Get a notification/event/signal when a Qt widget gets focus

Question

In Qt, when a widget receives focus, how can get a notification about it, so I can execute some custom code? Is there a signal or an event for that?

Solution 1

There is a "focusChanged" signal sent when the focus changes, introduced in Qt 4.1.
It has two arguments, he widget losing focus and the one gaining focus:

void QApplication::focusChanged(QWidget * old, QWidget * now)

Solution 2

Qt Designer isn't designed for this level of WYSIWYG programming.

Do it in C++:

class LineEdit : public QLineEdit
{
    virtual void focusInEvent( QFocusEvent* )
    {}
};

Solution 3

I'd have to play with it, but just looking at the QT Documentation, there is a "focusInEvent". This is an event handler.

Here's how you find information about.... Open up "QT Assistant". Go to the Index. Put in a "QLineEdit". There is a really useful link called "List of all members, including inherited members" on all the Widget pages. This list is great, because it even has the inherited stuff.

I did a quick search for "Focus" and found all the stuff related to focus for this Widget.

Solution 4

You have hit on of the weird splits in QT, if you look at the documentation focusInEvent is not a slot it is a protected function, you can override it if you are implementing a subclass of your widget. If you you just want to catch the event coming into your widget you can use QObject::installEventFilter it let's you catch any kind of events.

For some odd reason the developers of Trolltech decided to propagate UI events via two avenues, signals/slots and QEvent

Solution 5

Just in case anybody looking for two QMainWindow focus change . You can use

if(e->type() == QEvent::WindowActivate)
{
    //qDebug() << "Focus IN " << obj << e ;

}

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