Qt中，事件被封装成一个个对象

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QWidget7         window.show();8         return app.exec(); // 进入Qpplication事件循环，见section 29 }

section2

1 int QApplication::exec()2 {3 #ifndef QT_NO_ACCESSIBILITY4     QAccessible::setRootObject(qApp);5 #endif    //简单的交给QCoreApplication来处理事件循环=〉section 36     return QCoreApplication::exec();7 }

section3

(!QCoreApplicationPrivate::checkInstance(          -     //得到当前Thread数据       QThreadData *threadData = self->d_func()->      (threadData !=         qWarning(, self->metaObject()->          -       (!threadData->         qWarning(          -       threadData->quitNow =       self->d_func()->in_exec =      self->d_func()->aboutToQuitEmitted =       returnCode =     threadData->quitNow =               self->d_func()->in_exec =           (!self->d_func()->             emit self->         self->d_func()->aboutToQuitEmitted =          sendPostedEvents(        }

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 loop20     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 #else28     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 below37         locker.relock();38         QEventLoop *eventLoop = d->threadData->eventLoops.pop();39         Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");40         Q_UNUSED(eventLoop); // --release warning41         d->inExec = false;42         --d->threadData->loopLevel;43 44         throw;45     }46 #endif47 48     // copied above49     locker.relock();50     QEventLoop *eventLoop = d->threadData->eventLoops.pop();51     Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");52     Q_UNUSED(eventLoop); // --release warning53     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 69 }

// 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 break108                 break;109             }110             retVal = true;111         }112 113         // still nothing - wait for message or signalled objects114         canWait = (!retVal115                    && !d->interrupt116                    && (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 events135         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 85          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 Widget16             // => Section 2-217             result = widget->translateMouseEvent(msg);        // mouse event18         ...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QMouseEvent6           // ==> 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     else15         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_DEBUG14     d->checkReceiverThread(receiver);15 #endif16 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 event13     // 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 is12         // a responsibility of input method. reset() will be13         // called by mouseHandler() of input method if necessary14         // via mousePressEvent() of text widgets.15 #if 016         resetInputContext();17 #endif18         mousePressEvent((QMouseEvent*)event);19         break;20 21         ...22 23 }