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QT | about how to use EventFilter

2022-07-26 09:11:00 【InfoQ】

Preface ：

Qt The provided event filter is an operation composed of two functions to monitor the events of itself or other components in a component . These two functions are installEventFilter() and eventFilter(), All are QObject Functions in class .

Usage method ：

1. Install filters on components that need to be monitored , for example ：

ui->textEdit->installEventFilter(this);// Install event filters on this window for editing parts

To use an event filter on a part , First use its installEventFilter() Function to install the event filter , The parameters of this function indicate the monitoring object . Parameter is this Indicate to be in this window / Monitoring in components textEdit Events .

2. Re implement in the monitoring part eventFilter() function ：

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

bool Widget::eventFilter(QObject *obj, QEvent event)
{
 if(obj == ui->textEdit)
 {
 if(event->type == QEvent::Wheel){
 return true;
 }
 else{
 return false;
 }
 }
 else{
 return QWidget::eventFilter(obj, event);
 }
}

obj Is the object of the event ,event For events .

If you want to process a specific event , And I don't want it to be processed later in the delivery process , So return true, Otherwise return to false. If you don't need to deal with this event , So long to return to this event , Leave it to the superior parts .

Why use filters ：

occasionally , We need to listen for events of multiple components , If you don't use a filter , Then score and subclass each component , Then re implement their corresponding event handling functions , It's going to be a lot of trouble , In order to avoid causing such trouble , Directly listen to the events of these components in the parent window and handle them accordingly , It will be very convenient .

Send events ：

Qt In addition to monitoring events , It also provides the function of sending an event ：

bool QCoreApplication::sendEvent(QObject *receiver, QEvent *event);

void QCoreApplication::postEvent(QObject *receiver, QEvent *event, int priority = Qt::NormalEventPriority);

Used to MFC Children's shoes should think of these two functions SendMessage and PostMessage Well , They are indeed similar .

sendEvent and postEvent The difference between ：

sendEvent() The given event will be processed immediately ; and postEvent() The event will be placed in the waiting queue , Next time Qt Only when the main event loop is running will it be processed .

sendEvent() Medium QEvent The formation parameters cannot be automatically deleted after the event is sent , So you need to create on the stack QEvent object ; and postEvent() Medium QEvent Object parameters must be created on the heap （ For example, using new）, When an event is sent, the event queue will automatically delete it .

Send an event ：

QKeyEvent myEvent(QEvent::KeyPress, Qt::Key_Up, Qt::NoModify);
qApp->sendEvent(ui->spinBox, &myEvent);// Send keyboard events to spinbox parts

there qApp yes QApplication Global pointer to the object , Each application can only use one QApplication object , Equivalent to using QApplication::sendEvent().

Qt You can also use custom events , This event needs inheritance QEvent class .

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