Qt源码阅读(四) 事件循环

事件系统

文章为本人理解，如有理解不到位之处，烦请各位指正。

Qt的事件循环，应该是所有Qter都避不开的一个点，所以，这篇博客，咱们来了解源码中一些关于Qt中事件循环的部分。
先抛出几个疑问，根据源代码，下面一一进行解析。

1. 事件循环是什么？
2. 事件是怎么产生的？
3. 事件是如何处理的？

什么是事件循环？

对于Qt事件循环个人理解是，事件循环是一个队列去循环处理事件。当队列中有事件时，则去处理事件，如果没有事件时，则会阻塞等待。

事件是如何产生的？

事件的产生可以分为两种：

1. 程序外部产生
2. 程序内部产生

程序外部所产生的事件主要是指系统产生的事件，比如说鼠标按下（MouseButtonPress）、按键按下（KeyPress）等，Qt捕捉系统的事件，然后将系统事件封装成自己的QEvent类，再将事件发送出去。

程序内部产生的事件主要指我们在代码里，手动创建一个事件，然后将事件通过sendEvent/postEvent，来发送到事件循环中。而sendEvent和postEvent区别又在于一个是阻塞的(sendEvent)一个是非阻塞的(postEvent)。

我们结合源码分析，看一下sendEvent和postEvent分别干了什么导致一个是阻塞的一个是非阻塞的。

sendEvent

完整源码如下：

bool QCoreApplication::sendEvent(QObject *receiver, QEvent *event)
{
    
    
	// sendEvent是阻塞调用
    Q_TRACE(QCoreApplication_sendEvent, receiver, event, event->type());

    if (event)
        event->spont = false;
    return notifyInternal2(receiver, event);
}

可以看到，sendEvent是调用了notifyInternal2这个函数

bool QCoreApplication::notifyInternal2(QObject *receiver, QEvent *event)
{
    
    
	...
    // Qt enforces the rule that events can only be sent to objects in
    // the current thread, so receiver->d_func()->threadData is
    // equivalent to QThreadData::current(), just without the function
    // call overhead.
    // 事件只能在同一个线程被send
    QObjectPrivate *d = receiver->d_func();
    QThreadData *threadData = d->threadData;
    QScopedScopeLevelCounter scopeLevelCounter(threadData);
    if (!selfRequired)
        return doNotify(receiver, event);
    return self->notify(receiver, event);
}

进一步跟踪到其doNotify函数

static bool doNotify(QObject *receiver, QEvent *event)
{
    
    
    if (receiver == nullptr) {
    
                            // serious error
        qWarning("QCoreApplication::notify: Unexpected null receiver");
        return true;
    }

#ifndef QT_NO_DEBUG
	// 检查接受线程与当前是否同线程
    QCoreApplicationPrivate::checkReceiverThread(receiver);
#endif

	// QWidget类必须用QApplication
    return receiver->isWidgetType() ? false : QCoreApplicationPrivate::notify_helper(receiver, event);
}

再到QCoreApplicationPrivate::notify_helper

bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)
{
    
    
    // Note: when adjusting the tracepoints in here
    // consider adjusting QApplicationPrivate::notify_helper too.
    Q_TRACE(QCoreApplication_notify_entry, receiver, event, event->type());
    bool consumed = false;
    bool filtered = false;
    Q_TRACE_EXIT(QCoreApplication_notify_exit, consumed, filtered);

    // send to all application event filters (only does anything in the main thread)
    if (QCoreApplication::self
        && receiver->d_func()->threadData.loadRelaxed()->thread.loadAcquire() == mainThread()
        && QCoreApplication::self->d_func()->sendThroughApplicationEventFilters(receiver, event)) {
    
    
        filtered = true;
        return filtered;
    }
    // send to all receiver event filters
    if (sendThroughObjectEventFilters(receiver, event)) {
    
    
        filtered = true;
        return filtered;
    }

    // deliver the event
    // 直接调用对象的event函数，所以是阻塞的
    consumed = receiver->event(event);
    return consumed;
}

然后我们可以看到主要有几个流程：

1. 判断QCoreApplication有没有安装事件过滤器，有就把信号发送到事件过滤器里，由事件过滤器对事件进行处理。

   // send to all application event filters (only does anything in the main thread)
   if (QCoreApplication::self
       && receiver->d_func()->threadData.loadRelaxed()->thread.loadAcquire() == mainThread()
       && QCoreApplication::self->d_func()->sendThroughApplicationEventFilters(receiver, event)) {
         
         
       filtered = true;
       return filtered;
   }
   

2. 判断事件接受对象，有没有安装事件过滤器，有就将信号发送到事件过滤器。

   // send to all receiver event filters
   if (sendThroughObjectEventFilters(receiver, event)) {
         
         
       filtered = true;
       return filtered;
   }
   

   具体遍历事件接受对象所安装的事件过滤器的代码如下：

   bool QCoreApplicationPrivate::sendThroughObjectEventFilters(QObject *receiver, QEvent *event)
   {
         
         
       if (receiver != QCoreApplication::instance() && receiver->d_func()->extraData) {
         
         
           for (int i = 0; i < receiver->d_func()->extraData->eventFilters.size(); ++i) {
         
         
               QObject *obj = receiver->d_func()->extraData->eventFilters.at(i);
               if (!obj)
                   continue;
               if (obj->d_func()->threadData != receiver->d_func()->threadData) {
         
         
                   qWarning("QCoreApplication: Object event filter cannot be in a different thread.");
                   continue;
               }
               if (obj->eventFilter(receiver, event))
                   return true;
           }
       }
       return false;
   }
   

   我们可以看到，只要事件被一个事件过滤器所成功处理，那么后续的事件过滤器就不会被响应。同时，参看Qt帮助手册中有提及到：

   If multiple event filters are installed on a single object, the filter that was installed last is activated first.

   后插入的事件过滤器会被优先响应。 具体安装事件过滤器，我们在后面进行分析。

3. 直接调用事件接受对象的event函数进行处理。因为是直接调用的对象的event，所以说，sendEvent函数会阻塞等待。

       // deliver the event
       // 直接调用对象的event函数，所以是阻塞的
       consumed = receiver->event(event);
       return consumed
   

postEvent

完整代码如下：

void QCoreApplication::postEvent(QObject *receiver, QEvent *event, int priority)
{
    
    
    Q_TRACE_SCOPE(QCoreApplication_postEvent, receiver, event, event->type());

	// 事件的接收者不能为空
    if (receiver == nullptr) {
    
    
        qWarning("QCoreApplication::postEvent: Unexpected null receiver");
        delete event;
        return;
    }

	// 对事件接受对象所在线程的事件处理列表上锁
    auto locker = QCoreApplicationPrivate::lockThreadPostEventList(receiver);
    if (!locker.threadData) {
    
    
        // posting during destruction? just delete the event to prevent a leak
        delete event;
        return;
    }

    QThreadData *data = locker.threadData;

    // if this is one of the compressible events, do compression
    // 将重复的事件，进行压缩
    if (receiver->d_func()->postedEvents
        && self && self->compressEvent(event, receiver, &data->postEventList)) {
    
    
        Q_TRACE(QCoreApplication_postEvent_event_compressed, receiver, event);
        return;
    }

    if (event->type() == QEvent::DeferredDelete)
        receiver->d_ptr->deleteLaterCalled = true;

    if (event->type() == QEvent::DeferredDelete && data == QThreadData::current()) {
    
    
        // remember the current running eventloop for DeferredDelete
        // events posted in the receiver's thread.

        // Events sent by non-Qt event handlers (such as glib) may not
        // have the scopeLevel set correctly. The scope level makes sure that
        // code like this:
        //     foo->deleteLater();
        //     qApp->processEvents(); // without passing QEvent::DeferredDelete
        // will not cause "foo" to be deleted before returning to the event loop.

        // If the scope level is 0 while loopLevel != 0, we are called from a
        // non-conformant code path, and our best guess is that the scope level
        // should be 1. (Loop level 0 is special: it means that no event loops
        // are running.)
        int loopLevel = data->loopLevel;
        int scopeLevel = data->scopeLevel;
        if (scopeLevel == 0 && loopLevel != 0)
            scopeLevel = 1;
        static_cast<QDeferredDeleteEvent *>(event)->level = loopLevel + scopeLevel;
    }

    // delete the event on exceptions to protect against memory leaks till the event is
    // properly owned in the postEventList
    QScopedPointer<QEvent> eventDeleter(event);
    Q_TRACE(QCoreApplication_postEvent_event_posted, receiver, event, event->type());
    data->postEventList.addEvent(QPostEvent(receiver, event, priority));
    eventDeleter.take();
    event->posted = true;
    ++receiver->d_func()->postedEvents;
    data->canWait = false;
    locker.unlock();

    QAbstractEventDispatcher* dispatcher = data->eventDispatcher.loadAcquire();
    if (dispatcher)
        dispatcher->wakeUp();
}

1. 判断事件接收对象是否为空

   // 事件的接收者不能为空
   if (receiver == nullptr) {
         
         
       qWarning("QCoreApplication::postEvent: Unexpected null receiver");
       delete event;
       return;
   }
   

2. 将事件接收对象所在线程的post事件列表上锁，如果已经被锁了，就把事件删除掉，并返回，防止泄露。

   // 对事件接受对象所在线程的事件处理列表上锁
   auto locker = QCoreApplicationPrivate::lockThreadPostEventList(receiver);
   if (!locker.threadData) {
         
         
       // posting during destruction? just delete the event to prevent a leak
       delete event;
       return;
   }
   

3. 将一些可以压缩的事件进行压缩，及多个事件压缩成只推送最后的一个事件。Qt界面的update就是这个操作，为了防止多次刷新导致卡顿，短时间内多次的调用update可能只会刷新一次

   // if this is one of the compressible events, do compression
   // 将重复的事件，进行压缩
   if (receiver->d_func()->postedEvents
       && self && self->compressEvent(event, receiver, &data->postEventList)) {
         
         
       Q_TRACE(QCoreApplication_postEvent_event_compressed, receiver, event);
       return;
   }
   

4. 将事件插入接收对象所在线程的post事件列表中，并唤醒线程的事件调度器，来进行事件的处理。所以postEvent是非阻塞的，因为其只是把事件插入了线程的事件列表，唤醒事件调度器之后便返回。

       // delete the event on exceptions to protect against memory leaks till the event is
       // properly owned in the postEventList
       QScopedPointer<QEvent> eventDeleter(event);
       Q_TRACE(QCoreApplication_postEvent_event_posted, receiver, event, event->type());
       data->postEventList.addEvent(QPostEvent(receiver, event, priority));
       eventDeleter.take();
       event->posted = true;
       ++receiver->d_func()->postedEvents;
       data->canWait = false;
       locker.unlock();
   
       QAbstractEventDispatcher* dispatcher = data->eventDispatcher.loadAcquire();
       if (dispatcher)
           dispatcher->wakeUp();
   

事件是如何处理的？

在Qt中，事件的接收者都是QObject，而QObject中事件处理是调用event函数。如果当时对象不处理某个事件，就会将其转发到父类的event进行处理。
而事件的处理，主要分为三个部分：

1. 先是由事件循环遍历事件
2. 然后判断事件接受对象有没有安装事件过滤器(installEventFilter)，有安装的话，就把事件丢给事件过滤器(eventFilter)进行处理。
3. 如果没有安装事件过滤器或者事件过滤器对该事件不进行处理的话，那么，事件将会进一步转发到event函数里进行处理。

所以，在这一章节，我们同样一步一步的分析这三个点。

事件循环是怎么遍历的？

int main(int argc, char *argv[])
{
    
    
    QApplication a(argc, argv);

    MainWindow w;
    w.show();
    return a.exec();
}

上面是一个经典的QtGUI程序的main函数，调用a.exec()

int QCoreApplication::exec()
{
    
    
    ...
    
    threadData->quitNow = false;
    QEventLoop eventLoop;
    self->d_func()->in_exec = true;
    self->d_func()->aboutToQuitEmitted = false;
    int returnCode = eventLoop.exec();
    
    ...
}

而看QApplication::exec的源码，实际上就是开启了一个事件循环（QEventLoop）。同样，我们去看QEventLoop::exec的源码，进一步看处理事件的步骤是什么。

int QEventLoop::exec(ProcessEventsFlags flags)
{
    
    
    ...

    while (!d->exit.loadAcquire())
        processEvents(flags | WaitForMoreEvents | EventLoopExec);

    ref.exceptionCaught = false;
    return d->returnCode.loadRelaxed();
}

上面可以看到，QEvenLoop::exec里，是一个while循环，循环的去调用processEvent，而且设置了WaitForMoreEvents就是说，如果没有事件，就阻塞等待。

void QCoreApplication::processEvents(QEventLoop::ProcessEventsFlags flags, int ms)
{
    
    
    // ### Qt 6: consider splitting this method into a public and a private
    //           one, so that a user-invoked processEvents can be detected
    //           and handled properly.
    QThreadData *data = QThreadData::current();
    if (!data->hasEventDispatcher())
        return;
    QElapsedTimer start;
    start.start();
    while (data->eventDispatcher.loadRelaxed()->processEvents(flags & ~QEventLoop::WaitForMoreEvents)) {
    
    
        if (start.elapsed() > ms)
            break;
    }
}

阅读processEvent，其调用了线程的事件调度器QAbstrctEventDispatcher，而这个类是一个抽象基类，根据不同的平台，有不同的实现，我们以windows下(QEventDispatcherWin32)的为例，接着分析事件处理的流程。

bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
{
    
    
    Q_D(QEventDispatcherWin32);

	...

    // To prevent livelocks, send posted events once per iteration.
    // QCoreApplication::sendPostedEvents() takes care about recursions.
    sendPostedEvents();

    ...
}

void QEventDispatcherWin32::sendPostedEvents()
{
    
    
    Q_D(QEventDispatcherWin32);

    if (d->sendPostedEventsTimerId != 0)
        KillTimer(d->internalHwnd, d->sendPostedEventsTimerId);
    d->sendPostedEventsTimerId = 0;

    // Allow posting WM_QT_SENDPOSTEDEVENTS message.
    d->wakeUps.storeRelaxed(0);

    QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData.loadRelaxed());
}

可以看到，事件调度器最终还是调用了QCoreApplication的sendPostEvents

void QCoreApplicationPrivate::sendPostedEvents(QObject *receiver, int event_type,
                                               QThreadData *data)
{
    
    
    if (event_type == -1) {
    
    
        // we were called by an obsolete event dispatcher.
        event_type = 0;
    }

    if (receiver && receiver->d_func()->threadData != data) {
    
    
        qWarning("QCoreApplication::sendPostedEvents: Cannot send "
                 "posted events for objects in another thread");
        return;
    }

    ...

    // Exception-safe cleaning up without the need for a try/catch block
    struct CleanUp {
    
    
        QObject *receiver;
        int event_type;
        QThreadData *data;
        bool exceptionCaught;

        inline CleanUp(QObject *receiver, int event_type, QThreadData *data) :
            receiver(receiver), event_type(event_type), data(data), exceptionCaught(true)
        {
    
    }
        inline ~CleanUp()
        {
    
    
            if (exceptionCaught) {
    
    
                // since we were interrupted, we need another pass to make sure we clean everything up
                data->canWait = false;
            }

            --data->postEventList.recursion;
            if (!data->postEventList.recursion && !data->canWait && data->hasEventDispatcher())
                data->eventDispatcher.loadRelaxed()->wakeUp();

            // clear the global list, i.e. remove everything that was
            // delivered.
            if (!event_type && !receiver && data->postEventList.startOffset >= 0) {
    
    
                const QPostEventList::iterator it = data->postEventList.begin();
                data->postEventList.erase(it, it + data->postEventList.startOffset);
                data->postEventList.insertionOffset -= data->postEventList.startOffset;
                Q_ASSERT(data->postEventList.insertionOffset >= 0);
                data->postEventList.startOffset = 0;
            }
        }
    };
    CleanUp cleanup(receiver, event_type, data);

    while (i < data->postEventList.size()) {
    
    
       ...

        // first, we diddle the event so that we can deliver
        // it, and that no one will try to touch it later.
        pe.event->posted = false;
        QEvent *e = pe.event;
        QObject * r = pe.receiver;

        --r->d_func()->postedEvents;
        Q_ASSERT(r->d_func()->postedEvents >= 0);

        // next, update the data structure so that we're ready
        // for the next event.
        const_cast<QPostEvent &>(pe).event = nullptr;

        locker.unlock();
        const auto relocker = qScopeGuard([&locker] {
    
     locker.lock(); });

        QScopedPointer<QEvent> event_deleter(e); // will delete the event (with the mutex unlocked)

        // after all that work, it's time to deliver the event.
        QCoreApplication::sendEvent(r, e);

        // careful when adding anything below this point - the
        // sendEvent() call might invalidate any invariants this
        // function depends on.
    }

    cleanup.exceptionCaught = false;
}

我们一个一个的分块分析：

1. 判断是否在一个线程

   if (receiver && receiver->d_func()->threadData != data) {
         
         
       qWarning("QCoreApplication::sendPostedEvents: Cannot send "
                "posted events for objects in another thread");
       return;
   }
   

2. 一个有意思的异常安全的处理，不需要try/catch块

   // Exception-safe cleaning up without the need for a try/catch block
   struct CleanUp {
         
         
       QObject *receiver;
       int event_type;
       QThreadData *data;
       bool exceptionCaught;
   
       inline CleanUp(QObject *receiver, int event_type, QThreadData *data) :
           receiver(receiver), event_type(event_type), data(data), exceptionCaught(true)
       {
         
         }
       inline ~CleanUp()
       {
         
         
           if (exceptionCaught) {
         
         
               // since we were interrupted, we need another pass to make sure we clean everything up
               data->canWait = false;
           }
   
           --data->postEventList.recursion;
           if (!data->postEventList.recursion && !data->canWait && data->hasEventDispatcher())
               data->eventDispatcher.loadRelaxed()->wakeUp();
   
           // clear the global list, i.e. remove everything that was
           // delivered.
           if (!event_type && !receiver && data->postEventList.startOffset >= 0) {
         
         
               const QPostEventList::iterator it = data->postEventList.begin();
               data->postEventList.erase(it, it + data->postEventList.startOffset);
               data->postEventList.insertionOffset -= data->postEventList.startOffset;
               Q_ASSERT(data->postEventList.insertionOffset >= 0);
               data->postEventList.startOffset = 0;
           }
       }
   };
   CleanUp cleanup(receiver, event_type, data);
   

定义了一个结构体CleanUp，结构体的析构函数(~CleanUp)保存了函数退出时需要执行的清理操作。然后在栈上创建了一个结构体对象，遍历事件列表时，异常退出，那么就会调用自动调用~CleanUp的析构函数。

3. 将事件发送出去(sendEvent)

   while (i < data->postEventList.size()) {
         
         
          ...
   
           // first, we diddle the event so that we can deliver
           // it, and that no one will try to touch it later.
           pe.event->posted = false;
           QEvent *e = pe.event;
           QObject * r = pe.receiver;
   
           --r->d_func()->postedEvents;
           Q_ASSERT(r->d_func()->postedEvents >= 0);
   
           // next, update the data structure so that we're ready
           // for the next event.
           const_cast<QPostEvent &>(pe).event = nullptr;
   
           locker.unlock();
           const auto relocker = qScopeGuard([&locker] {
         
          locker.lock(); });
   
           QScopedPointer<QEvent> event_deleter(e); // will delete the event (with the mutex unlocked)
   
           // after all that work, it's time to deliver the event.
           QCoreApplication::sendEvent(r, e);
   
           // careful when adding anything below this point - the
           // sendEvent() call might invalidate any invariants this
           // function depends on.
       }
   

可以看到，核心还是调用sendEvent将事件发送出去，而前面我们对sendEvent的源码分析我们可以看到，事件先是经过事件过滤器，再经过对象的event函数，来进行事件的处理。所以就引出我们的下一个话题：事件过滤器

事件过滤器

在实际应用中，我们经常要将某一个窗口部件的某个事件如鼠标滑轮滚动拦截，然后执行我们自己想要的操作。这个时候，我们就可以用到事件过滤器(EventFilter**) **
首先，我们需要自己编写一个eventFilter函数，

bool Class::eventFilter(QObject* watcher, QEvent* event)
{
    
    
	//以过滤鼠标滚轮事件为例
    if (object == m_watcherObject && event->type() == QEvent::Wheel) {
    
    
    	// do something
        return true;       
    }

    QWidget::eventFilter(watcher, event);
}

然后，我们需要为要拦截的某个窗口部件，安装事件过滤器

void Class::initUI() 
{
    
    
	QWidget* m_watcherObject = new QWidget(this);
    // 为对象安装一个事件过滤器
	m_watcherObject->installEventFilterr(this);
}

initUI();

那么一个对象安装的多个事件过滤器，会以什么样的顺序触发呢？我们在前面的讲过，后安装的事件过滤器会先触发，这一点，我们可以在源码里得到佐证:

void QObject::installEventFilter(QObject *obj)
{
    
    
    Q_D(QObject);
    if (!obj)
        return;
    if (d->threadData != obj->d_func()->threadData) {
    
    
        qWarning("QObject::installEventFilter(): Cannot filter events for objects in a different thread.");
        return;
    }

    if (!d->extraData)
        d->extraData = new QObjectPrivate::ExtraData;

    // clean up unused items in the list
    d->extraData->eventFilters.removeAll((QObject*)nullptr);
    d->extraData->eventFilters.removeAll(obj);
    d->extraData->eventFilters.prepend(obj);
}

可以清楚的看到，事件过滤器，是以prepend的形式被添加进事件过滤器列表的。
那么，当有鼠标滚轮事件触发的时候，我们可以看到sendEvent会优先走到事件过滤器里，如果eventFilter返回一个true，那么事件就不会被继续派发，否则，将会将事件发送到其他的事件过滤器里进行处理，如果其他的事件过滤器均对该事件不进行处理，那么事件将会继续往下派发，走到事件的处理函数event

event

接下来，就到了事件处理的最后一站，event函数，这个函数比较简单，我们可以自己重写这个函数，对事件进行自定义的处理。

bool Class::event(QEvent *e)
{
    
    
    switch (e->type()) {
    
    
    case QEvent::Whell:
        // do something
        return true;

    default:
        if (e->type() >= QEvent::User) {
    
    
            customEvent(e);
            break;
        }
        return false;
    }
    return true;
}

夹带私货时间

1. 之前有说到processEvent，添加一个小经验。当我们有时候不得不在主线程循环执行很耗时的操作的时候，这个时候，界面就会刷新不过来，就会导致界面卡顿，影响使用。但是，我们可以在这个循环里，手动调用qApp->processEvent()，这样就可以手动调用处理掉所有的事件，就可以解决卡顿的问题。