How to catch enter key

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There are many different situations where you can use the enter key, e.g. to start a search action. But implementing something like this is not that easy - Qt catches enter keys before you even get the event.

Solution

Fortunately, Qt allows to reimplement the general event catching method. You need a new class with a method like this:

bool eventFilter(QObject* obj, QEvent* event);

That's everything:

class keyEnterReceiver : public QObject
{
    Q_OBJECT
protected:
    bool eventFilter(QObject* obj, QEvent* event);
};

Now, we have to implement the method:

bool keyEnterReceiver::eventFilter(QObject* obj, QEvent* event)
{
    if (event->type()==QEvent::KeyPress) {
        QKeyEvent* key = static_cast<QKeyEvent*>(event);
        if ( (key->key()==Qt::Key_Enter) || (key->key()==Qt::Key_Return) ) {
            //Enter or return was pressed
        } else {
            return QObject::eventFilter(obj, event);
        }
        return true;
    } else {
        return QObject::eventFilter(obj, event);
    }
    return false;
}

That was quiet fast - so here is a detailled explanation:

Key pressed?

First, we check if any key was pressed. If not, it is a event that has nothing to do with keys - and Qt should handle it:

bool keyEnterReceiver::eventFilter(QObject* obj, QEvent* event)
{
    if(event->type()==QEvent::KeyPress) {
        …
    } else {
        return QObject::eventFilter(obj, event);
    }
    return false;
}

Convertion

We got a QEvent as a parameter. To read out which key was pressed, we need to convert the QEvent to a QKeyEvent:

QKeyEvent* key = static_cast<QKeyEvent*>(event);

Enter/Return or another key?

That's it. Now we only have to check whether it was "our" enter key or another key we are not interested in:

if ( (key->key()==Qt::Key_Enter) || (key->key()==Qt::Key_Return) ) {
    //Enter or return was pressed
} else {
    return QObject::eventFilter(obj, event);
}
return true;

Finally, we can install our event handler:

keyEnterReceiver* key = new keyEnterReceiver();
aWidgetInAnotherClass->installEventFilter(key);