QT event cycle

1. In general, our event cycle is made up of exec() To open it , Take the following example ：

QCoreApplicaton::exec()
QApplication::exec()
QDialog::exec()
QThread::exec()
QDrag::exec()
QMenu::exec()

These all start the event cycle , The event loop is first an infinite “ loop ”, The program is in exec() There's an infinite loop inside , Can make you follow exec() The following code won't get a chance to run , Until the program starts from exec() Jump out of . from exec() When jumping out , The event loop is terminated .QEventLoop::quit() Can terminate the event loop .

An event loop is actually similar to an event queue , Handle the listed events in turn , When the time is finished and the time cycle is not over , It's actually more like an unoccupied CPU The event for(;;) loop .

Its essence is actually to redistribute time slices in the form of queues .
　　
2. Event loops can be nested , When in a sub event loop , The events in the parent event loop are actually in an interrupted state , When the sub loop jumps out exec Then the events in the parent loop can be executed .

Of course , This does not mean that when executing a sub loop , Similarly, the interface response in the parent loop will be interrupted , Because often there will be most of the events of the parent loop in the child loop , perform QMessageBox::exec(),QEventLoop::exec() When , Although these exec() Interrupted main() Medium QApplication::exec(), But because of GUI The response of the interface has been included in the sub loop , therefore GUI The interface is still responsive .

3. If a sub event loop is still valid , But its parent loop is forced to jump out , At this point, the parent loop does not immediately execute the jump out , Instead, wait for the sub event loop to jump out , The parent loop will jump out

Take a few examples , For example, if you want to wait for the main thread 100ms, Never use sleep Well , That would lead to GUI When the interface stops responding , But you can avoid this with event loops ：

 QEventLoop loop;
 QTimer::singleShot(100, &loop, SLOT(quit()));
 loop.exec();

also , For example, for a slot function , After triggering, a... Will pop up dialog, But if you write like this , The window will flash by ：

 void ****::mySLot{
    
     QDialog dlg;
     dlg.show();
 }

Of course, you can use here will dlg Change to a static member , To solve this problem by prolonging the life span , But it can also be used here eventLoop To solve this problem ：

void ****::mySLot{
    
     QDialog dlg;
     dlg.show();
     QEventLoop loop;
     connect(&dlg, SIGNAL(finished(int)), &loop, SLOT(quit()));
     loop.exec(QEventLoop::ExcludeUserInputEvents);
 }

turn ：https://www.cnblogs.com/-wang-cheng/p/4973021.html