QPushButton staying pressed too long

[A Former User]

@MaxDevI Can you please show us the actual declaration of your "aFunction" and the actual line with "QFuture.." ?

[MaxDevI]

@Wieland

I just got the Qtconcurrent to run, but it gives me Microsoft Visual C++ Runtime Library Debug Error saying

"ASSERT failure in QCoreApplication::sendEvent: Cannot send events to objects owned by a different thread..."

[A Former User]

Seems like your aFunction is interacting with objects that live in the GUI thread.

[MaxDevI]

@Wieland yes it is, can I do anything to fix this other than rewriting?

[A Former User]

Might be easy to fix without rewriting it. What exactly do you do that causes the problem?

[MaxDevI]

@Wieland There is so much going on in the function it is pretty hard to explain. It goes through many loops looking through widgets and checking their values, and updating other widgets on other tabs that correspond.

Let's say I have tab A, B, C with information on each tab. If I press the button on tab A, it will scan tab A for relative information (predetermined) to send over the tab B and C. Then it goes to tab B and C and enters the information from A, then returns to tab A.

This is why I get problems when using processEvents() because of the tab switching.

[A Former User]

@MaxDevI Ok, that's a problem that actually cannot be fixed simply because one must never directly interact with GUI elements (widgets) from a thread other then the main thread (aka GUI thread) in Qt. That's simply not supported. But you still can send signals from other threads to the widgets on the GUI thread. They will be processed asynchronously by default. So, yeah, looks like you'll have to refactor your code a bit :-/

[MaxDevI]

@Wieland Can I send a signal from the other thread to the GUI thread which will set the button to unpressed immediately after it is pressed? Seems like it would work somehow?

[MaxDevI]

@Wieland Maybe I could set the pushbutton to unpressed before making the computations which cause the delay???

[MaxDevI]

@Wieland I did it!

It was a workaround, no doubt about it, but it seems to work! Here is what I had to do.

1. I connect GUI object to the class object which was performing the long operation. I create my own signal and slot. I call something like this in my GUI class when I instantiate the longOperationObject.

connect(longOperationObject, SIGNAL(myLongOpSignal()), this, SLOT(myGUISlot());

2. BEFORE performing the long operation in the longOperationObject, I emit myLongOpSignal() which calls myGUISlot(). Then, right BEFORE the long operation, I call QCoreApplication::processEvents(). Then I perform the long operation.

emit myLongOpSignal();
QCoreApplication::processEvents();
longOpFunction(...);

3. Now, the magic happens in myGUISlot. In order to make the push button look pressed and released as normal, I set the stylesheet of the button to be pressed. Then I call processEvents again. I wait any amount of time I want using std::clock_t, then I set the button stylesheet back to pressed! In this example the button stays pressed for .5 seconds.

void GUI::myGUISlot(){
    button->setStyleSheet("pressed format...");
    QCoreApplication::processEvents();
    std::clock_t start;
    double duration;
    start = std::clock();
    duration = ( std::clock() - start ) / (double) CLOCKS_PER_SEC;
    while(duration <= .5){

        duration = ( std::clock() - start ) / (double) CLOCKS_PER_SEC;
    }
    button->setStyleSheet("normal un-pressed format...");
}

Quite the work around...but hey, it works for now. Thanks @Wieland for your help.

[A Former User]

@MaxDevI The main thing is that it works ;-)