Qt中事件分发源代码剖析

Qt中事件传递顺序：

1)首先看看下面一段示例代码：

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);
}

9)从上面第8步的代码可以看出事件传递

传递的顺序是：首先传递给全局的事件过滤器，再传递给目标对象的事件过滤器，最终传递给目标对象。