Qt中事件傳遞順序：

1)首先看看下面一段示例程式碼：

1. int main(int argc, char *argv[])  
2. {  
3.     QApplication a(argc, argv);  
4.     MouseEvent w;  
5.     w.show();  
6.     returna.exec();
7. }  

2)a.exec進入事件迴圈，呼叫的是QApplication::exec()；

1. int QApplication::exec()  
2. {  
3.     return
   QGuiApplication::exec();
4. }  

3)QApplication::exec()呼叫的是QGuiApplication::exec()；

1. int QGuiApplication::exec()  
2. {  
3. #ifndef QT_NO_ACCESSIBILITY
4.     QAccessible::setRootObject(qApp);  
5. #endif
6.     returnQCoreApplication::exec();
7. }  

4)QGuiApplication::exec()呼叫的是QCoreApplication::exec()；

1. int QCoreApplication::exec()  
2. {  
3.     if (!QCoreApplicationPrivate::checkInstance("exec"))  
4.         return -1;  
5.     QThreadData *threadData = self->d_func()->threadData;  
6.     if (threadData != QThreadData::current()) {  
7.         qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className());  
8.         return -1;  
9.     }  
10.     if (!threadData->eventLoops.isEmpty()) {  
11.         qWarning("QCoreApplication::exec: The event loop is already running");  
12.         return -1;  
13.     }  
14.     threadData->quitNow = false;  
15.     QEventLoop eventLoop;  
16.     self->d_func()->in_exec = true;  
17.     self->d_func()->aboutToQuitEmitted = false;  
18.     int returnCode = eventLoop.exec();
19.     threadData->quitNow = false;  
20.     if (self) {  
21.         self->d_func()->in_exec = false;  
22.         if (!self->d_func()->aboutToQuitEmitted)  
23.             emit self->aboutToQuit(QPrivateSignal());  
24.         self->d_func()->aboutToQuitEmitted = true;  
25.         sendPostedEvents(0, QEvent::DeferredDelete);  
26.     }  
27.     return returnCode;  
28. }  

5)QCoreApplication::exec()呼叫eventLoop.exec()進行事件迴圈；

1. int QEventLoop::exec(ProcessEventsFlags flags)  
2. {  
3.     Q_D(QEventLoop);  
4.     //we need to protect from race condition with QThread::exit
5.     QMutexLocker locker(&static_cast<QThreadPrivate *>(QObjectPrivate::get(d->threadData->thread))->mutex);  
6.     if (d->threadData->quitNow)  
7.         return -1;  
8.     if (d->inExec) {  
9.         qWarning("QEventLoop::exec: instance %p has already called exec()", this);  
10.         return -1;  
11.     }  
12.     struct LoopReference {  
13.         QEventLoopPrivate *d;  
14.         QMutexLocker &locker;  
15.         bool exceptionCaught;  
16.         LoopReference(QEventLoopPrivate *d, QMutexLocker &locker) : d(d), locker(locker), exceptionCaught(true)  
17.         {  
18.             d->inExec = true;  
19.             d->exit = false;  
20.             ++d->threadData->loopLevel;  
21.             d->threadData->eventLoops.push(d->q_func());  
22.             locker.unlock();  
23.         }  
24.         ~LoopReference()  
25.         {  
26.             if (exceptionCaught) {  
27.                 qWarning("Qt has caught an exception thrown from an event handler. Throwing\n"
28.                          "exceptions from an event handler is not supported in Qt. You must\n"
29.                          "reimplement QApplication::notify() and catch all exceptions there.\n");  
30.             }  
31.             locker.relock();  
32.             QEventLoop *eventLoop = d->threadData->eventLoops.pop();  
33.             Q_ASSERT_X(eventLoop == d->q_func(), "QEventLoop::exec()", "internal error");  
34.             Q_UNUSED(eventLoop); // --release warning
35.             d->inExec = false;  
36.             --d->threadData->loopLevel;  
37.         }  
38.     };  
39.     LoopReference ref(d, locker);  
40.     // remove posted quit events when entering a new event loop
41.     QCoreApplication *app = QCoreApplication::instance();  
42.     if (app && app->thread() == thread())  
43.         QCoreApplication::removePostedEvents(app, QEvent::Quit);  
44.     while (!d->exit)  
45.         processEvents(flags | WaitForMoreEvents | EventLoopExec);  
46.     ref.exceptionCaught = false;  
47.     return d->returnCode;  
48. }  

6)eventLoop.exec()呼叫QCoreApplication的processEvents進行事件分發；

7)呼叫notify進行分發

QCoreApplication::sendEvent、QCoreApplication::postEvent和QCoreApplication::sendPostedEvents都呼叫notify進行事件分發；

1. bool QCoreApplication::notify(QObject *receiver, QEvent *event)  
2. {  
3.     Q_D(QCoreApplication);  
4.     // no events are delivered after ~QCoreApplication() has started
5.     if (QCoreApplicationPrivate::is_app_closing)  
6.         returntrue;  
7.     if (receiver == 0) {                        // serious error
8.         qWarning("QCoreApplication::notify: Unexpected null receiver");  
9.         returntrue;  
10.     }  
11. #ifndef QT_NO_DEBUG
12.     d->checkReceiverThread(receiver);  
13. #endif
14.     return receiver->isWidgetType() ? false :d->notify_helper(receiver, event);  
15. }  

8)notify呼叫notify_helper進行事件分發；

1. bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)  
2. {  
3.     // send to all application event filters
4.     if (sendThroughApplicationEventFilters(receiver, event))  
5.         returntrue;  
6.     // send to all receiver event filters
7.     if (sendThroughObjectEventFilters(receiver, event))  
8.         returntrue;  
9.     // deliver the event
10.     returnreceiver->event(event);  
11. }  

9)從上面第8步的程式碼可以看出事件傳遞

傳遞的順序是：首先傳遞給全域性的事件過濾器，再傳遞給目標物件的事件過濾器，最終傳遞給目標物件。