在Qt中，事件被封裝成一個個對象，所有的事件均繼承自抽象類QEvent. 接下來依次談談Qt中有誰來產生、分發、接受和處理事件。
本篇來介紹Qt 事件 處理機制。深入了解事件 處理系統對于每個學習Qt 人來說非常重要，可以說，Qt 是以事件 驅動的UI工具集。 大家熟知Signals/Slots在多線程的實現也依賴于Qt 的事件 處理機制
在Qt 中，事件 被封裝成一個個對象，所有的事件 均繼承自抽象類QEvent.? 接下來依次談談Qt 中有誰來產生、分發、接受和處理事件 ：
1、誰來產生事件： 最容易想到的是我們的輸入設備，比如鍵盤、鼠標產生的
keyPressEvent，keyReleaseEvent，mousePressEvent，mouseReleaseEvent事件（他們被封裝成QMouseEvent和QKeyEvent），這些事件來自于底層的操作系統，它們以異步的形式通知Qt事件處理系統，后文會仔細道來。當然Qt自己也會產生很多事件，比如QObject::startTimer()會觸發QTimerEvent.
2、誰來接受和處理事件：答案是QObject。在Qt的內省機制剖析一文已經介紹QObject
3、誰來負責分發事件：對于non-GUI的Qt程序，是由QCoreApplication負責將QEvent分發給QObject的子類-Receiver.
接下來，將通過對代碼的解析來看看QT是利用event loop從事件隊列中獲取用戶輸入事件，又是如何將事件轉義成QEvents，并分發給相應的QObject處理。
section1
1 #include <QApplication>
2 #include "widget.h"
3 int main(int argc, char *argv[])
4 {
5 QApplication app(argc, argv);
6 Widget window; // Widget 繼承自QWidget
7 window.show();
8 return app.exec(); // 進入Qpplication事件循環，見section 2
9 }
section2
1 int QApplication::exec()
2 {
3 #ifndef QT_NO_ACCESSIBILITY
4 QAccessible::setRootObject(qApp);
5 #endif //簡單的交給QCoreApplication來處理事件循環=〉section?3
6 return QCoreApplication::exec();
7 }
section3
1 int QCoreApplication::exec()2 {3 if (!QCoreApplicationPrivate::checkInstance("exec"))4 return -1;5 //得到當前Thread數據??6 QThreadData *threadData = self->d_func()->threadData;7 if (threadData != QThreadData::current()) {8 qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className());9 return -1;
10 }//檢查event?loop是否已經創建?
11 if (!threadData->eventLoops.isEmpty()) {
12 qWarning("QCoreApplication::exec: The event loop is already running");
13 return -1;
14 }
15
16 threadData->quitNow = false;
17 QEventLoop eventLoop;
18 self->d_func()->in_exec = true;
19 self->d_func()->aboutToQuitEmitted = false;//委任QEventLoop?處理事件隊列循環?==>?Section?4
20 int returnCode = eventLoop.exec();
21 threadData->quitNow = false;
22 if (self) {
23 self->d_func()->in_exec = false;
24 if (!self->d_func()->aboutToQuitEmitted)
25 emit self->aboutToQuit();
26 self->d_func()->aboutToQuitEmitted = true;
27 sendPostedEvents(0, QEvent::DeferredDelete);
28 }
29
30 return returnCode;
31 }
1 int QEventLoop::exec(ProcessEventsFlags flags)2 {3 Q_D(QEventLoop); //訪問QEventloop私有類實例d4 //we need to protect from race condition with QThread::exit5 QMutexLocker locker(&static_cast<QThreadPrivate *>(QObjectPrivate::get(d->threadData->thread))->mutex);6 if (d->threadData->quitNow)7 return -1;8 9 if (d->inExec) {
10 qWarning("QEventLoop::exec: instance %p has already called exec()", this);
11 return -1;
12 }
13 d->inExec = true;
14 d->exit = false;
15 ++d->threadData->loopLevel;
16 d->threadData->eventLoops.push(this);
17 locker.unlock();
18
19 // remove posted quit events when entering a new event loop
20 QCoreApplication *app = QCoreApplication::instance();
21 if (app && app->thread() == thread())
22 QCoreApplication::removePostedEvents(app, QEvent::Quit);
23 //這里的實現代碼不少，最為重要的是以下幾行?
24 #if defined(QT_NO_EXCEPTIONS)
25 while (!d->exit)
26 processEvents(flags | WaitForMoreEvents | EventLoopExec);
27 #else
28 try {
29 while (!d->exit) //只要沒有遇見exit，循環派發事件?
30 processEvents(flags | WaitForMoreEvents | EventLoopExec);
31 } catch (...) {
32 qWarning("Qt has caught an exception thrown from an event handler. Throwing\n"
33 "exceptions from an event handler is not supported in Qt. You must\n"
34 "reimplement QApplication::notify() and catch all exceptions there.\n");
35
36 // copied from below
37 locker.relock();
38 QEventLoop *eventLoop = d->threadData->eventLoops.pop();
39 Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");
40 Q_UNUSED(eventLoop); // --release warning
41 d->inExec = false;
42 --d->threadData->loopLevel;
43
44 throw;
45 }
46 #endif
47
48 // copied above
49 locker.relock();
50 QEventLoop *eventLoop = d->threadData->eventLoops.pop();
51 Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");
52 Q_UNUSED(eventLoop); // --release warning
53 d->inExec = false;
54 --d->threadData->loopLevel;
55
56 return d->returnCode;
57 }
section5
1 bool QEventLoop::processEvents(ProcessEventsFlags flags)
2 {
3 Q_D(QEventLoop);
4 if (!d->threadData->eventDispatcher)
5 return false;
6 if (flags & DeferredDeletion)
7 QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete);
8 return d->threadData->eventDispatcher->processEvents(flags); //將事件派發給與平臺相關的QAbstractEventDispatcher子類?=>Section?6
9 }
//?Section?6，QTDIR\src\corelib\kernel\qeventdispatcher_win.cpp??? ? //?這段代碼是完成與windows平臺相關的windows?c++。?以跨平臺著稱的Qt同時也提供了對Symiban,Unix等平臺的消息派發支持??? ? //?其事現分別封裝在QEventDispatcherSymbian和QEventDispatcherUNIX??? ? //?QEventDispatcherWin32派生自QAbstractEventDispatcher
1 bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)2 {3 Q_D(QEventDispatcherWin32);4 5 if (!d->internalHwnd)6 createInternalHwnd();7 8 d->interrupt = false;9 emit awake();10 11 bool canWait;12 bool retVal = false;13 bool seenWM_QT_SENDPOSTEDEVENTS = false;14 bool needWM_QT_SENDPOSTEDEVENTS = false;15 do {16 DWORD waitRet = 0;17 HANDLE pHandles[MAXIMUM_WAIT_OBJECTS - 1];18 QVarLengthArray<MSG> processedTimers;19 while (!d->interrupt) {20 DWORD nCount = d->winEventNotifierList.count();21 Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);22 23 MSG msg;24 bool haveMessage;25 26 if (!(flags & QEventLoop::ExcludeUserInputEvents) && !d->queuedUserInputEvents.isEmpty()) {27 // process queued user input events28 haveMessage = true;29 msg = d->queuedUserInputEvents.takeFirst(); //從處理用戶輸入隊列中取出一條事件，處理隊列里面的用戶輸入事件30 } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {31 // process queued socket events32 haveMessage = true;33 msg = d->queuedSocketEvents.takeFirst(); //?從處理socket隊列中取出一條事件,處理隊列里面的socket事件34 } else {35 haveMessage = PeekMessage(&msg, 0, 0, 0, PM_REMOVE);36 if (haveMessage && (flags & QEventLoop::ExcludeUserInputEvents)37 && ((msg.message >= WM_KEYFIRST38 && msg.message <= WM_KEYLAST)39 || (msg.message >= WM_MOUSEFIRST40 && msg.message <= WM_MOUSELAST)41 || msg.message == WM_MOUSEWHEEL42 || msg.message == WM_MOUSEHWHEEL43 || msg.message == WM_TOUCH44 #ifndef QT_NO_GESTURES45 || msg.message == WM_GESTURE46 || msg.message == WM_GESTURENOTIFY47 #endif48 || msg.message == WM_CLOSE)) {49 // queue user input events for later processing50 haveMessage = false;51 d->queuedUserInputEvents.append(msg); //?用戶輸入事件入隊列，待以后處理?52 }53 if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers)54 && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) {55 // queue socket events for later processing56 haveMessage = false;57 d->queuedSocketEvents.append(msg); ?//?socket?事件入隊列，待以后處理???58 }59 }60 if (!haveMessage) {61 // no message - check for signalled objects62 for (int i=0; i<(int)nCount; i++)63 pHandles[i] = d->winEventNotifierList.at(i)->handle();64 waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, 0, QS_ALLINPUT, MWMO_ALERTABLE);65 if ((haveMessage = (waitRet == WAIT_OBJECT_0 + nCount))) {66 // a new message has arrived, process it67 continue;68 }69 }70 if (haveMessage) {71 #ifdef Q_OS_WINCE72 // WinCE doesn't support hooks at all, so we have to call this by hand :(73 (void) qt_GetMessageHook(0, PM_REMOVE, (LPARAM) &msg);74 #endif75 76 if (d->internalHwnd == msg.hwnd && msg.message == WM_QT_SENDPOSTEDEVENTS) {77 if (seenWM_QT_SENDPOSTEDEVENTS) {78 // when calling processEvents() "manually", we only want to send posted79 // events once80 needWM_QT_SENDPOSTEDEVENTS = true;81 continue;82 }83 seenWM_QT_SENDPOSTEDEVENTS = true;84 } else if (msg.message == WM_TIMER) {85 // avoid live-lock by keeping track of the timers we've already sent86 bool found = false;87 for (int i = 0; !found && i < processedTimers.count(); ++i) {88 const MSG processed = processedTimers.constData()[i];89 found = (processed.wParam == msg.wParam && processed.hwnd == msg.hwnd && processed.lParam == msg.lParam);90 }91 if (found)92 continue;93 processedTimers.append(msg);94 } else if (msg.message == WM_QUIT) {95 if (QCoreApplication::instance())96 QCoreApplication::instance()->quit();97 return false;98 }99
100 if (!filterEvent(&msg)) {
101 TranslateMessage(&msg); //將事件打包成message調用Windows?API派發出去
102 DispatchMessage(&msg); //分發一個消息給窗口程序。消息被分發到回調函數，將消息傳遞給windows系統，windows處理完畢，會調用回調函數?=>?section?7?
103 }
104 } else if (waitRet < WAIT_OBJECT_0 + nCount) {
105 d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));
106 } else {
107 // nothing todo so break
108 break;
109 }
110 retVal = true;
111 }
112
113 // still nothing - wait for message or signalled objects
114 canWait = (!retVal
115 && !d->interrupt
116 && (flags & QEventLoop::WaitForMoreEvents));
117 if (canWait) {
118 DWORD nCount = d->winEventNotifierList.count();
119 Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);
120 for (int i=0; i<(int)nCount; i++)
121 pHandles[i] = d->winEventNotifierList.at(i)->handle();
122
123 emit aboutToBlock();
124 waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, INFINITE, QS_ALLINPUT, MWMO_ALERTABLE | MWMO_INPUTAVAILABLE);
125 emit awake();
126 if (waitRet < WAIT_OBJECT_0 + nCount) {
127 d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));
128 retVal = true;
129 }
130 }
131 } while (canWait);
132
133 if (!seenWM_QT_SENDPOSTEDEVENTS && (flags & QEventLoop::EventLoopExec) == 0) {
134 // when called "manually", always send posted events
135 QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);
136 }
137
138 if (needWM_QT_SENDPOSTEDEVENTS)
139 PostMessage(d->internalHwnd, WM_QT_SENDPOSTEDEVENTS, 0, 0);
140
141 return retVal;
142 }
//?Section?7?windows窗口回調函數?定義在QTDIR\src\gui\kernel\qapplication_win.cpp?
1 extern "C" LRESULT QT_WIN_CALLBACK QtWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
2 {
3 ...
4 //將消息重新封裝成QEvent的子類QMouseEvent ==> Section 8
5 result = widget->translateMouseEvent(msg);
6 ...
7 }
從Section 1~Section7，?Qt 進入QApplication的event loop，經過層層委任，最終QEventloop的processEvent將通過與平臺相關的QAbstractEventDispatcher的子類QEventDispatcherWin32獲得用戶的用戶輸入事件 ，并將其打包成message后，通過標準Windows API ，把消息傳遞給了Windows OS，Windows OS得到通知后回調QtWndProc,? 至此事件 的分發與處理完成了一半的路程。
在下文中，我們將進一步討論當我們收到來在Windows的回調后，事件 又是怎么一步步打包成QEvent并通過QApplication分發給最終事件 的接受和處理者QObject::event
事件 的產生、分發、接受和處理，并以視窗系統鼠標點擊QWidget為例，對代碼進行了剖析，向大家分析了Qt 框架如何通過Event事件 交給視窗系統處理，函數調用棧如下：
1 main(int, char **)
2 QApplication::exec()
3 QCoreApplication::exec()
4 QEventLoop::exec(ProcessEventsFlags )
5 QEventLoop::processEvents(ProcessEventsFlags )
6 QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags)
本文將介紹Qt app在視窗系統回調后，事件 又是怎么一步步通過QApplication分發給最終事件的接受和處理者QWidget::event, （QWidget繼承Object,重載其虛函數event)，以下所有的討論都將嵌入在源碼之中。
1 QT_WIN_CALLBACK QtWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
2 bool QETWidget::translateMouseEvent(const MSG &msg)
3 bool QApplicationPrivate::sendMouseEvent(...)
4 inline bool QCoreApplication::sendSpontaneousEvent(QObject *receiver, QEvent *event)
5 bool QCoreApplication::notifyInternal(QObject *receiver, QEvent *event)
6 bool QApplication::notify(QObject *receiver, QEvent *e)
7 bool QApplicationPrivate::notify_helper(QObject *receiver, QEvent * e)
8 bool QWidget::event(QEvent *event)
section7 == section2-1
1 QT_WIN_CALLBACK QtWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) 2 {3 ...4 //檢查message是否屬于Qt可轉義的鼠標事件5 if (qt_is_translatable_mouse_event(message)) {6 if (QApplication::activePopupWidget() != 0) { // in popup mode7 POINT curPos = msg.pt;8 //取得鼠標點擊坐標所在的QWidget指針，它指向我們在main創建的widget實例9 QWidget* w = QApplication::widgetAt(curPos.x, curPos.y);
10 if (w)
11 widget = (QETWidget*)w;
12 }
13
14 if (!qt_tabletChokeMouse) {
15 //對，就在這里。Windows的回調函數將鼠標事件分發回給了Qt Widget
16 // => Section 2-2
17 result = widget->translateMouseEvent(msg); // mouse event
18 ...
19 }
//?Section?2-2??$QTDIR\src\gui\kernel\qapplication_win.cpp??? ? //該函數所在與Windows平臺相關，主要職責就是把已windows格式打包的鼠標事件解包、翻譯成QApplication可識別的QMouseEvent,QWidget.
1 bool QETWidget::translateMouseEvent(const MSG &msg)
2 {
3 //.. 這里很長的代碼給以忽略
4 // 讓我們看一下sendMouseEvent的聲明
5 // widget是事件的接受者； e是封裝好的QMouseEvent
6 // ==> Section 2-3
7 res = QApplicationPrivate::sendMouseEvent(target, &e, alienWidget, this, &qt_button_down, qt_last_mouse_receiver);
8 }
//?Section?2-3?$QTDIR\src\gui\kernel\qapplication.cpp??
1 bool QApplicationPrivate::sendMouseEvent(QWidget *receiver, QMouseEvent *event,2 QWidget *alienWidget, QWidget *nativeWidget,3 QWidget **buttonDown, QPointer<QWidget> &lastMouseReceiver,4 bool spontaneous)5 {6 ...7 //至此與平臺相關代碼處理完畢8 //MouseEvent默認的發送方式是spontaneous, 所以將執行9 //sendSpontaneousEvent。 sendSpontaneousEvent() 與 sendEvent的代碼實現幾乎相同
10 //除了將QEvent的屬性spontaneous標記不同。 這里是解釋什么spontaneous事件：如果事件由應用程序之外產生的，比如一個系統事件。
11 //顯然MousePress事件是由視窗系統產生的一個的事件（詳見上文Section 1~ Section 7),因此它是 spontaneous事件
12 if (spontaneous)
13 result = QApplication::sendSpontaneousEvent(receiver, event);
14 else
15 result = QApplication::sendEvent(receiver, event);
16
17 ...
18
19 return result;
20 }
//?Section?2-4?C:\Qt\4.7.1-Vs\src\corelib\kernel\qcoreapplication.h
1 inline bool QCoreApplication::sendSpontaneousEvent(QObject *receiver, QEvent *event)
2 {
3 //將event標記為自發事件
4 //進一步調用 2-5 QCoreApplication::notifyInternal
5 if (event)
6 event->spont = true;
7 return self ? self->notifyInternal(receiver, event) : false;
8 }
//?Section?2-5:??$QTDIR\gui\kernel\qapplication.cpp?????
1 bool QCoreApplication::notifyInternal(QObject *receiver, QEvent *event)2 {3 // 幾行代碼對于Qt Jambi (QT Java綁定版本） 和QSA (QT Script for Application)的支持4 5 ...6 7 // 以下代碼主要意圖為Qt強制事件只能夠發送給當前線程里的對象，也就是說receiver->d_func()->threadData應該等于QThreadData::current()。8 //注意，跨線程的事件需要借助Event Loop來派發9 QObjectPrivate *d = receiver->d_func();
10 QThreadData *threadData = d->threadData;
11 ++threadData->loopLevel;
12
13 //哇，終于來到大名鼎鼎的函數QCoreApplication::nofity()了 ==> Section 2-6
14 QT_TRY {
15 returnValue = notify(receiver, event);
16 } QT_CATCH (...) {
17 --threadData->loopLevel;
18 QT_RETHROW;
19 }
20
21 ...
22
23 return returnValue;
24 }
//?Section?2-6:??$QTDIR\gui\kernel\qapplication.cpp??? ? //?QCoreApplication::notify和它的重載函數QApplication::notify在Qt的派發過程中起到核心的作用，Qt的官方文檔時這樣說的： //任何線程的任何對象的所有事件在發送時都會調用notify函數。
1 bool QCoreApplication::notify(QObject *receiver, QEvent *event)2 {3 Q_D(QCoreApplication);4 // no events are delivered after ~QCoreApplication() has started5 if (QCoreApplicationPrivate::is_app_closing)6 return true;7 8 if (receiver == 0) { // serious error9 qWarning("QCoreApplication::notify: Unexpected null receiver");
10 return true;
11 }
12
13 #ifndef QT_NO_DEBUG
14 d->checkReceiverThread(receiver);
15 #endif
16
17 return receiver->isWidgetType() ? false : d->notify_helper(receiver, event);
18 }
notify 調用 notify_helper()
1 bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)2 {3 // send to all application event filters4 if (sendThroughApplicationEventFilters(receiver, event))5 return true;6 // 向事件過濾器發送該事件，這里介紹一下Event Filters. 事件過濾器是一個接受即將發送給目標對象所有事件的對象。 7 //如代碼所示它開始處理事件在目標對象行動之前。過濾器的QObject::eventFilter（）實現被調用，能接受或者丟棄過濾8 //允許或者拒絕事件的更進一步的處理。如果所有的事件過濾器允許更進一步的事件處理，事件將被發送到目標對象本身。9 //如果他們中的一個停止處理，目標和任何后來的事件過濾器不能看到任何事件。
10 if (sendThroughObjectEventFilters(receiver, event))
11 return true;
12 // deliver the event
13 // 遞交事件給receiver => Section 2-8
14 return receiver->event(event);
15 }
//?Section?2-8??$QTDIR\gui\kernel\qwidget.cpp??
//?QApplication通過notify及其私有類notify_helper,將事件最終派發給了QObject的子類-?QWidget.
1 bool QWidget::event(QEvent *event)2 {3 ...4 5 switch (event->type()) {6 case QEvent::MouseMove:7 mouseMoveEvent((QMouseEvent*)event);8 break;9
10 case QEvent::MouseButtonPress:
11 // Don't reset input context here. Whether reset or not is
12 // a responsibility of input method. reset() will be
13 // called by mouseHandler() of input method if necessary
14 // via mousePressEvent() of text widgets.
15 #if 0
16 resetInputContext();
17 #endif
18 mousePressEvent((QMouseEvent*)event);
19 break;
20
21 ...
22
23 }
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