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 }