详解 QT 源码之 Qt 事件机制原理

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;  
} 

小结：关于详解 QT 源码之 Qt 事件机制原理的内容介绍完了，基本属于代码实现的内容，最后希望本文对你有帮助！