9、Qt 事件处理机制

原文地址：http://mobile.51cto.com/symbian-272812.htm

在Qt中，事件被封装成一个个对象，所有的事件均继承自抽象类QEvent. 接下来依次谈谈Qt中有谁来产生、分发、接受和处理事件。

本篇来介绍Qt 事件处理机制。深入了解事件处理系统对于每个学习Qt人来说非常重要，可以说，Qt是以事件驱动的UI工具集。大家熟知Signals/Slots在多线程的实现也依赖于Qt的事件处理机制

在Qt中，事件被封装成一个个对象，所有的事件均继承自抽象类QEvent. 接下来依次谈谈Qt中有谁来产生、分发、接受和处理事件：

1、谁来产生事件：最容易想到的是我们的输入设备，比如键盘、鼠标产生的

keyPressEvent，keyReleaseEvent，mousePressEvent，mouseReleaseEvent事件（他们被封装成QMouseEvent和QKeyEvent），这些事件来自于底层的操作系统，它们以异步的形式通知Qt事件处理系统，后文会仔细道来。当然Qt自己也会产生很多事件，比如QObject::startTimer()会触发QTimerEvent.
用户的程序可还以自己定制事件

2、谁来接受和处理事件：答案是QObject。在Qt的内省机制剖析一文已经介绍QObject
类是整个Qt对象模型的心脏，事件处理机制是QObject三大职责（内存管理、内省(intropection)与事件处理制）之一。任何一个想要接受并处理事件的对象均须继承自QObject，可以选择重载QObject::event()函数或事件的处理权转给父类。

3、谁来负责分发事件：对于non-GUI的Qt程序，是由QCoreApplication负责将QEvent分发给QObject的子类-Receiver.
对于Qt GUI程序，由QApplication来负责

接下来，将通过对代码的解析来看看QT是利用event loop从事件队列中获取用户输入事件，又是如何将事件转义成QEvents，并分发给相应的QObject处理。

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 继承自QWidget

7         window.show();

8         return app.exec(); // 进入Qpplication事件循环，见section 2

9 }

section2

1 int QApplication::exec()

2 {

3 #ifndef QT_NO_ACCESSIBILITY

4     QAccessible::setRootObject(qApp);

5 #endif    //简单的交给QCoreApplication来处理事件循环=〉section 3

6     return QCoreApplication::exec();

7 }

section3

 1 int QCoreApplication::exec()

 2 {

 3     if (!QCoreApplicationPrivate::checkInstance("exec"))

 4         return -1;

 5     //得到当前Thread数据  

 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     }
       //检查event loop是否已经创建 

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 处理事件队列循环 ==> 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 }

section4

 1 int QEventLoop::exec(ProcessEventsFlags flags)

 2 {

 3     Q_D(QEventLoop);  //访问QEventloop私有类实例d

 4     //we need to protect from race condition with QThread::exit

 5     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     //这里的实现代码不少，最为重要的是以下几行 

24 #if defined(QT_NO_EXCEPTIONS)

25     while (!d->exit)

26         processEvents(flags | WaitForMoreEvents | EventLoopExec);

27 #else

28     try {

29         while (!d->exit)  //只要没有遇见exit，循环派发事件 

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);  //将事件派发给与平台相关的QAbstractEventDispatcher子类 =>Section 6

9 }

// Section 6，QTDIR\src\corelib\kernel\qeventdispatcher_win.cpp
// 这段代码是完成与windows平台相关的windows c++。以跨平台著称的Qt同时也提供了对Symiban,Unix等平台的消息派发支持
// 其事现分别封装在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 events

 28                 haveMessage = true;

 29                 msg = d->queuedUserInputEvents.takeFirst(); //从处理用户输入队列中取出一条事件，处理队列里面的用户输入事件

 30             } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {

 31                 // process queued socket events

 32                 haveMessage = true;

 33                 msg = d->queuedSocketEvents.takeFirst();  // 从处理socket队列中取出一条事件,处理队列里面的socket事件

 34             } else {

 35                 haveMessage = PeekMessage(&msg, 0, 0, 0, PM_REMOVE);

 36                 if (haveMessage && (flags & QEventLoop::ExcludeUserInputEvents)

 37                     && ((msg.message >= WM_KEYFIRST

 38                          && msg.message <= WM_KEYLAST)

 39                         || (msg.message >= WM_MOUSEFIRST

 40                             && msg.message <= WM_MOUSELAST)

 41                         || msg.message == WM_MOUSEWHEEL

 42                         || msg.message == WM_MOUSEHWHEEL

 43                         || msg.message == WM_TOUCH

 44 #ifndef QT_NO_GESTURES

 45                         || msg.message == WM_GESTURE

 46                         || msg.message == WM_GESTURENOTIFY

 47 #endif

 48                         || msg.message == WM_CLOSE)) {

 49                     // queue user input events for later processing

 50                     haveMessage = false;

 51                     d->queuedUserInputEvents.append(msg);  // 用户输入事件入队列，待以后处理 

 52                 }

 53                 if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers)

 54                     && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) {

 55                     // queue socket events for later processing

 56                     haveMessage = false;

 57                     d->queuedSocketEvents.append(msg);     // socket 事件入队列，待以后处理   

 58                 }

 59             }

 60             if (!haveMessage) {

 61                 // no message - check for signalled objects

 62                 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 it

 67                     continue;

 68                 }

 69             }

 70             if (haveMessage) {

 71 #ifdef Q_OS_WINCE

 72                 // 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 #endif

 75

 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 posted

 79                         // events once

 80                         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 sent

 86                     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);   //将事件打包成message调用Windows API派发出去

102                     DispatchMessage(&msg);    //分发一个消息给窗口程序。消息被分发到回调函数，将消息传递给windows系统，windows处理完毕，会调用回调函数 => 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窗口回调函数定义在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         //将消息重新封装成QEvent的子类QMouseEvent ==> Section 8

5          result = widget->translateMouseEvent(msg);

6          ...

7 }

从Section 1~Section7， Qt进入QApplication的event loop，经过层层委任，最终QEventloop的processEvent将通过与平台相关的QAbstractEventDispatcher的子类QEventDispatcherWin32获得用户的用户输入事件，并将其打包成message后，通过标准Windows API ，把消息传递给了Windows OS，Windows OS得到通知后回调QtWndProc, 至此事件的分发与处理完成了一半的路程。

在下文中，我们将进一步讨论当我们收到来在Windows的回调后，事件又是怎么一步步打包成QEvent并通过QApplication分发给最终事件的接受和处理者QObject::event

事件的产生、分发、接受和处理，并以视窗系统鼠标点击QWidget为例，对代码进行了剖析，向大家分析了Qt框架如何通过Event
Loop处理进入处理消息队列循环，如何一步一步委派给平台相关的函数获取、打包用户输入事件交给视窗系统处理，函数调用栈如下：

1 main(int, char **)

2 QApplication::exec()

3 QCoreApplication::exec()

4 QEventLoop::exec(ProcessEventsFlags )

5 QEventLoop::processEvents(ProcessEventsFlags )

6 QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags)

本文将介绍Qt app在视窗系统回调后，事件又是怎么一步步通过QApplication分发给最终事件的接受和处理者QWidget::event, （QWidget继承Object,重载其虚函数event)，以下所有的讨论都将嵌入在源码之中。

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     //检查message是否属于Qt可转义的鼠标事件

 5     if (qt_is_translatable_mouse_event(message)) {

 6         if (QApplication::activePopupWidget() != 0) { // in popup mode

 7             POINT curPos = msg.pt;

 8             //取得鼠标点击坐标所在的QWidget指针，它指向我们在main创建的widget实例

 9             QWidget* w = QApplication::widgetAt(curPos.x, curPos.y);

10             if (w)

11                 widget = (QETWidget*)w;

12         }

13

14         if (!qt_tabletChokeMouse) {

15             //对，就在这里。Windows的回调函数将鼠标事件分发回给了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
//该函数所在与Windows平台相关，主要职责就是把已windows格式打包的鼠标事件解包、翻译成QApplication可识别的QMouseEvent,QWidget.

1 bool QETWidget::translateMouseEvent(const MSG &msg)

2 {

3           //.. 这里很长的代码给以忽略

4           // 让我们看一下sendMouseEvent的声明

5           // widget是事件的接受者； e是封装好的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     //至此与平台相关代码处理完毕

 8     //MouseEvent默认的发送方式是spontaneous, 所以将执行

 9     //sendSpontaneousEvent。 sendSpontaneousEvent() 与 sendEvent的代码实现几乎相同

10     //除了将QEvent的属性spontaneous标记不同。 这里是解释什么spontaneous事件：如果事件由应用程序之外产生的，比如一个系统事件。

11      //显然MousePress事件是由视窗系统产生的一个的事件（详见上文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       //将event标记为自发事件

4      //进一步调用 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     // 几行代码对于Qt Jambi (QT Java绑定版本） 和QSA (QT Script for Application)的支持

 4

 5     ...

 6

 7     // 以下代码主要意图为Qt强制事件只能够发送给当前线程里的对象，也就是说receiver->d_func()->threadData应该等于QThreadData::current()。

 8     //注意，跨线程的事件需要借助Event Loop来派发

 9     QObjectPrivate *d = receiver->d_func();

10     QThreadData *threadData = d->threadData;

11     ++threadData->loopLevel;

12

13     //哇，终于来到大名鼎鼎的函数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和它的重载函数QApplication::notify在Qt的派发过程中起到核心的作用，Qt的官方文档时这样说的：
//任何线程的任何对象的所有事件在发送时都会调用notify函数。

 1 bool QCoreApplication::notify(QObject *receiver, QEvent *event)

 2 {

 3     Q_D(QCoreApplication);

 4     // no events are delivered after ~QCoreApplication() has started

 5     if (QCoreApplicationPrivate::is_app_closing)

 6         return true;

 7

 8     if (receiver == 0) {                        // serious error

 9         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 调用 notify_helper()
// Section 2-7: $QTDIR\gui\kernel\qapplication.cpp

 1 bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)

 2 {

 3     // send to all application event filters

 4     if (sendThroughApplicationEventFilters(receiver, event))

 5         return true;

 6      // 向事件过滤器发送该事件，这里介绍一下Event Filters. 事件过滤器是一个接受即将发送给目标对象所有事件的对象。

 7     //如代码所示它开始处理事件在目标对象行动之前。过滤器的QObject::eventFilter（）实现被调用，能接受或者丢弃过滤

 8     //允许或者拒绝事件的更进一步的处理。如果所有的事件过滤器允许更进一步的事件处理，事件将被发送到目标对象本身。

 9     //如果他们中的一个停止处理，目标和任何后来的事件过滤器不能看到任何事件。

10     if (sendThroughObjectEventFilters(receiver, event))

11         return true;

12     // deliver the event

13     // 递交事件给receiver  => Section 2-8

14     return receiver->event(event);

15 }

// Section 2-8 $QTDIR\gui\kernel\qwidget.cpp

// QApplication通过notify及其私有类notify_helper,将事件最终派发给了QObject的子类- 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 }

转自：http://www.cnblogs.com/lfsblack/p/5338668.html

巴特西

9、Qt 事件处理机制

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