Qt Invoke Slot Another Thread

• Qt Call Slot In Different Thread
• Qt Call Slot From Another Thread
• Qt Invoke Slot Another Thread Holder
• Qt Call Method From Another Thread
• Qt Invoke Slot Another Thread Tool

Direct ConnectionThe 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 ConnectionThe slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.

QtUtils provides convenience functions for accessing Qt objects in a thread safe way.Qt requires that all GUI objects exist in the MainThread and that access to these objects is only made from the MainThread (see Qt documentation).This, while understandable, imposes significant limits on Python applications where threading is easy.While there are solutions using Qt signals, slots and a QThread, these require significant boiler plate code that we believe is unnecessary.

Note

There is some debate as to whether using Python threads with any part of the Qt library is safe, however this has been recently challenged. The QtUtils library only instantiates a QEvent and calls QCoreApplication.postEvent() from a Python thread. It seems likely that as long as the underlying Python threading implementation matches the underlying Qt threading implementation for your particular platform, that there is no issue with how we have written this library. While we have not observed any issues with our library (and we have used it extensively on Windows, OSX and Ubuntu), this does not mean all platforms will behave in the same way. If this matters to you, we suggest you confirm the underlying thread implementation for your build of Python and Qt.

Examples¶

1. This is rather intuitive and easy to used. But when SLOTS and Qt event loop are used in the worker thread, some users do it wrong. Hughes, one of the Qt core developers, recommend that use worker objects by moving them to the thread using QObject::moveToThread. Unfortunately, some users went on a crusade against the former usage.
2. QObject::killTimer: Timers cannot be stopped from another thread Stopped worker process in Thread 0x15b8 After closing the window: QObject::QObject: Timers cannot be stopped from another thread @mrjj case would have a signal from the simulator, but I just get the signal from pushing the stop button.

We utilise the Qt event loop to execute arbitrary methods in the MainThread by posting a Qt event to the MainThread from a secondary thread.QtUtils provides a function called inmain which takes a reference to a method to execute in the MainThread, followed by any arguments to be passed to the method.

A call to inmain blocks the calling thread until the Qt event loop can process our message, execute the specified method and return the result.For situations where you don't wait to wait for the result, or you wish to do some other processing while waiting for the result, QtUtils provides the inmain_later function.This works in the same way as inmain, but returns a reference to a Python Queue object immediately.The result can be retrieved from this queue at any time, as shown in the following example:

This of course works directly with Qt methods as well as user defined functions/methods.For example:

As you can see, the change between a direct call to a Qt method, and doing it in a thread safe way, is very simple:

We also provide decorators so that you can ensure the decorated method always runs in the MainThread regardless of the calling thread.This is particularly useful when combined with Python properties.

QtUtils also provides a convenience function for launching a Python thread in daemon mode.inthread(target_method,arg1,arg2,..kwarg1=False,kwargs2=7,..)

Exception handling¶

Typically, exceptions are raised in the calling thread.However, inmain_later and the associated decorator will also raise the exception in the MainThread as there is no guarantee that the results will ever be read from the calling thread.

Using QtUtils from the MainThread¶

When using inmain, or the associated decorator, QtUtils will bypass the Qt Event loop as just immediately execute the specified method.This avoids the obvious deadlock where the calling code is being executed by the Qt event loop, and is now waiting for the Qt event loop to execute the next event (which won't ever happen because it is blocked waiting for the next event by the calling code).inmain_later still posts an event to the Qt event loop when used from the MainThread.This is useful if you want to execute something asynchronously from the MainThread (for example, asynchronously update the text of a label) but we recommend you do not attempt to read the result of such a call as you risk creating a deadlock.

What if I want to wait for user input in a thread?¶

If you want your thread to wait for user input, then this is not the library for you!We suggest you check out how to Wait in thread for user input from GUI for a Qt solution and/or Python threading events for a Python solution.

API reference¶

An event containing a request for a function call.

An event handler which calls the function held within a CallEvent.

event(self, QEvent) → bool[source]¶

Processes the result of qtutils.invoke_in_main.inmain_later().

This function takes the queue returned by inmain_later and blocksuntil a result is obtained. If an exception occurred when executing thefunction in the MainThread, it is raised again here (it is also raised in theMainThread). If no exception was raised, the result from the execution of thefunction is returned.

queue – The Python Queue object returned by inmain_later

The result from executing the function specified in the call toinmain_later

Execute a function in the main thread. Wait for it to completeand return its return value.

This function queues up a custom QEvent to the Qt event loop.This event executes the specified function fn in the PythonMainThread with the specified arguments and keyword arguments, and returns the result to the calling thread.

This function can be used from the MainThread, but such use will just directly call the function, bypassing the Qt event loop.

• fn – A reference to the function or method to run in the MainThread.

• *args – Any arguments to pass to fn when it is called from theMainThread.

• **kwargs – Any keyword arguments to pass to fn when it is calledfrom the MainThread

The result of executing fn(*args,**kwargs)

A decorator which enforces the execution of the decorated thread to occur in the MainThread.

This decorator wraps the decorated function or method in eitherqtutils.invoke_in_main.inmain() orqtutils.invoke_in_main.inmain_later().

This of course works directly with Qt methods as well as user defined functions/methods.For example:

As you can see, the change between a direct call to a Qt method, and doing it in a thread safe way, is very simple:

We also provide decorators so that you can ensure the decorated method always runs in the MainThread regardless of the calling thread.This is particularly useful when combined with Python properties.

QtUtils also provides a convenience function for launching a Python thread in daemon mode.inthread(target_method,arg1,arg2,..kwarg1=False,kwargs2=7,..)

Exception handling¶

Typically, exceptions are raised in the calling thread.However, inmain_later and the associated decorator will also raise the exception in the MainThread as there is no guarantee that the results will ever be read from the calling thread.

Using QtUtils from the MainThread¶

When using inmain, or the associated decorator, QtUtils will bypass the Qt Event loop as just immediately execute the specified method.This avoids the obvious deadlock where the calling code is being executed by the Qt event loop, and is now waiting for the Qt event loop to execute the next event (which won't ever happen because it is blocked waiting for the next event by the calling code).inmain_later still posts an event to the Qt event loop when used from the MainThread.This is useful if you want to execute something asynchronously from the MainThread (for example, asynchronously update the text of a label) but we recommend you do not attempt to read the result of such a call as you risk creating a deadlock.

What if I want to wait for user input in a thread?¶

If you want your thread to wait for user input, then this is not the library for you!We suggest you check out how to Wait in thread for user input from GUI for a Qt solution and/or Python threading events for a Python solution.

API reference¶

An event containing a request for a function call.

An event handler which calls the function held within a CallEvent.

event(self, QEvent) → bool[source]¶

Processes the result of qtutils.invoke_in_main.inmain_later().

This function takes the queue returned by inmain_later and blocksuntil a result is obtained. If an exception occurred when executing thefunction in the MainThread, it is raised again here (it is also raised in theMainThread). If no exception was raised, the result from the execution of thefunction is returned.

queue – The Python Queue object returned by inmain_later

The result from executing the function specified in the call toinmain_later

Execute a function in the main thread. Wait for it to completeand return its return value.

This function queues up a custom QEvent to the Qt event loop.This event executes the specified function fn in the PythonMainThread with the specified arguments and keyword arguments, and returns the result to the calling thread.

This function can be used from the MainThread, but such use will just directly call the function, bypassing the Qt event loop.

• fn – A reference to the function or method to run in the MainThread.

• *args – Any arguments to pass to fn when it is called from theMainThread.

• **kwargs – Any keyword arguments to pass to fn when it is calledfrom the MainThread

The result of executing fn(*args,**kwargs)

A decorator which enforces the execution of the decorated thread to occur in the MainThread.

This decorator wraps the decorated function or method in eitherqtutils.invoke_in_main.inmain() orqtutils.invoke_in_main.inmain_later().

• wait_for_return – Specifies whether to use inmain (ifTrue) or inmain_later (ifFalse).

• exceptions_in_main – Specifies whether the exceptions should be raisedin the main thread or not. This is ignored ifwait_for_return=True. If this isFalse, then exceptions may be silenced ifyou do not explicitly useqtutils.invoke_in_main.get_inmain_result().

The decorator returns a function that has wrapped the decorated functionin the appropriate call to inmain or inmain_later (ifyou are unfamiliar with how decorators work, please see the Pythondocumentation).

When calling the decorated function, the result is either the result ofthe function executed in the MainThread (if wait_for_return=True)or a Python Queue to be used withqtutils.invoke_in_main.get_inmain_result() at a later time.

Queue up the executing of a function in the main thread and return immediately.

This function queues up a custom QEvent to the Qt event loop.This event executes the specified function fn in the PythonMainThread with the specified arguments and keyword arguments, and returnsa Python Queue which will eventually hold the result from the executing offn. To access the result, use qtutils.invoke_in_main.get_inmain_result().

This function can be used from the MainThread, but such use will just directly call the function, bypassing the Qt event loop.

Qt Call Slot In Different Thread

• fn – A reference to the function or method to run in the MainThread.

• *args – Any arguments to pass to fn when it is called from theMainThread.

• **kwargs – Any keyword arguments to pass to fn when it is calledfrom the MainThread

A Python Queue which will eventually hold the result(fn(*args,**kwargs),exception) whereexception=[type,value,traceback].

A convenience function for starting a Python thread.

This function launches a Python thread in Daemon mode, and returns areference to the running thread object.

• f – A reference to the target function to be executed in the Python thread.

• *args – Any arguments to pass to f when it is executed in thenew thread.

• **kwargs – Any keyword arguments to pass to f when it is executedin the new thread.

A reference to the (already running) Python thread object

how in BOOST send a signal in a thread and have the corresponding slot executed in another thread?

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:

Petit Casino Rond Point Saint Etienne a deposit to get your hands on 50 wager-free spins. Read our full review. Prize pool: 100% up to £1000 + 50 free spins. Petit Casino Rond Point Saint Etienne, stockton poker room, slots capital sibnup, midnight diamonds free slot. Leo Vegas Casino - Welcome Bonus 100%. Petit casino rond point saint etienne Assistir 007 casino royale online dublado armageddon Site remediation and development columbus, oh – hull hollywood casino columbus site remediation and development the new casino in april 2011 and opened the facility for business in october 2012.

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.

Qt Call Slot From Another Thread

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 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.

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.

Qt Invoke Slot Another Thread Holder

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.

Crown casino melbourne online induction. 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.

Qt Call Method From Another Thread

[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

Qt Invoke Slot Another Thread Tool

• THX for this very helpfull sample ! Since boost::signal is deprecated I have to use boost::signals2::signal<>.