分析Qt原始碼一則想自己在開發學習中有積累，同時自己也一直有一種理念，使用她那麼就更深入的認識她。 如果有分析不正確的，還煩請各位看官指正。

• 事件分發器建立
  在QCoreApplication建構函式中

    if (!QCoreApplicationPrivate::eventDispatcher)
        d->createEventDispatcher();

接著

void QGuiApplicationPrivate::createEventDispatcher()
{
    Q_ASSERT(!eventDispatcher);

    if (platform_integration == 0)
        createPlatformIntegration();

    // The platform integration should not mess with the event dispatcher
    Q_ASSERT(!eventDispatcher);

    eventDispatcher = platform_integration->createEventDispatcher();
}
 

如果是windows平臺

QAbstractEventDispatcher * QWindowsIntegration::createEventDispatcher() const
{
    return new QWindowsGuiEventDispatcher;
}

而這裡的QWindowsGuiEventDispatcher是QEventDispatcherWin32的子類

QWindowsGuiEventDispatcher::QWindowsGuiEventDispatcher(QObject *parent) :
    QEventDispatcherWin32(parent), m_flags(0)
{
    setObjectName(QStringLiteral("QWindowsGuiEventDispatcher"));
    createInternalHwnd(); // QTBUG-40881: Do not delay registering timers, etc. for QtMfc.
}

 

• 這裡每一個事件分發器建立後都會建立一個內部視窗，這個視窗雖然不顯示，但是卻是非常重要的，一些自定義訊息都是在這個視窗的過程回撥中處理

void QEventDispatcherWin32::createInternalHwnd()
{
    Q_D(QEventDispatcherWin32);

    if (d->internalHwnd)
        return;
    d->internalHwnd = qt_create_internal_window(this);

    installMessageHook();

    // start all normal timers
    for (int i = 0; i < d->timerVec.count(); ++i)
        d->registerTimer(d->timerVec.at(i));
}

內部視窗的過程函式：
LRESULT QT_WIN_CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp)
{
    if (message == WM_NCCREATE)
        return true;

    MSG msg;
    msg.hwnd = hwnd;
    msg.message = message;
    msg.wParam = wp;
    msg.lParam = lp;
    QAbstractEventDispatcher* dispatcher = QAbstractEventDispatcher::instance();
    long result;
    if (!dispatcher) {
        if (message == WM_TIMER)
            KillTimer(hwnd, wp);
        return 0;
    } else if (dispatcher->filterNativeEvent(QByteArrayLiteral("windows_dispatcher_MSG"), &msg, &result)) {
        return result;
    }

#ifdef GWLP_USERDATA
    QEventDispatcherWin32 *q = (QEventDispatcherWin32 *) GetWindowLongPtr(hwnd, GWLP_USERDATA);
#else
    QEventDispatcherWin32 *q = (QEventDispatcherWin32 *) GetWindowLong(hwnd, GWL_USERDATA);
#endif
    QEventDispatcherWin32Private *d = 0;
    if (q != 0)
        d = q->d_func();

    if (message == WM_QT_SOCKETNOTIFIER) {
        // socket notifier message
        int type = -1;
        switch (WSAGETSELECTEVENT(lp)) {
        case FD_READ:
        case FD_ACCEPT:
            type = 0;
            break;
        case FD_WRITE:
        case FD_CONNECT:
            type = 1;
            break;
        case FD_OOB:
            type = 2;
            break;
        case FD_CLOSE:
            type = 3;
            break;
        }
        if (type >= 0) {
            Q_ASSERT(d != 0);
            QSNDict *sn_vec[4] = { &d->sn_read, &d->sn_write, &d->sn_except, &d->sn_read };
            QSNDict *dict = sn_vec[type];

            QSockNot *sn = dict ? dict->value(wp) : 0;
            if (sn) {
                d->doWsaAsyncSelect(sn->fd, 0);
                d->active_fd[sn->fd].selected = false;
                d->postActivateSocketNotifiers();
                if (type < 3) {
                    QEvent event(QEvent::SockAct);
                    QCoreApplication::sendEvent(sn->obj, &event);
                } else {
                    QEvent event(QEvent::SockClose);
                    QCoreApplication::sendEvent(sn->obj, &event);
                }
            }
        }
        return 0;
    } else if (message == WM_QT_ACTIVATENOTIFIERS) {
        Q_ASSERT(d != 0);

        // register all socket notifiers
        for (QSFDict::iterator it = d->active_fd.begin(), end = d->active_fd.end();
             it != end; ++it) {
            QSockFd &sd = it.value();
            if (!sd.selected) {
                d->doWsaAsyncSelect(it.key(), sd.event);
                sd.selected = true;
            }
        }
        d->activateNotifiersPosted = false;
        return 0;
    } else if (message == WM_QT_SENDPOSTEDEVENTS
               // we also use a Windows timer to send posted events when the message queue is full
               || (message == WM_TIMER
                   && d->sendPostedEventsWindowsTimerId != 0
                   && wp == (uint)d->sendPostedEventsWindowsTimerId)) {
        const int localSerialNumber = d->serialNumber.load();
        if (localSerialNumber != d->lastSerialNumber) {
            d->lastSerialNumber = localSerialNumber;
            q->sendPostedEvents();
        }
        return 0;
    } else if (message == WM_TIMER) {
        Q_ASSERT(d != 0);
        d->sendTimerEvent(wp);
        return 0;
    }

    return DefWindowProc(hwnd, message, wp, lp);
}
 

我們需要區分的是這裡的內部視窗和應用建立的Native視窗的區別(Native視窗後面原始碼分析再說)
這裡處理內部建立

• socket訊息，也是Qt內部自定義的訊息WM_QT_SOCKETNOTIFIER
• postEvent到一些物件的訊息，也是自定義訊息WM_QT_SENDPOSTEDEVENTS
• 通過QObject註冊的定時器訊息WM_TIMER

最終轉成事件直接傳送到相應的物件處理。

• 現在有了事件分發器這個訊息處理髮動機，發動機是如何啟動的呢？
  在QApplication的exec()或者QEventLoop的exec()啟動了事件迴圈
  繼續跟蹤程式碼

void QEventLoop::processEvents(ProcessEventsFlags flags, int maxTime)
{
    Q_D(QEventLoop);
    if (!d->threadData->eventDispatcher.load())
        return;

    QElapsedTimer start;
    start.start();
    while (processEvents(flags & ~WaitForMoreEvents)) {
        if (start.elapsed() > maxTime)
            break;
    }
}

bool QEventLoop::processEvents(ProcessEventsFlags flags)
{
    Q_D(QEventLoop);
    if (!d->threadData->eventDispatcher.load())
        return false;
    return d->threadData->eventDispatcher.load()->processEvents(flags);
}

最後還是事件分發器一直處理事件：

bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
{
    Q_D(QEventDispatcherWin32);

    if (!d->internalHwnd) {
        createInternalHwnd();
        wakeUp(); // trigger a call to sendPostedEvents()
    }

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

    bool canWait;
    bool retVal = false;
    bool seenWM_QT_SENDPOSTEDEVENTS = false;
    bool needWM_QT_SENDPOSTEDEVENTS = false;
    do {
        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
                haveMessage = true;
                msg = d->queuedUserInputEvents.takeFirst();
            } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {
                // process queued socket events
                haveMessage = true;
                msg = d->queuedSocketEvents.takeFirst();
            } else {
                haveMessage = PeekMessage(&msg, 0, 0, 0, PM_REMOVE);
                if (haveMessage) {
                    if ((flags & QEventLoop::ExcludeUserInputEvents)
                        && ((msg.message >= WM_KEYFIRST
                             && msg.message <= WM_KEYLAST)
                            || (msg.message >= WM_MOUSEFIRST
                                && msg.message <= WM_MOUSELAST)
                            || msg.message == WM_MOUSEWHEEL
                            || msg.message == WM_MOUSEHWHEEL
                            || msg.message == WM_TOUCH
#ifndef QT_NO_GESTURES
                            || msg.message == WM_GESTURE
                            || msg.message == WM_GESTURENOTIFY
#endif
                            || msg.message == WM_CLOSE)) {
                        // queue user input events for later processing
                        d->queuedUserInputEvents.append(msg);
                        continue;
                    }
                    if ((flags & QEventLoop::ExcludeSocketNotifiers)
                        && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) {
                        // queue socket events for later processing
                        d->queuedSocketEvents.append(msg);
                        continue;
                    }
                }
            }
            if (!haveMessage) {
                // no message - check for signalled objects
                for (int i=0; i<(int)nCount; i++)
                    pHandles[i] = d->winEventNotifierList.at(i)->handle();
                waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, 0, QS_ALLINPUT, MWMO_ALERTABLE);
                if ((haveMessage = (waitRet == WAIT_OBJECT_0 + nCount))) {
                    // a new message has arrived, process it
                    continue;
                }
            }
            if (haveMessage) {
                // WinCE doesn't support hooks at all, so we have to call this by hand :(
                if (!d->getMessageHook)
                    (void) qt_GetMessageHook(0, PM_REMOVE, (LPARAM) &msg);

                if (d->internalHwnd == msg.hwnd && msg.message == WM_QT_SENDPOSTEDEVENTS) {
                    if (seenWM_QT_SENDPOSTEDEVENTS) {
                        // when calling processEvents() "manually", we only want to send posted
                        // events once
                        needWM_QT_SENDPOSTEDEVENTS = true;
                        continue;
                    }
                    seenWM_QT_SENDPOSTEDEVENTS = true;
                } else 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;
                }

                if (!filterNativeEvent(QByteArrayLiteral("windows_generic_MSG"), &msg, 0)) {
                    TranslateMessage(&msg);
                    DispatchMessage(&msg);
                }
            } else if (waitRet - WAIT_OBJECT_0 < nCount) {
                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
        canWait = (!retVal
                   && !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 | MWMO_INPUTAVAILABLE);
            emit awake();
            if (waitRet - WAIT_OBJECT_0 < nCount) {
                d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));
                retVal = true;
            }
        }
    } while (canWait);

    if (!seenWM_QT_SENDPOSTEDEVENTS && (flags & QEventLoop::EventLoopExec) == 0) {
        // when called "manually", always send posted events
        sendPostedEvents();
    }

    if (needWM_QT_SENDPOSTEDEVENTS)
        PostMessage(d->internalHwnd, WM_QT_SENDPOSTEDEVENTS, 0, 0);

    return retVal;
}

注意到關鍵的Windows訊息迴圈的API：
PeekMessage(&msg, 0, 0, 0, PM_REMOVE);
從訊息佇列裡面取訊息
TranslateMessage(&msg);
轉化訊息
DispatchMessage(&msg);
分發訊息到視窗過程函式

那麼源源不斷的視窗訊息被分發到過程函式中進行處理。
上面我們說到了一些自定義的訊息被內部視窗過程函式接收，後面講我們會講到視窗的建立和視窗過程函式，視窗訊息、然後轉成事件被髮送到相應的widget。