QT | event system qevent

Preface ：

stay Qt in , Events are treated as an object , Inherited from QEvent class . Events and signals are not the same concept , For example, click the button on the interface , At this time, a mouse event is generated （QMouseEvent）, This event is not caused by the button , But because the button was pressed , So the button will send a click signal （clicked()）, This signal is generated by the button .

The flow of events is ：

  When an event occurs , There will be a QEvent object , This QEvent The object will be passed to the current part event() function . If the current part does not have an event filter installed eventFilter, Will be event Function into the corresponding event function （xxxEvent） In the middle .

Events are mainly divided into two categories ：

• Occurs when interacting with a user （ For example, press the mouse :mousePressEvent）;

• It happens automatically （ Like timer events ：timerEvent）;

QObject Subclass instances can receive and process events .

The handling of events ：

When we receive an event , When events need to be handled , There are usually the following methods ：

• Method 1 ： Re implement event functions .
    For example, re implement the paintEvent()、mousePressEvent() And so on . At this time, the most commonly used method , However, it can only be used to deal with specific events of specific parts .
• for example ：
   void paintEvent(QPaintEvent *event);

• Method 2 ： Re actualize notify() function .
    This function is very powerful , Provides complete control , You can get events before the event filter gets them . But it can only handle one event at a time .

• Method 3 ： towards QApplication Object .
    Because a program has only one QApplication object , So the function and use of this realization notify() The function is the same , The advantage is that you can handle multiple events at the same time .

• Method four ： Re actualize event() function .
   QObject Class event() Function can get the event before it reaches the default event handler function .
• for example ：
   bool event(QEvent *event);

• Method five ： Install the event filter on the object .
    Using event filters, you can handle different events of different subcomponents in one interface class at the same time .
• for example ：
   bool eventFilter(QObject *obj, QEvent *event);

The most commonly used method is method one 、 The second is method 5 . Because method 2 needs to inherit from QApplication class , Method 3 uses a global event filter , This will slow down the transmission of events , So although these two methods are very powerful , But it's rarely used .

The transmission of events ：

In a multi-level window , The event is passed to the specified widget first , That is, first pass it to the widget that gets the focus . But if the event function of this part calls ignore() The function ignores this event , Then this event will be passed to the parent part of this part .

When the function is re implemented , Be sure to call the corresponding event handler function of the parent class to realize the default operation . for example :

• stay paintEvent() At the end of the function, call ：QWidget::paintEvent(event);

• stay event() At the end of the function, call ：return QWidget::event(event);

• stay eventFilter() Call at the end of function ：return QWidget::eventFilter(obj, event);

The sequence of events is ：

  First, the event filter , Then there's the focus part event() function , Finally, the event handling function of the focus part ; If the focus component ignores the event , Then the event handling function of the parent part will be executed .