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9、Qt 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。
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
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 420 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 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 = http://www.mamicode.com/d->threadData;>- // 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 }转自：http://www.cnblogs.com/lfsblack/p/5338668.html
9、Qt 事件处理机制
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