CODEWITHSUNDEEP
c++multithreadingqt5signals-slots
Seb
Seb

Reputation: 3213

Qt/C++ how to wait a slot when signal emitted

I have developed an app in Qt/C++, I have used signal/slot mechanism to interact between 2 threads. the first thread run the UI/TreeWidget and the second one run the framework

I got an issue on one action.

In the UI side, before starting my action, I'm connect signal/slot between UI and framework such as below in the treewidget.cpp

connect(&m_Framework, &Framework::RequestIfNameExist, this, &TreeWidget::RequestIfNameExist);
connect(this, &TreeWidget::SendAnswerIfNameExist, &m_Framework, &Framework::NotifIfNameExist);

The framework, start and send the RequestIfNameExist:

emit RequestIfNameExist(tmpname, item, fileInfo.isDir());

while(WaitingResponse == false){
    usleep(200);
}

I have added a loop because I need to wait the feedback. Strange things is that in the treewidget.cpp, I never enter in 

void TreeWidget::RequestIfNameExist(QString name, TreeWidgetItem *parent, bool isFolder) {
#ifdef PULS_LOG
    QLOG_INFO() << "[TreeWidget] [RequestIfNameExist] ";
#endif
    emit SendAnswerIfNameExist(isNameExist(name, parent), isFolder);
}

I never access to RequestIfNameExist in the TreeWidget but the signal is emitted.

I have also put a while loop in the framework to wait the feedback from TreeWidget

void Framework::NotifIfNameExist(QTreeWidgetItem *item, bool isFolder){

    if(item != NULL)
        item->isFolder = isFolder;

    WaitingResponse = true;

}

Any idea why the signal emitted by framework never arrived on the treewidget ? is it coming from the while ??

Is there a way to not use while such as a "wait event" + timeout

Thanks

Upvotes: 1

Views: 5530

Answers (1)

Jeremy Friesner
Jeremy Friesner

Reputation: 73041

My first thought is that having either thread block until an operation in the other thread completes is a poor design -- it partially defeats the purpose of having multiple threads, which is to allow multiple operations to run in parallel. It's also liable to result in deadlocks if you're not careful (e.g. if both threads decide to emit-and-wait at approximately the same time!)

A better design would have the initiating method do the emit RequestIfNameExit and then return immediately, so that the initiating thread's event loop can continue running as usual during the operation. Then, when the other thread has done its work, it responds by emitting its own response-signal, causing the appropriate/connected slot-method in the first thread to be called, at which point the results are handled back in the first thread.

That said, if you insist on wanting to block the execution of the signal-emitting thread inside a method, until the other thread has finished executing the associated slot-method, you can get that behavior by setting the signal/slot connection's type to be Qt::BlockingQueuedConnection (the connection type can be specified via an optional extra argument to connect()). If you do that, then you emit call won't return until the slot-method (in the other thread) has finished executing. Given that, you can get the results from the other thread by passing a pointer to a data object as one of the arguments in the signal/slot method signature, and having the other thread populate that data object as necessary. When the emit returns you can just examine the contents of that data object to see the results.

Upvotes: 5

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