Qt源码阅读(四) 事件循环

技术分享 3年前 (2023-03-31) 0 999+

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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。

事件系统

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。

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：

程序外部产生
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：

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; }

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。

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(); }

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; }

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; }

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; }

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：

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：

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; }

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。

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

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。

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