研一的时候开始使用Qt，感觉用Qt开发图形界面比MFC的一套框架来方便的多。后来由于项目的需要，也没有再接触Qt了。现在要重新拾起来，于是要从基础学起。
Now，开始学习Qt事件处理机制。
先给出原文的链接：Qt 事件处理机制
因为这篇文章写得特别好，将Qt的事件处理机制能够阐述的清晰有条理，并且便于学习。于是就装载过来了（本文做了排版，并删减了一些冗余的东西，希望原主勿怪），以供学习之用。
简介
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。Qt是以事件驱动UI工具集。Signals/Slots在多线程中的实现也是依赖于Qt的事件处理机制。在Qt中，事件被封装成一个个对象，所有的事件都继承抽象基类QEvent。
Qt事件处理机制
产生事件：输入设备，键盘鼠标等。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。用户的程序还可以自定义事件。
事件的接受和处理者：QObject类使整个Qt对象模型的核心，事件处理机制是QObject三大职责（内存管理、内省(intropection)与事件处理机制）之一。任何一个想要接受并处理事件的对象必须继承QObject，可以选择重载QObject::event()函数或事件的处理权转交给父类。
事件的派送者：对于non-GUI的Qt程序，由QCoreApplication负责将QEvent分发给QObject的子类-Receiver；对于GUI程序，则由QApplication负责派送。
Qt源码分析
Qt利用event loop从事件队列中获取用户的输入事件，并将事件转义成QEvents，分发给相应的QObject处理，这中间共有七个阶段。如下分析：
section 1
#include <QApplication> #include "widget.h" int main(int argc, char *argv[]) { QApplication app(argc, argv); Widget window; // Widget 继承自QWidget window.show(); return app.exec(); // 进入Qpplication事件循环，见section 2 }
section 2
int QApplication::exec() { #ifndef QT_NO_ACCESSIBILITY QAccessible::setRootObject(qApp); #endif //简单的交给QCoreApplication来处理事件循环=〉section 3 return QCoreApplication::exec(); }
section 3
int QCoreApplication::exec() { if (!QCoreApplicationPrivate::checkInstance("exec")) return -1; //得到当前Thread数据 QThreadData *threadData = self->d_func()->threadData; if (threadData != QThreadData::current()) { qWarning("%s::exec: Must be called from the main thread", self->metaObject()->className()); return -1; } //检查event loop是否已经创建 if (!threadData->eventLoops.isEmpty()) { qWarning("QCoreApplication::exec: The event loop is already running"); return -1; } threadData->quitNow = false; QEventLoop eventLoop; self->d_func()->in_exec = true; self->d_func()->aboutToQuitEmitted = false; //委任QEventLoop 处理事件队列循环 ==> Section 4 int returnCode = eventLoop.exec(); threadData->quitNow = false; if (self) { self->d_func()->in_exec = false; if (!self->d_func()->aboutToQuitEmitted) emit self->aboutToQuit(); self->d_func()->aboutToQuitEmitted = true; sendPostedEvents(0, QEvent::DeferredDelete); } return returnCode; }
section 4
int QEventLoop::exec(ProcessEventsFlags flags) { Q_D(QEventLoop); //访问QEventloop私有类实例d //we need to protect from race condition with QThread::exit QMutexLocker locker(&static_cast<QThreadPrivate *>(QObjectPrivate::get(d->threadData->thread))->mutex); if (d->threadData->quitNow) return -1; if (d->inExec) { qWarning("QEventLoop::exec: instance %p has already called exec()", this); return -1; } d->inExec = true; d->exit = false; ++d->threadData->loopLevel; d->threadData->eventLoops.push(this); locker.unlock(); // remove posted quit events when entering a new event loop QCoreApplication *app = QCoreApplication::instance(); if (app && app->thread() == thread()) QCoreApplication::removePostedEvents(app, QEvent::Quit); //这里的实现代码不少，最为重要的是以下几行 #if defined(QT_NO_EXCEPTIONS) while (!d->exit) processEvents(flags | WaitForMoreEvents | EventLoopExec); #else try { while (!d->exit) //只要没有遇见exit，循环派发事件 processEvents(flags | WaitForMoreEvents | EventLoopExec); } catch (...) { qWarning("Qt has caught an exception thrown from an event handler. Throwing\n" "exceptions from an event handler is not supported in Qt. You must\n" "reimplement QApplication::notify() and catch all exceptions there.\n"); // copied from below locker.relock(); QEventLoop *eventLoop = d->threadData->eventLoops.pop(); Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error"); Q_UNUSED(eventLoop); // --release warning d->inExec = false; --d->threadData->loopLevel; throw; } #endif // copied above locker.relock(); QEventLoop *eventLoop = d->threadData->eventLoops.pop(); Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error"); Q_UNUSED(eventLoop); // --release warning d->inExec = false; --d->threadData->loopLevel; return d->returnCode; }
section 5
bool QEventLoop::processEvents(ProcessEventsFlags flags) { Q_D(QEventLoop); if (!d->threadData->eventDispatcher) return false; if (flags & DeferredDeletion) QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete); return d->threadData->eventDispatcher->processEvents(flags); //将事件派发给与平台相关的QAbstractEventDispatcher子类 =>Section 6 }
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 ，分别封装在QEventDispatcherSymbian和QEventDIspatcherUNIX。
QEventDispatcherWin32继承QAbstractEventDispatcher。
bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags) { Q_D(QEventDispatcherWin32); if (!d->internalHwnd) createInternalHwnd(); d->interrupt = false; emit awake(); bool canWait; bool retVal = false; bool seenWM_QT_SENDPOSTEDEVENTS = false; bool needWM_QT_SENDPOSTEDEVENTS = false; do { DWORD waitRet = 0; HANDLE pHandles[MAXIMUM_WAIT_OBJECTS - 1]; QVarLengthArray<MSG> processedTimers; while (!d->interrupt) { DWORD nCount = d->winEventNotifierList.count(); Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1); MSG msg; bool haveMessage; if (!(flags & QEventLoop::ExcludeUserInputEvents) && !d->queuedUserInputEvents.isEmpty()) { // process queued user input events haveMessage = true; 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 } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) { // process queued socket events haveMessage = true; 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 } else { haveMessage = PeekMessage(&msg, 0, 0, 0, PM_REMOVE); if (haveMessage && (flags & QEventLoop::ExcludeUserInputEvents) && ((msg.message >= WM_KEYFIRST && msg.message <= WM_KEYLAST) || (msg.message >= WM_MOUSEFIRST && msg.message <= WM_MOUSELAST) || msg.message == WM_MOUSEWHEEL || msg.message == WM_MOUSEHWHEEL || msg.message == WM_TOUCH #ifndef QT_NO_GESTURES || msg.message == WM_GESTURE || msg.message == WM_GESTURENOTIFY #endif || msg.message == WM_CLOSE)) { // queue user input events for later processing haveMessage = false; d->queuedUserInputEvents.append(msg); // 用户输入事件入队列，待以后处理 } if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers) && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) { // queue socket events for later processing haveMessage = false; d->queuedSocketEvents.append(msg); // socket 事件入队列，待以后处理 } } if (!haveMessage) { // no message - check for signalled objects for (int i=0; i<(int)nCount; i++) pHandles[i] = d->winEventNotifierList.at(i)->handle(); waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, 0, QS_ALLINPUT, MWMO_ALERTABLE); if ((haveMessage = (waitRet == WAIT_OBJECT_0 + nCount))) { // a new message has arrived, process it continue; } } if (haveMessage) { #ifdef Q_OS_WINCE // WinCE doesn't support hooks at all, so we have to call this by hand :( (void) qt_GetMessageHook(0, PM_REMOVE, (LPARAM) &msg); #endif if (d->internalHwnd == msg.hwnd && msg.message == WM_QT_SENDPOSTEDEVENTS) { if (seenWM_QT_SENDPOSTEDEVENTS) { // when calling processEvents() "manually", we only want to send posted // events once needWM_QT_SENDPOSTEDEVENTS = true; continue; } seenWM_QT_SENDPOSTEDEVENTS = true; } else if (msg.message == WM_TIMER) { // avoid live-lock by keeping track of the timers we've already sent bool found = false; for (int i = 0; !found && i < processedTimers.count(); ++i) { const MSG processed = processedTimers.constData()[i]; found = (processed.wParam == msg.wParam && processed.hwnd == msg.hwnd && processed.lParam == msg.lParam); } if (found) continue; processedTimers.append(msg); } else if (msg.message == WM_QUIT) { if (QCoreApplication::instance()) QCoreApplication::instance()->quit(); return false; } if (!filterEvent(&msg)) { TranslateMessage(&msg); //将事件打包成message调用Windows API派发出去 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 } } else if (waitRet < WAIT_OBJECT_0 + nCount) { d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0)); } else { // nothing todo so break break; } retVal = true; } // still nothing - wait for message or signalled objects canWait = (!retVal && !d->interrupt && (flags & QEventLoop::WaitForMoreEvents)); if (canWait) { DWORD nCount = d->winEventNotifierList.count(); Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1); for (int i=0; i<(int)nCount; i++) pHandles[i] = d->winEventNotifierList.at(i)->handle(); emit aboutToBlock(); waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, INFINITE, QS_ALLINPUT, MWMO_ALERTABLE | MWMO_INPUTAVAILABLE); emit awake(); if (waitRet < WAIT_OBJECT_0 + nCount) { d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0)); retVal = true; } } } while (canWait); if (!seenWM_QT_SENDPOSTEDEVENTS && (flags & QEventLoop::EventLoopExec) == 0) { // when called "manually", always send posted events QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData); } if (needWM_QT_SENDPOSTEDEVENTS) PostMessage(d->internalHwnd, WM_QT_SENDPOSTEDEVENTS, 0, 0); return retVal; }
section 7
windows窗口回调函数，定义在QTDIR\src\gui\kernel\qapplication_win.cpp
extern "C" LRESULT QT_WIN_CALLBACK QtWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) { ... //将消息重新封装成QEvent的子类QMouseEvent ==> Section 8 result = widget->translateMouseEvent(msg); ... }
section1~7的过程：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AbstractEventDispatcher的子类QEventDispatcherWin32获得用户的输入事件，并将其打包成message后，通过标准的Windows API传递给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。
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)
下面介绍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。
QT_WIN_CALLBACK QtWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) bool QETWidget::translateMouseEvent(const MSG &msg) bool QApplicationPrivate::sendMouseEvent(...) inline bool QCoreApplication::sendSpontaneousEvent(QObject *receiver, QEvent *event) bool QCoreApplication::notifyInternal(QObject *receiver, QEvent *event) bool QApplication::notify(QObject *receiver, QEvent *e) bool QApplicationPrivate::notify_helper(QObject *receiver, QEvent * e) bool QWidget::event(QEvent *event)
section 2-1 (section 8)
QT_WIN_CALLBACK QtWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) { ... //检查message是否属于Qt可转义的鼠标事件 if (qt_is_translatable_mouse_event(message)) { if (QApplication::activePopupWidget() != 0) { // in popup mode POINT curPos = msg.pt; //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 QWidget* w = QApplication::widgetAt(curPos.x, curPos.y); if (w) widget = (QETWidget*)w; } if (!qt_tabletChokeMouse) { //对，就在这里。Windows的回调函数将鼠标事件分发回给了Qt Widget // => Section 2-2 result = widget->translateMouseEvent(msg); // mouse event ... }
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。
bool QETWidget::translateMouseEvent(const MSG &msg) { //.. 这里很长的代码给以忽略 // 让我们看一下sendMouseEvent的声明 // widget是事件的接受者； e是封装好的QMouseEvent // ==> Section 2-3 res = QApplicationPrivate::sendMouseEvent(target, &e, alienWidget, this, &qt_button_down, qt_last_mouse_receiver); }
section 2-3 $QTDIR\src\gui\kernel\qapplication.cpp
bool QApplicationPrivate::sendMouseEvent(QWidget *receiver, QMouseEvent *event, QWidget *alienWidget, QWidget *nativeWidget, QWidget **buttonDown, QPointer<QWidget> &lastMouseReceiver, bool spontaneous) { ... //至此与平台相关代码处理完毕 //MouseEvent默认的发送方式是spontaneous, 所以将执行 //sendSpontaneousEvent。 sendSpontaneousEvent() 与 sendEvent的代码实现几乎相同 //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。 //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 if (spontaneous) result = QApplication::sendSpontaneousEvent(receiver, event); else result = QApplication::sendEvent(receiver, event); ... return result; }
section 2-4 C:\Qt\4.7.1-Vs\src\corelib\kernel\qcoreapplication.h
inline bool QCoreApplication::sendSpontaneousEvent(QObject *receiver, QEvent *event) { //将event标记为自发事件 //进一步调用 2-5 QCoreApplication::notifyInternal if (event) event->spont = true; return self ? self->notifyInternal(receiver, event) : false; }
section 2-5: $QTDIR\gui\kernel\qapplication.cpp
bool QCoreApplication::notifyInternal(QObject *receiver, QEvent *event) { // 几行代码对于Qt Jambi (QT Java绑定版本） 和QSA (QT Script for Application)的支持 ... // 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()。 //注意，跨线程的事件需要借助Event Loop来派发 QObjectPrivate *d = receiver->d_func(); QThreadData *threadData = d->threadData; ++threadData->loopLevel; //哇，终于来到大名鼎鼎的函数QCoreApplication::nofity()了 ==> Section 2-6 QT_TRY { returnValue = notify(receiver, event); } QT_CATCH (...) { --threadData->loopLevel; QT_RETHROW; } ... return returnValue; }
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。
bool QCoreApplication::notify(QObject *receiver, QEvent *event) { Q_D(QCoreApplication); // no events are delivered after ~QCoreApplication() has started if (QCoreApplicationPrivate::is_app_closing) return true; if (receiver == 0) { // serious error qWarning("QCoreApplication::notify: Unexpected null receiver"); return true; } #ifndef QT_NO_DEBUG d->checkReceiverThread(receiver); #endif return receiver->isWidgetType() ? false : d->notify_helper(receiver, event); }
section 2-7: $QTDIR\gui\kernel\qapplication.cpp
notify 调用 notify_helper()
bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event) { // send to all application event filters if (sendThroughApplicationEventFilters(receiver, event)) return true; // 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。 //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 //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。 //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。 if (sendThroughObjectEventFilters(receiver, event)) return true; // deliver the event // 递交事件给receiver => Section 2-8 return receiver->event(event); }
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.
bool QWidget::event(QEvent *event) { ... switch (event->type()) { case QEvent::MouseMove: mouseMoveEvent((QMouseEvent*)event); break; case QEvent::MouseButtonPress: // Don't reset input context here. Whether reset or not is // a responsibility of input method. reset() will be // called by mouseHandler() of input method if necessary // via mousePressEvent() of text widgets. #if 0 resetInputContext(); #endif mousePressEvent((QMouseEvent*)event); break; ... }
写代码是一种艺术，甚于蒙娜丽莎的微笑。
