Qt user resize event ends (stops)

Question

I have a QWidget and i need to do some actions (refresh a picture in widget) when resize event ends. How can i catch this action? I need to catch moment when user ENDs all his resize actions by releasing mouse button. It is not a good practice in my application to refresh image every pixel resized. It should calls only when mouse released and resize actions ends.

I am just tried to reimplement QMouseReleaseEvent to catch it, but it do not works when user presses on the border of widget to resize it. It means does not working in our situation.

Then i was tried to create my own QSizeGrip and insert it on the bottom of my widget, but reimplemented event QMouseReleaseEvent again did not work in it. Event did not generates any time user released mouse. I do not know why.

Anybody can help me with that problem?

Thanks in advance.

Answer 1

The timeout method is a decent idea, but if the user is resizing and then pauses for longer than the timer's interval then you end up not getting the true "user is done resizing the window" event. Setting the interval longer makes that situation less likely, but by doing that, you end up having a long delay between the time the user finished resizing and the time your function gets called. In my search for a solution, I found quite a few people solving it using the timer method, so apparently it's reliable enough for some use cases, but I think it's a bit hacky.

I like mhstnsc's idea, so after implementing it, I decided to add some code here that might be of use to someone trying to do something similar. You could easily adapt it to catch the "user is done moving the window" event by making a m_bUserIsMoving flag and overriding "void MainWindow::moveEvent(QMoveEvent* pEvent)". I'm using it to save a config file whenever the user finishes resizing or moving the window, so that the last position will always be saved even if the app is killed in an unclean manner.

// constructor
MainWindow::MainWindow(QWidget* pParent, Qt::WindowFlags flags) : QMainWindow(pParent, flags)
{
    m_bUserIsResizing = false;
    qApp->installEventFilter(this);
}

// this will be called when any event in the application occurs
bool MainWindow::eventFilter(QObject* pObj, QEvent* pEvent)
{
    // We need to check for both types of mouse release, because it can vary on which type happens when resizing.
    if ((pEvent->type() == QEvent::MouseButtonRelease) || (pEvent->type() == QEvent::NonClientAreaMouseButtonRelease)) {
        QMouseEvent* pMouseEvent = dynamic_cast<QMouseEvent*>(pEvent);
        if ((pMouseEvent->button() == Qt::MouseButton::LeftButton) && m_bUserIsResizing) {
            printf("Gotcha!\n");
            m_bUserIsResizing = false; // reset user resizing flag
        }
    }
    return QObject::eventFilter(pObj, pEvent); // pass it on without eating it
}

// override from QWidget that triggers whenever the user resizes the window
void MainWindow::resizeEvent(QResizeEvent* pEvent) { m_bUserIsResizing = true; }

It's slightly more complicated than the timer, but more robust.

Answer 2

I've do it in this way:

1. inherit my class from QWidget
2. define private variable int timerId = 0
3. overload QWidget::resizeEvent and QObject::timerEvent

---

void MapLoader::resizeEvent(QResizeEvent *){
    if (timerId){
        killTimer(timerId);
        timerId = 0;
    }
    timerId = startTimer(5000/*delay beetween ends of resize and your action*/);
}

void MapLoader::timerEvent(QTimerEvent *te){
    /*your actions here*/
    killTimer(te->timerId());
    timerId = 0;
}