剖析Qt的事件机制原理（源代码级别）

1.先看看QTDIR/src/corelib/kernel/qcoreapplication.cpp

int QCoreApplication::exec()
{
if (!QCoreApplicationPrivate::checkInstance("exec"))
return -1;
//获取线程数据
QThreadData *threadData = self->d_func()->threadData;
//判断是否在主线程创建
if (threadData != QThreadData::current()) {
qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className());
return -1;
}
//判断eventLoop是否已经创建
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;
//创建eventLoop
int returnCode = eventLoop.exec();
threadData->quitNow = false;
if (self) {
self->d_func()->in_exec = false;
//退出程序
emit self->aboutToQuit();
sendPostedEvents(0, QEvent::DeferredDelete);
}
return returnCode;
}

2.再看看qeventloop.cpp

int QEventLoop::exec(ProcessEventsFlags flags)
{
Q_D(QEventLoop);
if (d->threadData->quitNow)
return -1;
//已经调用过exec了。
if (d->inExec) {
qWarning("QEventLoop::exec: instance %p has already called exec()", this);
return -1;
}
d->inExec = true;
d->exit = false;
++d->threadData->loopLevel;
//将事件类对象压入线程结构体中
d->threadData->eventLoops.push(this);

// remove posted quit events when entering a new event loop
// 这句不用翻译了把！
if (qApp->thread() == thread())
QCoreApplication::removePostedEvents(qApp, QEvent::Quit);

#if defined(QT_NO_EXCEPTIONS)
while (!d->exit)
//这里才是关键,我们还要继续跟踪进去。
processEvents(flags | WaitForMoreEvents);
#else
try {
while (!d->exit)
processEvents(flags | WaitForMoreEvents);
} catch (...) {
//如果使用了EXCEPTION，则继续对下一条时间进行处理。
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");
throw;
}
#endif
//退出eventloop前，将时间对象从线程结构中取出。
QEventLoop *eventLoop = d->threadData->eventLoops.pop();
Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");
Q_UNUSED(eventLoop); // --release warning

d->inExec = false;
--d->threadData->loopLevel;
//退出事件循环。
return d->returnCode;
}

3.processEvents函数：

bool QEventLoop::processEvents(ProcessEventsFlags flags)
{
Q_D(QEventLoop);
//判断事件分派器是否为空。
if (!d->threadData->eventDispatcher)
return false;
if (flags & DeferredDeletion)
QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete);
//调用不同平台下的事件分派器来处理事件。
return d->threadData->eventDispatcher->processEvents(flags);
}

4.processEvents是在QAbstractEventDispatcher类中定义的纯虚方法。在QEventDispatcherWin32类有processEvents的实现。

bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
{
Q_D(QEventDispatcherWin32);
//内部数据创建。registerClass注册窗口类，createwindow创建窗体。
//注册socket notifiers，启动所有的normal timers
if (!d->internalHwnd)
createInternalHwnd();

d->interrupt = false;
emit awake();

bool canWait;
bool retVal = false;
do {
QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);

DWORD waitRet = 0;
HANDLE pHandles[MAXIMUM_WAIT_OBJECTS - 1];
QVarLengthArray processedTimers;
while (!d->interrupt) {
DWORD nCount = d->winEventNotifierList.count();
Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);

MSG msg;
bool haveMessage;

if (!(flags & QEventLoop::ExcludeUserInputEvents) && !d->queuedUserInputEvents.isEmpty()) {
// process queued user input events处理用户输入事件，放入队列中。
haveMessage = true;
msg = d->queuedUserInputEvents.takeFirst();
} else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {
// process queued socket events 处理socket事件，放入队列中。
haveMessage = true;
msg = d->queuedSocketEvents.takeFirst();
} else {
//从消息队列中取消息，同PeekMessage
haveMessage = winPeekMessage(&msg, 0, 0, 0, PM_REMOVE);
if (haveMessage && (flags & QEventLoop::ExcludeUserInputEvents)
&& ((msg.message >= WM_KEYFIRST
&& msg.message <= WM_KEYLAST)
|| (msg.message >= WM_MOUSEFIRST
&& msg.message <= WM_MOUSELAST)
|| msg.message == WM_MOUSEWHEEL)) {
// queue user input events for later processing
haveMessage = false;
d->queuedUserInputEvents.append(msg);
}
if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers)
&& (msg.message == WM_USER && msg.hwnd == d->internalHwnd)) {
// queue socket events for later processing
haveMessage = false;
d->queuedSocketEvents.append(msg);
}
}
if (!haveMessage) {
// no message - check for signalled objects
for (int i=0; i<(int)nCount; i++)
pHandles[i] = d->winEventNotifierList.at(i)->handle();
//注册signal--slot。
waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, 0, QS_ALLINPUT, MWMO_ALERTABLE);
if ((haveMessage = (waitRet == WAIT_OBJECT_0 + nCount))) {
// a new message has arrived, process it
continue;
}
}
//事件队列中有事件需要处理。
if (haveMessage) {
//处理timer事件
if (msg.message == WM_TIMER) {
// avoid live-lock by keeping track of the timers we've already sent
bool found = false;
for (int i = 0; !found && i < processedTimers.count(); ++i) {
const MSG processed = processedTimers.constData()[i];
found = (processed.wParam == msg.wParam && processed.hwnd == msg.hwnd && processed.lParam == msg.lParam);
}
if (found)
continue;
processedTimers.append(msg);
} else if (msg.message == WM_QUIT) {
if (QCoreApplication::instance())
QCoreApplication::instance()->quit();
return false;
}
//消息分发处理。
if (!filterEvent(&msg)) {
TranslateMessage(&msg);
QT_WA({
DispatchMessage(&msg);
} , {
DispatchMessageA(&msg);
});
}
} else if (waitRet >= WAIT_OBJECT_0 && waitRet < WAIT_OBJECT_0 + nCount) {
//处理signal--slot
d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));
} else {
// nothing todo so break
break;
}
retVal = true;
}

// still nothing - wait for message or signalled objects
QThreadData *data = d->threadData;
canWait = (!retVal
&& data->canWait
&& !d->interrupt
&& (flags & QEventLoop::WaitForMoreEvents));
if (canWait) {
DWORD nCount = d->winEventNotifierList.count();
Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);
for (int i=0; i<(int)nCount; i++)
pHandles[i] = d->winEventNotifierList.at(i)->handle();

emit aboutToBlock();
waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, INFINITE, QS_ALLINPUT, MWMO_ALERTABLE);
emit awake();
if (waitRet >= WAIT_OBJECT_0 && waitRet < WAIT_OBJECT_0 + nCount) {
d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));
retVal = true;
}
}
} while (canWait);

return retVal;
}