理解Qt消息機制刻不容緩，那我們從對比傳統的windows消息處理機制對比來說起；
只有知道QT底層的消息處理、對我們理解并學習Qt有很大幫助；
下面我將對windows程序與Qt對比，并在核心代碼處并給出注釋進行對比、方便學習。

注意重點看代碼中的注視進行對比：！

注意重點看代碼中的注視進行對比：！

注意重點看代碼中的注視進行對比：！

一：windows程序的消息處理
windows程序的處理大概一致
如下：
1.0 windows 消息處理機制：

??? int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
??????????? PSTR szCmdLine, int iCmdShow)
??? {
??????? static TCHAR szAppName[] = TEXT("Hello");
??????? HWND?? hwnd;
??????? MSG??? msg;
??????? WNDCLASS wndclass;
??????? //fill wndclass
??????? wndclass.lpfnWndProc? = WndProc;
??????? ...
??????? RegisterClass(&wndclass);
??????? hwnd = CreateWindow( .... );????? // creation parameters
??????? ShowWindow(hwnd, iCmdShow);
??????? UpdateWindow(hwnd);
??????? while(GetMessage(&msg, NULL, 0, 0))? { //這一塊位置得到消息
??????????? TranslateMessage(&msg);//轉換消息
??????????? DispatchMessage(&msg);//分發消息到系統處理
??????? }
??????? return msg.wParam;
??? }

1.1 這是分發消息的回調函數熟悉windows程序應該不難看懂

??? LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
??? {
??????? HDC hdc;
??????? PAINTSTRUCT ps;
??????? RECT rect;
??????? switch(message) {
??????????? case WM_CREATE:
??????????????? return 0;
??????????? case WM_PAINT://重繪、比如窗口大小拉伸
??????????????? ...
??????????????? return 0;
??????????? case WM_DESTROY:
??????????????? PostQuitMessage(0);
??????????????? return 0;
??????? }
??????? return DefWindowProc(hwnd, message, wParam, lParam); // windows 操作系統內部的消息機制、系統調用函數；
??? }

二 Qt消息處理機制
2.0 Qt的消息機制


QEventDispatcherWin32：
注冊窗口類別，并創建一個隱藏窗口 （QEventDispatcherWin32_Internal_WidgetXXXX）
窗口的回調函數 qt_internal_proc()
安裝WH_GETMESSAGE類型的鉤子函數 qt_GetMessageHook()

??? bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
??? if (!filterNativeEvent(QByteArrayLiteral("windows_generic_MSG"), &msg, 0))? 與上面的消息循環：的while一樣 、得到過濾所有消息
??? {
?? ??? ?TranslateMessage(&msg);//轉換消息
?? ??? ?DispatchMessage(&msg); //分發消息
??? }

DispatchMessage(&msg); //分發消息
分發消息的或回調函數、這個與windows程序的CALLBACK WndProc一樣
LRESULT QT_WIN_CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp)
2.1 下面就是從系統獲得的消息后Qt封裝消息后所作的事情

這個就是Qt的消息回調：

??? LRESULT QT_WIN_CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp)
??? {
??????? if (message == WM_NCCREATE)
??????????? return true;
?? ??? ?// MSG windows 消息結構：
??????? 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();
?? ??? ?// 下面 WM_QT_SOCKETNOTIFIER socket Qt 事件底層的處理機制、處理網絡的消息事件
??????? if (message == WM_QT_SOCKETNOTIFIER) {
??????????? // socket notifier message
??????????? int type = -1;
??????????? switch (WSAGETSELECTEVENT(lp)) { //在非阻塞模式下利用socket事件的消息機制，Server端與Client端之間的通信處于異步狀態下
??????????? case FD_READ: //socket 文件描述符 read 、有數據到達時發生
??????????? case FD_ACCEPT: //socket 文件描述符 接收連接 、 作為客戶端連接成功時發生
??????????????? type = 0;
??????????????? break;
??????????? case FD_WRITE: //socket 文件描述符寫 、有數據發送時產生
??????????? case FD_CONNECT:? //socket 文件描述符發起連接 、 作為服務端等待連接成功時發生?? ?
??????????????? type = 1;
??????????????? break;
??????????? case FD_OOB: //socket 文件描述符收到數據 、 收到外帶數據時發生
??????????????? type = 2;
??????????????? break;
??????????? case FD_CLOSE: //socket 文件描述符關閉斷開連接 、套接口關閉時發生
??????????????? 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 == nullptr) {
??????????????????? d->postActivateSocketNotifiers();
??????????????? } else {
??????????????????? Q_ASSERT(d->active_fd.contains(sn->fd));
??????????????????? QSockFd &sd = d->active_fd[sn->fd];
??????????????????? if (sd.selected) {
??????????????????????? Q_ASSERT(sd.mask == 0);
??????????????????????? d->doWsaAsyncSelect(sn->fd, 0);
??????????????????????? sd.selected = false;
??????????????????? }
??????????????????? d->postActivateSocketNotifiers();
??? ?
??????????????????? // Ignore the message if a notification with the same type was
??????????????????? // received previously. Suppressed message is definitely spurious.
??????????????????? const long eventCode = WSAGETSELECTEVENT(lp);
??????????????????? if ((sd.mask & eventCode) != eventCode) {
??????????????????????? sd.mask |= eventCode;
??????????????????????? QEvent event(type < 3 ? QEvent::SockAct : QEvent::SockClose);
??????????????????????? QCoreApplication::sendEvent(sn->obj, &event);
??????????????????? }
??????????????? }
??????????? }
??????????? return 0;
??????? } else if (message == WM_QT_ACTIVATENOTIFIERS) { // 處理postEvent事件
??????????? Q_ASSERT(d != 0);
?? ??? ??? ?// postEvent() 事件，因為是隊列的存儲的方式，郵遞發送消息、直接返回
?? ??? ??? ?
??????????? // Postpone activation if we have unhandled socket notifier messages
??????????? // in the queue. WM_QT_ACTIVATENOTIFIERS will be posted again as a result of
??????????? // event processing.
??????????? MSG msg;//異步調用
??????????? if (!PeekMessage(&msg, d->internalHwnd,
???????????????????????????? WM_QT_SOCKETNOTIFIER, WM_QT_SOCKETNOTIFIER, PM_NOREMOVE)
??????????????? && d->queuedSocketEvents.isEmpty()) { // d->queuedSocketEvents 消息隊列如果消息有很多的時候都會加入這個隊列、這個就是關于Qt::connect();參數：地址
??????????????? // 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);
??????????????????????? // allow any event to be accepted
??????????????????????? sd.mask = 0;
??????????????????????? sd.selected = true;
??????????????????? }
??????????????? }
??????????? }
??????????? d->activateNotifiersPosted = false;
??????????? return 0;
??????? } else if (message == WM_QT_SENDPOSTEDEVENTS? // 處理 Qt sendPostEvent()發送事件
?????????????????? // we also use a Windows timer to send posted events when the message queue is full
?? ??? ??? ??? ??? // WM_QT_SENDPOSTEDEVENTS : 這個消息是我們Qt程序大部分走的事件，特殊情況除外。
?????????????????? || (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();//因為sendevent是同步所以直接進入進行調用
??????????? }
??????????? return 0;
??????? } else if (message == WM_TIMER) {//系統定時器超時
??????????? Q_ASSERT(d != 0);
??????????? d->sendTimerEvent(wp);
??????????? return 0;
??????? }
??? ?
??????? return DefWindowProc(hwnd, message, wp, lp); // 這個與windows程序一樣的地方。
??? }

另外還有一點很重要的：大家都知道Qt消息處理比windows處理的塊，Qt程序通過一通處理才調用DefWindowProc傳給系統所以Qt系統的消息機制是比windows的消息慢的原因之一；
不過他也有它優點：qt的信號與槽函數讓我們讓我們編程更方便。
2.3 消息全局通知事件

另外我們都知道我們開發的系統、所有的消息都會到這個類QApplication::notify事件過濾所有的事件：
其實QApplication繼承QGuiApplication類；

??? bool QGuiApplication::notify(QObject *object, QEvent *event)
??? {
??????? if (object->isWindowType())
??????????? QGuiApplicationPrivate::sendQWindowEventToQPlatformWindow(static_cast<QWindow *>(object), event);
??????? return QCoreApplication::notify(object, event);
??? }

2.4 消息的組裝

下面是所有的消息類型處理：在我們開發的系統中別人使用processEvent發送消息是比較高效的。
原因：這個消息直達經過的處理的少。

??? void QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent *e)
??? {
??????? switch(e->type) {
??????? case QWindowSystemInterfacePrivate::FrameStrutMouse:
??????? case QWindowSystemInterfacePrivate::Mouse:
??????????? QGuiApplicationPrivate::processMouseEvent(static_cast<QWindowSystemInterfacePrivate::MouseEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::Wheel:
??????????? QGuiApplicationPrivate::processWheelEvent(static_cast<QWindowSystemInterfacePrivate::WheelEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::Key:
??????????? QGuiApplicationPrivate::processKeyEvent(static_cast<QWindowSystemInterfacePrivate::KeyEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::Touch:
??????????? QGuiApplicationPrivate::processTouchEvent(static_cast<QWindowSystemInterfacePrivate::TouchEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::GeometryChange:
??????????? QGuiApplicationPrivate::processGeometryChangeEvent(static_cast<QWindowSystemInterfacePrivate::GeometryChangeEvent*>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::Enter:
??????????? QGuiApplicationPrivate::processEnterEvent(static_cast<QWindowSystemInterfacePrivate::EnterEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::Leave:
??????????? QGuiApplicationPrivate::processLeaveEvent(static_cast<QWindowSystemInterfacePrivate::LeaveEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::ActivatedWindow:
??????????? QGuiApplicationPrivate::processActivatedEvent(static_cast<QWindowSystemInterfacePrivate::ActivatedWindowEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::WindowStateChanged:
??????????? QGuiApplicationPrivate::processWindowStateChangedEvent(static_cast<QWindowSystemInterfacePrivate::WindowStateChangedEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::WindowScreenChanged:
??????????? QGuiApplicationPrivate::processWindowScreenChangedEvent(static_cast<QWindowSystemInterfacePrivate::WindowScreenChangedEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::ApplicationStateChanged: {
??????????? QWindowSystemInterfacePrivate::ApplicationStateChangedEvent * changeEvent = static_cast<QWindowSystemInterfacePrivate::ApplicationStateChangedEvent *>(e);
??????????? QGuiApplicationPrivate::setApplicationState(changeEvent->newState, changeEvent->forcePropagate); }
??????????? break;
??????? case QWindowSystemInterfacePrivate::FlushEvents: {
??????????? QWindowSystemInterfacePrivate::FlushEventsEvent *flushEventsEvent = static_cast<QWindowSystemInterfacePrivate::FlushEventsEvent *>(e);
??????????? QWindowSystemInterface::deferredFlushWindowSystemEvents(flushEventsEvent->flags); }
??????????? break;
??????? case QWindowSystemInterfacePrivate::Close:
??????????? QGuiApplicationPrivate::processCloseEvent(
??????????????????? static_cast<QWindowSystemInterfacePrivate::CloseEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::ScreenOrientation:
??????????? QGuiApplicationPrivate::reportScreenOrientationChange(
??????????????????? static_cast<QWindowSystemInterfacePrivate::ScreenOrientationEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::ScreenGeometry:
??????????? QGuiApplicationPrivate::reportGeometryChange(
??????????????????? static_cast<QWindowSystemInterfacePrivate::ScreenGeometryEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::ScreenLogicalDotsPerInch:
??????????? QGuiApplicationPrivate::reportLogicalDotsPerInchChange(
??????????????????? static_cast<QWindowSystemInterfacePrivate::ScreenLogicalDotsPerInchEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::ScreenRefreshRate:
??????????? QGuiApplicationPrivate::reportRefreshRateChange(
??????????????????? static_cast<QWindowSystemInterfacePrivate::ScreenRefreshRateEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::ThemeChange:
??????????? QGuiApplicationPrivate::processThemeChanged(
??????????????????????? static_cast<QWindowSystemInterfacePrivate::ThemeChangeEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::Expose:
??????????? QGuiApplicationPrivate::processExposeEvent(static_cast<QWindowSystemInterfacePrivate::ExposeEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::Tablet:
??????????? QGuiApplicationPrivate::processTabletEvent(
??????????????????????? static_cast<QWindowSystemInterfacePrivate::TabletEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::TabletEnterProximity:
??????????? QGuiApplicationPrivate::processTabletEnterProximityEvent(
??????????????????????? static_cast<QWindowSystemInterfacePrivate::TabletEnterProximityEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::TabletLeaveProximity:
??????????? QGuiApplicationPrivate::processTabletLeaveProximityEvent(
??????????????????????? static_cast<QWindowSystemInterfacePrivate::TabletLeaveProximityEvent *>(e));
??????????? break;
??? #ifndef QT_NO_GESTURES
??????? case QWindowSystemInterfacePrivate::Gesture:
??????????? QGuiApplicationPrivate::processGestureEvent(
??????????????????????? static_cast<QWindowSystemInterfacePrivate::GestureEvent *>(e));
??????????? break;
??? #endif
??????? case QWindowSystemInterfacePrivate::PlatformPanel:
??????????? QGuiApplicationPrivate::processPlatformPanelEvent(
??????????????????????? static_cast<QWindowSystemInterfacePrivate::PlatformPanelEvent *>(e));
??????????? break;
??????? case QWindowSystemInterfacePrivate::FileOpen:
??????????? QGuiApplicationPrivate::processFileOpenEvent(
??????????????????????? static_cast<QWindowSystemInterfacePrivate::FileOpenEvent *>(e));
??????????? break;
??? #ifndef QT_NO_CONTEXTMENU
??????????? case QWindowSystemInterfacePrivate::ContextMenu:
??????????? QGuiApplicationPrivate::processContextMenuEvent(
??????????????????????? static_cast<QWindowSystemInterfacePrivate::ContextMenuEvent *>(e));
??????????? break;
??? #endif
??????? case QWindowSystemInterfacePrivate::EnterWhatsThisMode:
??????????? QGuiApplication::postEvent(QGuiApplication::instance(), new QEvent(QEvent::EnterWhatsThisMode));
??????????? break;
??????? default:
??????????? qWarning() << "Unknown user input event type:" << e->type;
??????????? break;
??????? }
??? }

2.5 常用的消息過濾事件

通過2.4組裝的消息最終到達、此處。經過這些之后:之前的windows消息現在都已映射為QT類型的消息、進入Qt消息處理機制中來、才有了我們的消息過濾等等消息事件鍵盤鼠標等等；

??? virtual void mouseDoubleClickEvent(QMouseEvent *event)
??? virtual void mouseMoveEvent(QMouseEvent *event)
??? virtual void mousePressEvent(QMouseEvent *event)
??? virtual void mouseReleaseEvent(QMouseEvent *event)
??? virtual void moveEvent(QMoveEvent *event)
??? virtual bool nativeEvent(const QByteArray &eventType, void *message, long *result)
??? virtual void paintEvent(QPaintEvent *event)
??? virtual void resizeEvent(QResizeEvent *event)
??? virtual void showEvent(QShowEvent *event)
??? virtual void tabletEvent(QTabletEvent *event)
??? virtual void wheelEvent(QWheelEvent *event)

重新實現上面的消息事件做我們的功能；

總結：

通過windows消息的運行機制與Qt的消息機制進行了對比。其實Qt在windows平臺上的消息傳遞依賴的是windows消息機制。只不過在開始時進行注冊消息攔截等一些封裝處理、加工成Qt消息在Qt程序中進行傳遞。

總結

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