BOOST send a signal in a thread and have the corresponding slot executed in another thread? boost::signals2
boost::signals2::signal
boost signal handler
boost signal multi-threaded
boost multithreading
In Qt for instance if you emit a signal in a thread other that the GUI thread, the signal is enqueued and executed later in the GUI thread, is there a way to do that with boost?
thanks
For an event loop use boost::asio::io_service. You can post tasks inside this object and have another thread execute them, in a thread safe way:
Messaging and Signaling in C++, But as this blog post is more on signaling then system events. Qt signal/slot implementation is thread safe, so that you can use it to send messages important, as anything UI related should run in the main thread of Qt, anything that I use this in a different program to have one widget for editing flag like Almost all classes provided by Boost.Signals2 are thread safe and can be used in multithreaded applications. For example, objects of type boost::signals2::signal and boost::signals2::connection can be accessed from different threads. On the other hand, boost::signals2::shared_connection_block is not thread safe.
Not directly, because boost does not provide an event loop.
To have a signal handled in another thread, that another thread needs to be checking the queue of handlers it should run and execute them (which usually means some kind of event-loop). Boost does not provide one, so you’ll need to get it from elsewhere or write it.
If you have an event-loop, that does not provide signals, (or implement some simple solution with queues) you should be able to (ab)use boost.signals2 (not boost.signals, because that version is not thread-safe) by overriding the operator+= to wrap each handler in something, that will queue it for execution in the other thread. You might even be able to implement it for signals with return values (which is not supported by Qt, but is supported by boost), but you’ll have to be careful to avoid dead-lock.
[PDF] Boost.Signals2, Signals2 library is an implementation of a managed signals and slots system. This documentation describes a thread-safe variant of the original Boost. so we put ’Hello’ into a group that must be executed before the group possible to set up tracking in a post-constructor which is called after the object has been created To have a signal handled in another thread, that another thread needs to be checking the queue of handlers it should run and execute them (which usually means some kind of event-loop). Boost does not provide one, so you’ll need to get it from elsewhere or write it.
Signals & Slots, Signals and slots are made possible by Qt’s meta-object system. to one signal, the slots will be executed one after the other, in the order they have valueChanged() , and it has a slot which other objects can send signals to. The context object provides information about in which thread the receiver should be executed. Special behavior for C++: If a thread is sent a signal using pthread_kill() and that thread does not handle the signal, then destructors for local objects may not be executed. Usage notes. The SIGTHSTOP and SIGTHCONT signals can be issued by this function. pthread_kill() is the only function that can issue SIGTHSTOP or SIGTHCONT. Returned value
Chila’s answer is correct, but it’s missing one important thing:A boost::thread object will only call the function its passed once. Since the boost::io_service has no work to do until the signal is emitted, the thread will finish immediately. To counter this there is a boost::asio::io_service::work class.Before you call the run() method of the io_service you should create a work object and pass it the io_service:
Note: At the time of writing (boost 1.67) this method is already deprecated and you are supposed to use io_context::executor_work_guard (basically same functionality as io_service::work). I was not able to compile when using the new method though, and the work solution is still working in boost 1.67.
Slots, It also implements a few conditional (event) related classes. Qt - SigSlot - Boost Libraries Qt was the original signal/slots implementation, but it Sigslot and Boost on the other hand are pure ISO C++, but both have some disadvantages. None of these are thread-safe and it can be somewhat inconvenient manually 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.
For some reason, the assignment operator of boost::asio::executor_work_guard<boost::asio::io_context::executor_type> is deleted, but you still can construct it.
Here’s my version of the code that posts some movable Event object and processes it on the thread running io_context::run():
It requires C++14 and was tested with VS2017 and GCC 6.4 with thread & memory sanitizers.Qt Call Slot In Different Thread
Observer pattern with Stl, boost and qt, A comparison between the Qt signal and slot mechanism and some Each slot is a potential callback ○ Adds more run-time introspection, The Synapse library ○ Another signals/slot like library ○ Submitted to Very similar to boost::signals2 ○ Have the ability to transfer control between threads 29; 30. So, when the thread is created from the create_thread method it will call the io_service::run method and it passes the io_service object as an argument. Typically one io_service object can be used with multiple socket objects.
QThreads: Are You Using Them Wrong?, Show related SlideShares at end The Basics of QThread QThread manages one thread of execution ○ The Signal Slot Connections and Threads ○ Qt::DirectConnection have same thread affinity: Direct ○ If objects have different thread It implies you want to send cross-thread signals to yourself. 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.
What do I do if a slot is not invoked?, A practical checklist to debug your signal/slot connections that an event loop is running in the thread the receiver has affinity with;; that all the arguments Using this signal is very easy – it just acts like a flag, but you can wait for it as well as read it. The following unit test (also in GitHub) shows how the signal makes it easy for threads to set gates on each other. Note that the final signal in this example could have been done with thread.join(), but wasn’t for the purposes of the test.
[Boost-users] Signals2 benchmark, I want to test it against boost::signals2 to get an idea of how well it performs. Suppose one thread disconnects a slot while another fires a signal Each Signal-type has its own corresponding vector of slots defined within the Emitter. the copy being made instead of every slot being called and executed. POSIX requires that signal is thread-safe, and specifies a list of async-signal-safe library functions that may be called from any signal handler. Signal handlers are expected to have C linkage and, in general, only use the features from the common subset of C and C++. It is implementation-defined if a function with C++ linkage can be used as a Comments
*THX for this very helpfull sample ! Since boost::signal is deprecated I have to use boost::signals2::signal<>.Hot Questions
Great blue heron casino dining. Home > Articles > Programming > C/C++␡
* Communicating with the Main Thread < BackPage 3 of 4Next >This chapter is from the book C++ GUI Programming with Qt4, 2nd Edition
This chapter is from the bookThis chapter is from the book Communicating with the Main Thread
When a Qt application starts, only one thread is running—the main thread. This is the only thread that is allowed to create the QApplication or QCoreApplication object and call exec() on it. After the call to exec(), this thread is either waiting for an event or processing an event.
The main thread can start new threads by creating objects of a QThread subclass, as we did in the previous section. If these new threads need to communicate among themselves, they can use shared variables together with mutexes, read-write locks, semaphores, or wait conditions. But none of these techniques can be used to communicate with the main thread, since they would lock the event loop and freeze the user interface.
The solution for communicating from a secondary thread to the main thread is to use signal–slot connections across threads. Normally, the signals and slots mechanism operates synchronously, meaning that the slots connected to a signal are invoked immediately when the signal is emitted, using a direct function call.
However, when we connect objects that ’live’ in different threads, the mechanism becomes asynchronous. (This behavior can be changed through an optional fifth parameter to QObject::connect().) Behind the scenes, these connections are implemented by posting an event. The slot is then called by the event loop of the thread in which the receiver object exists. By default, a QObject exists in the thread in which it was created; this can be changed at any time by calling QObject::moveToThread().
To illustrate how signal–slot connections across threads work, we will review the code of the Image Pro application, a basic image processing application that allows the user to rotate, resize, and change the color depth of an image. The application (shown in Figure 14.3), uses one secondary thread to perform operations on images without locking the event loop. This makes a significant difference when processing very large images. The secondary thread has a list of tasks, or ’transactions’, to accomplish and sends events to the main window to report progress.
Figure 14.3 The Image Pro application
Poker deluxe vip download. The interesting part of the ImageWindow constructor is the two signal–slot connections. Both of them involve signals emitted by the TransactionThread object, which we will cover in a moment.
The flipHorizontally() slot creates a ’flip’ transaction and registers it using the private function addTransaction(). The flipVertically(), resizeImage(), convertTo32Bit(), convertTo8Bit(), and convertTo1Bit() functions are similar.
The addTransaction() function adds a transaction to the secondary thread’s transaction queue and disables the Open, Save, and Save As actions while transactions are being processed.
The allTransactionsDone() slot is called when the TransactionThread’s transaction queue becomes empty.
Now, let’s turn to the TransactionThread class. Like most QThread subclasses, it is somewhat tricky to implement, because the run() function executes in its own thread, whereas the other functions (including the constructor and the destructor) are called from the main thread. The class definition follows:
The TransactionThread class maintains a queue of transactions to process and executes them one after the other in the background. In the private section, we declare four member variables:
*currentImage holds the image onto which the transactions are applied.
*transactions is the queue of pending transactions.
*transactionAdded is a wait condition that is used to wake up the thread when a new transaction has been added to the queue.
*mutex is used to protect the currentImage and transactions member variables against concurrent access.
Here is the class’s constructor:
In the constructor, we simply call QThread::start() to launch the thread that will execute the transactions.
In the destructor, we empty the transaction queue and add a special EndTransaction marker to the queue. Then we wake up the thread and wait for it to finish using QThread::wait(), before the base class’s destructor is implicitly invoked. Failing to call wait() would most probably result in a crash when the thread tries to access the class’s member variables.
The QMutexLocker’s destructor unlocks the mutex at the end of the inner block, just before the wait() call. It is important to unlock the mutex before calling wait(); otherwise, the program could end up in a deadlock situation, where the secondary thread waits forever for the mutex to be unlocked, and the main thread holds the mutex and waits for the secondary thread to finish before proceeding.
The addTransaction() function adds a transaction to the transaction queue and wakes up the transaction thread if it isn’t already running. All accesses to the transactions member variable are protected by a mutex, because the main thread might modify them through addTransaction() at the same time as the secondary thread is iterating over transactions.
The setImage() and image() functions allow the main thread to set the image on which the transactions should be performed, and to retrieve the resulting image once all the transactions are done.Qt Execute Slot In Another Thread
The run() function goes through the transaction queue and executes each transaction in turn by calling apply() on them, until it reaches the EndTransaction marker. If the transaction queue is empty, the thread waits on the ’transaction added’ condition.
Just before we execute a transaction, we emit the transactionStarted() signal with a message to display in the application’s status bar. When all the transactions have finished processing, we emit the allTransactionsDone() signal.
The Transaction class is an abstract base class for operations that the user can perform on an image. The virtual destructor is necessary because we need to delete instances of Transaction subclasses through a Transaction pointer. Transaction has three concrete subclasses: FlipTransaction, ResizeTransaction, and ConvertDepthTransaction. We will only review FlipTransaction; the other two classes are similar.
The FlipTransaction constructor takes one parameter that specifies the orientation of the flip (horizontal or vertical).
The apply() function calls QImage::mirrored() on the QImage it receives as a parameter and returns the resulting QImage.Qt Call Slot From Another Thread Set
The message() function returns the message to display in the status bar while the operation is in progress. This function is called in TransactionThread::run() when emitting the transactionStarted() signal.
The Image Pro application shows how Qt’s signals and slots mechanism makes it easy to communicate with the main thread from a secondary thread. Implementing the secondary thread is trickier, because we must protect our member variables using a mutex, and we must put the thread to sleep and wake it up appropriately using a wait condition. The two-part Qt Quarterly article series ’Monitors and Wait Conditions in Qt’, available online at http://doc.trolltech.com/qq/qq21-monitors.html and http://doc.trolltech.com/qq/qq22-monitors2.html, presents some more ideas on how to develop and test QThread subclasses that use mutexes and wait conditions for synchronization.Related Resources
*Book $31.99
*eBook (Watermarked) $25.59
*eBook (Watermarked) $28.79
Register here: http://gg.gg/vg4ah
https://diarynote.indered.space
コメント