Connecting signal slots across threads
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QThread Class | Qt Core
I know this should pretty much work "out of the box" if you pay attention to some details, which I tried to - but I aceoss have missed something. I have a worker thread which starts another worker thread let's call it subworker. The worker and the subworker need to communicate via signals and slots. I make all the connections from the worker thread, and all the connect statements return true when running also, there's no warnings reported in the debug output.
My debugging shows me that the beginWork slot is correctly hit, but after the acroxs from SubWorker is emitted, I'm not hitting the breakpoint inside the handleSignalFromSubWorker slot.
SGaist subWorkerObj is indeed a class member. I might have figured this out. My main application thread is starting my first worker thread but it has it's own loop in a run method, without calling exec. Would that mean that for this worker thread there's no event loop to process incoming signals? It's based off the basic "new Widget Application". The issue is with the connection between Worker and SubWorker.
"How to use QThread in the right way (Part 1)" — 1+1=10
Edit: the code here is deemed too complex and lacking event loops to actually process the slots, any late-comers to the thread, please don't spend too much time figuring it out. Code: MainWindow.Signals and slots were one of the distinguishing features that made Qt an exciting and innovative tool back in time. But sometimes you can teach new tricks to an old dog, and QObjects gained a new way to connect between signals and slots in Qt5, plus some extra features to connect to other functions which are not slots. Let’s review how to get the most of that feature. Jan 27, · I wanted to know what is the best practice to connect signal/slots between two QObjects created in the contructor of MainWindow but moved to different threads later default connections seems not connecting signal/slot across different threads between QObjects. Ask Question With multiple threads, it's generally better to use automatic. However, you are free to connect the Worker's slots to any signal, from any object, in any thread. It is safe to connect signals and slots across different threads, thanks to a mechanism called queued connections. Another way to make code run in a separate thread, is to subclass QThread and reimplement run(). For example.
In MainWindow. You're making things overly complicated.
You have three levels of thread and none of them has an active event loop. So signals and slots between them will not work as you think.Make sure you declare the signals and the slots in the respective header files. Can you send multiple work() signals to the sub worker? - is it getting stuck? Reply to Problem with signal-slot connection across threads [SOLVED] on Fri, GMT. Signals and slots were one of the distinguishing features that made Qt an exciting and innovative tool back in time. But sometimes you can teach new tricks to an old dog, and QObjects gained a new way to connect between signals and slots in Qt5, plus some extra features to connect to other functions which are not slots. Let’s review how to get the most of that feature. When a button is clicked, for example, it emits a “clicked()” signal. Signals do nothing alone, but once connected to a slot, the code in the slot will be executed whenever the signal is emitted. In the Python programs, every function is a slot. It is possible to connect one signal to multiple slots, and to connect slots consecutively.
Thanks SGaist. You have to guess the rest of the code from what I posted but it's built up that way for a reason and of course, the original design didn't consider using signals and slots at that point.
But I think I can make some changes and get an event loop running. As onTimeout which is a member of conneting Thread object, get called from the creating thread too.
As none of the member of QThread wcross are designed to be called from the worker thread. Though this works perfect, but you may have notice that, when event loop QThread::exec is used in the worker thread, the code in the QThread::run seems has nothing to do with QThread itself.
So can we move the object creation out of the QThread::runand at the same time, the slots of they will still be called by the QThread::run? If we only want to make use of QThread::execwhich has been called by QThread::run by default, there will be no need to subclass the QThread any more. In this example, both of the QTimer and Worker are moved to the sub-thread.
How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections
In fact, moving QTimer to sub-thread slots not required. Simply remove the line timer. Thanks to a mechanism called queued connections, it is safe to connect signals and slots across different threads. If all the across threads communication are done though signal connections, the usual multithreading precautions such as QMutex will no longer need to be taken.
A short history Long long ago, subclass QThread and reimplement its run function is the only recommended way of using QThread. QThread::run is the thread entry point From the Qt Documentation, we can see that Threads QThread instance represents a connecting and provides across means to start a thread, which will then execute the reimplementation of QThread::run. As QThread::run is the thread entry point, it is rather intuitive to use the Usage 1.
Problem with signal-slot connection across threads [SOLVED] | Qt Forum
Usage To run some code in a new thread, subclass QThread and reimplement its run function. From main thread: 0x13a8 From slits thread: 0xab8 Thread::stop called from main thread: 0x13a8.
From main thread: 0x13a4 From worker thread: 0x Thread::onTimeout get called from? From main thread: 0x From work thread: 0xfac Worker::onTimeout get called from? From main thread: 0x Worker::onTimeout get called from?
4 thoughts on “Connecting signal slots across threads”
In the first part , we saw that signals are just simple functions, whose body is generated by moc. They are just calling QMetaObject::activate , with an array of pointers to arguments on the stack. Here is the code of a signal, as generated by moc: from part 1.