Qt中postEvent和sendEvent函数
部分内容参考http://blog.csdn.net/lvmengzou/article/details/65450908
qt事件循环需要维护一个事件队列，在Qt的main函数中最后一般调用QApplication::exec()成员函数来保持程序对事件队列的处理，exec()的实质是不停调用processEvent()函数从队列中获取事件，并处理，然后删除，postEvent的作用就是发送一个事件到此队列中，由于删除队列中事件调用delete运算符，所以，postEvent()传递的事件一定要是动态分配的。
sendEvent()函数直接跳过事件循环队列，直接调用notify()函数发送事件到目标对象，并等待事件处理结果，所以其传递的事件直接在栈上分配即可，(postEvent()函数只负责添加事件到队列，不等待事件处理结果)。
sendEvent调用关系如下：
inline bool QCoreApplication::sendEvent(QObject *receiver, QEvent *event)
{ if (event) event->spont = false; return self ? self->notifyInternal(receiver, event) : false; }
QCoreApplication::notifyInternal(QObject * receiver, QEvent * event)
QCoreApplication::notify(QObject * receiver, QEvent *event)
QCoreApplicationPrivate::notify_helper(QObject * receiver, QEvent * event)
最终调用receiver->event(event);
PostEvent()函数只是单纯添加事件到postEventList
void QCoreApplication::postEvent(QObject *receiver, QEvent *event)
{
postEvent(receiver, event, Qt::NormalEventPriority);
}
void QCoreApplication::postEvent(QObject *receiver, QEvent *event, int priority)
{
...
QThreadData * volatile * pdata = &receiver->d_func()->threadData; //得到线程信息
QThreadData *data = *pdata;
if (!data) {
// posting during destruction? just delete the event to prevent a leak
delete event;
return;
}
// lock the post event mutex
data->postEventList.mutex.lock();
// if object has moved to another thread, follow it
while (data != *pdata) { //在这里判断receiver线程信息是否发生变化。(有可能是另外一个线程调用用receiver->moveToThread)
data->postEventList.mutex.unlock();
data = *pdata;
if (!data) {
// posting during destruction? just delete the event to prevent a leak
delete event;
return;
}
data->postEventList.mutex.lock();
}
//这里postEventList还是被锁着的。
// if this is one of the compressible events, do compression
if (receiver->d_func()->postedEvents
&& self && self->compressEvent(event, receiver, &data->postEventList)) {
data->postEventList.mutex.unlock();//这个事件有可能被压缩(实际上是发现队列中有这个事件还没有被处理，且这个事件是可以被压缩的，例如paintevent)
return;
}
event->posted = true;
++receiver->d_func()->postedEvents;
if (event->type() == QEvent::DeferredDelete && data == QThreadData::current()) {
// remember the current running eventloop for DeferredDelete
// events posted in the receiver's thread
event->d = reinterpret_cast(quintptr(data->loopLevel)); //receiver即将被析构？
}
//将事件添加到postEventList中，注意这里的优先级第一个最高，最后一个优先级最低
if (data->postEventList.isEmpty() || data->postEventList.last().priority >= priority) {
// optimization: we can simply append if the last event in
// the queue has higher or equal priority
data->postEventList.append(QPostEvent(receiver, event, priority));
} else {
// insert event in descending priority order, using upper
// bound for a given priority (to ensure proper ordering
// of events with the same priority)
QPostEventList::iterator begin = data->postEventList.begin()
+ data->postEventList.insertionOffset,
end = data->postEventList.end();
QPostEventList::iterator at = qUpperBound(begin, end, priority);
data->postEventList.insert(at, QPostEvent(receiver, event, priority));
}
data->canWait = false;
data->postEventList.mutex.unlock();//在这里解除锁
//receiver所在的线程调用eventDispatcher处理postEventList
if (data->eventDispatcher)
data->eventDispatcher->wakeUp();
}
如果想查看Qt中QApplication::exec()函数是如何传递事件到目标对象，并根据notify返回值和事件isAccept()的返回值决定是否传递事件到目标对象的父对象，可以重载目标对象的事件处理函数，并在事件处理函数中加入断点，开始调试，然后在可执行程序界面上触发事件处理函数，程序就会在事件处理函数的断点处停止，然后逐级返回，查看整个调用情况，从processEvent()函数到最终我们重载的事件处理函数，至少有20层函数调用。(调试之前要安装Qt源码组件)
