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。
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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：
- 先是由事件循环遍历事件
- 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。 - 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：
-
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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