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QT源碼解析(一) QT創建窗口程序、消息循環和WinMain函數
QT源碼解析(四）剖析Qt的事件機制原理
在用Qt寫Gui程序的時候，在main函數里面最后依據都是app.exec();很多書上對這句的解釋是，使Qt程序進入消息循環。下面我們就到exec()函數內部，來看一下他的實現原理。
Let's go!
首先來到QTDIR/src/corelib/kernel/qcoreapplication.cpp
int QCoreApplication::exec()
{
if (!QCoreApplicationPrivate::checkInstance("exec"))
return -1;
//獲取線程數據
QThreadData *threadData = self->d_func()->threadData;
//判斷是否在主線程創建
if (threadData != QThreadData::current()) {
qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className());
return -1;
}
//判斷eventLoop是否已經創建
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;
//創建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;
}
再來到qeventloop.cpp中。
int QEventLoop::exec(ProcessEventsFlags flags)
{
Q_D(QEventLoop);
if (d->threadData->quitNow)
return -1;
//已經調用過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;
//將事件類對象壓入線程結構體中
d->threadData->eventLoops.push(this);
// remove posted quit events when entering a new event loop
// 這句不用翻譯了把！
if (qApp->thread() == thread())
QCoreApplication::removePostedEvents(qApp, QEvent::Quit);
#if defined(QT_NO_EXCEPTIONS)
while (!d->exit)
//這里才是關鍵,我們還要繼續跟蹤進去。
processEvents(flags | WaitForMoreEvents);
#else
try {
while (!d->exit)
processEvents(flags | WaitForMoreEvents);
} catch (...) {
//如果使用了EXCEPTION，則繼續對下一條時間進行處理。
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，將時間對象從線程結構中取出。
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;
//退出事件循環。
return d->returnCode;
}
來到了processEvents函數：
bool QEventLoop::processEvents(ProcessEventsFlags flags)
{
Q_D(QEventLoop);
//判斷事件分派器是否為空。
if (!d->threadData->eventDispatcher)
return false;
if (flags & DeferredDeletion)
QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete);
//調用不同平台下的事件分派器來處理事件。
return d->threadData->eventDispatcher->processEvents(flags);
}
processEvents是在QAbstractEventDispatcher類中定義的純虛方法。在QEventDispatcherWin32類有processEvents的實現。
bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
{
Q_D(QEventDispatcherWin32);
//內部數據創建。registerClass注冊窗口類，createwindow創建窗體。
//注冊socket notifiers，啟動所有的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<MSG> 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處理用戶輸入事件，放入隊列中。
haveMessage = true;
msg = d->queuedUserInputEvents.takeFirst();
} else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {
// process queued socket events 處理socket事件，放入隊列中。
haveMessage = true;
msg = d->queuedSocketEvents.takeFirst();
} else {
//從消息隊列中取消息，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();
//注冊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;
}
}
//事件隊列中有事件需要處理。
if (haveMessage) {
//處理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;
}
//消息分發處理。
if (!filterEvent(&msg)) {
TranslateMessage(&msg);
QT_WA({
DispatchMessage(&msg);
} , {
DispatchMessageA(&msg);
});
}
} else if (waitRet >= WAIT_OBJECT_0 && waitRet < WAIT_OBJECT_0 + nCount) {
//處理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;
}
參考：http://blog.csdn.net/tingsking18/article/details/5528666