Qt中事件分发源码剖析

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Qt中事件传递顺序：

在一个应该程序中，会进入一个事件循环，接受系统产生的事件，而且进行分发，这些都是在exec中进行的。

以下举例说明：

1)首先看看以下一段演示样例代码：

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MouseEvent w;
w.show(); return a.exec();
}

2)a.exec进入事件循环，调用的是QApplication::exec()；

int QApplication::exec()
{
return QGuiApplication::exec();
}

3)QApplication::exec()调用的是QGuiApplication::exec()；

int QGuiApplication::exec()
{
#ifndef QT_NO_ACCESSIBILITY
QAccessible::setRootObject(qApp);
#endif
return QCoreApplication::exec();
}

4)QGuiApplication::exec()调用的是QCoreApplication::exec()；

int QCoreApplication::exec()
{
if (!QCoreApplicationPrivate::checkInstance("exec"))
return -1; QThreadData *threadData = self->d_func()->threadData;
if (threadData != QThreadData::current()) {
qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className());
return -1;
}
if (!threadData->eventLoops.isEmpty()) {
qWarning("QCoreApplication::exec: The event loop is already running");
return -1;
} threadData->quitNow = false;
QEventLoop eventLoop;
self->d_func()->in_exec = true;
self->d_func()->aboutToQuitEmitted = false;
int returnCode = eventLoop.exec();
threadData->quitNow = false;
if (self) {
self->d_func()->in_exec = false;
if (!self->d_func()->aboutToQuitEmitted)
emit self->aboutToQuit(QPrivateSignal());
self->d_func()->aboutToQuitEmitted = true;
sendPostedEvents(0, QEvent::DeferredDelete);
} return returnCode;
}

5)QCoreApplication::exec()调用eventLoop.exec()进行事件循环；

int QEventLoop::exec(ProcessEventsFlags flags)
{
Q_D(QEventLoop);
//we need to protect from race condition with QThread::exit
QMutexLocker locker(&static_cast<QThreadPrivate *>(QObjectPrivate::get(d->threadData->thread))->mutex);
if (d->threadData->quitNow)
return -1; if (d->inExec) {
qWarning("QEventLoop::exec: instance %p has already called exec()", this);
return -1;
} struct LoopReference {
QEventLoopPrivate *d;
QMutexLocker &locker; bool exceptionCaught;
LoopReference(QEventLoopPrivate *d, QMutexLocker &locker) : d(d), locker(locker), exceptionCaught(true)
{
d->inExec = true;
d->exit = false;
++d->threadData->loopLevel;
d->threadData->eventLoops.push(d->q_func());
locker.unlock();
} ~LoopReference()
{
if (exceptionCaught) {
qWarning("Qt has caught an exception thrown from an event handler. Throwing\n"
"exceptions from an event handler is not supported in Qt. You must\n"
"reimplement QApplication::notify() and catch all exceptions there.\n");
}
locker.relock();
QEventLoop *eventLoop = d->threadData->eventLoops.pop();
Q_ASSERT_X(eventLoop == d->q_func(), "QEventLoop::exec()", "internal error");
Q_UNUSED(eventLoop); // --release warning
d->inExec = false;
--d->threadData->loopLevel;
}
};
LoopReference ref(d, locker); // remove posted quit events when entering a new event loop
QCoreApplication *app = QCoreApplication::instance();
if (app && app->thread() == thread())
QCoreApplication::removePostedEvents(app, QEvent::Quit); while (!d->exit)
processEvents(flags | WaitForMoreEvents | EventLoopExec); ref.exceptionCaught = false;
return d->returnCode;
}

6)eventLoop.exec()调用QCoreApplication的processEvents进行事件分发；

7)调用notify进行分发

QCoreApplication::sendEvent、QCoreApplication::postEvent和QCoreApplication::sendPostedEvents都调用notify进行事件分发；

bool QCoreApplication::notify(QObject *receiver, QEvent *event)
{
Q_D(QCoreApplication);
// no events are delivered after ~QCoreApplication() has started
if (QCoreApplicationPrivate::is_app_closing)
return true; if (receiver == 0) { // serious error
qWarning("QCoreApplication::notify: Unexpected null receiver");
return true;
}#ifndef QT_NO_DEBUG
d->checkReceiverThread(receiver);
#endif return receiver->isWidgetType() ?false : d->notify_helper(receiver, event);
}

8)notify调用notify_helper进行事件分发；

bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)
{
// send to all application event filters
if (sendThroughApplicationEventFilters(receiver, event))
return true;
// send to all receiver event filters
if (sendThroughObjectEventFilters(receiver, event))
return true;
// deliver the event
return receiver->event(event);
}