Qt or any other lang. Event Loop

Question

I'm a c++ programmer on Qt platform.

I'm wondering, how does the event loop "knows" to which widget to send an event, mainly mouse/keyboard events? Is it done based on mouse coordinates and z-order? What about events from keyboard?

Thanks

score 3 · Accepted Answer · answered Jan 29 '15 at 20:01

The event loop doesn't know. This is done in other bits of code.

The term you're looking for with the keyboard is "focus". Exactly one window has focus, systemwide (or at least one window per keyboard on multi-user systems). The OS delivers the keystrokes to that window. Qt just finds the Qt object from the native window handle. Similarly, mouse clicks are mostly handled by the OS.

score 0 · Answer 2 · edited Jan 29 '15 at 20:21

It doesn't know.

When you want to capture an event, you must create an event filter that either captures the event, or allows it to be passed down.

Here's a very simple event filter that I created some time ago:

bool OGL_widget::eventFilter(QObject *obj, QEvent *event) {
    switch (event->type()) {
    case QEvent::KeyRelease:
    case QEvent::KeyPress: {
        QKeyEvent *key = static_cast<QKeyEvent*> (event);
        if (!key->isAutoRepeat())
            key_event_queue << *key;
    }
    break;
    case 1001:
    case 1002: {
        Savestate_event *save = static_cast<Savestate_event*> (event);
            save_event_queue << *save;
    }
    break;
    }

    return QObject::eventFilter(obj, event);
}

Take a look at this well written article of events at Qt docs.

Qt or any other lang. Event Loop

2 Answers2