QT 源码之 Qt 事件机制原理是本文要介绍的内容，在用Qt写Gui程序的时候，在main函数里面最后依据都是app.exec();很多书上对这句的解释是，使 Qt 程序进入消息循环。下面我们就到exec()函数内部，来看一下他的实现原理。
Let's go!
首先来到QTDIRsrccorelibkernelqcoreapplication.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; } 再来到qeventloop.cpp中。 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 " "exceptions from an event handler is not supported in Qt. You must " "reimplement QApplication::notify() and catch all exceptions there. "); 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; } 来到了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); } 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<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 *ddata = 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; }
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