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。
#include <QApplication> #include "widget.h" //Section 1 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() { //skip codes //简单的交给QCoreApplication来处理事件循环=〉section 3 return QCoreApplication::exec(); } // Section 3 int QCoreApplication::exec() { //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; } ... 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(); .... } return returnCode; } // Section 4 int QEventLoop::exec(ProcessEventsFlags flags) { //这里的实现代码不少，最为重要的是以下几行 Q_D(QEventLoop); // 访问QEventloop私有类实例d try { //只要没有遇见exit，循环派发事件 while (!d->exit) processEvents(flags | WaitForMoreEvents | EventLoopExec); } catch (...) {} } // Section 5 bool QEventLoop::processEvents(ProcessEventsFlags flags) { Q_D(QEventLoop); if (!d->threadData->eventDispatcher) return false; if (flags & DeferredDeletion) QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete); //将事件派发给与平台相关的QAbstractEventDispatcher子类 =>Section 6 return d->threadData->eventDispatcher->processEvents(flags); } #include <QApplication> #include "widget.h" //Section 1 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() { //skip codes //简单的交给QCoreApplication来处理事件循环=〉section 3 return QCoreApplication::exec(); } // Section 3 int QCoreApplication::exec() { //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; } ... 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(); .... } return returnCode; } // Section 4 int QEventLoop::exec(ProcessEventsFlags flags) { //这里的实现代码不少，最为重要的是以下几行 Q_D(QEventLoop); // 访问QEventloop私有类实例d try { //只要没有遇见exit，循环派发事件 while (!d->exit) processEvents(flags | WaitForMoreEvents | EventLoopExec); } catch (...) {} } // Section 5 bool QEventLoop::processEvents(ProcessEventsFlags flags) { Q_D(QEventLoop); if (!d->threadData->eventDispatcher) return false; if (flags & DeferredDeletion) QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete); //将事件派发给与平台相关的QAbstractEventDispatcher子类 =>Section 6 return d->threadData->eventDispatcher->processEvents(flags); } // 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. 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; //从处理用户输入队列中取出一条事件 msg = d->queuedUserInputEvents.takeFirst(); } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) { // 从处理socket队列中取出一条事件 haveMessage = true; msg = d->queuedSocketEvents.takeFirst(); } 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)) { // 用户输入事件入队列，待以后处理 haveMessage = false; d->queuedUserInputEvents.append(msg); } if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers) && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) { // socket 事件入队列，待以后处理 haveMessage = false; d->queuedSocketEvents.append(msg); } } .... if (!filterEvent(&msg)) { TranslateMessage(&msg); //将事件打包成message调用Windows API派发出去 //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 DispatchMessage(&msg); } } } } while (canWait); ... return retVal; } // 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. 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; //从处理用户输入队列中取出一条事件 msg = d->queuedUserInputEvents.takeFirst(); } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) { // 从处理socket队列中取出一条事件 haveMessage = true; msg = d->queuedSocketEvents.takeFirst(); } 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)) { // 用户输入事件入队列，待以后处理 haveMessage = false; d->queuedUserInputEvents.append(msg); } if (haveMessage && (flags & QEventLoop::ExcludeSocketNotifiers) && (msg.message == WM_QT_SOCKETNOTIFIER && msg.hwnd == d->internalHwnd)) { // socket 事件入队列，待以后处理 haveMessage = false; d->queuedSocketEvents.append(msg); } } .... if (!filterEvent(&msg)) { TranslateMessage(&msg); //将事件打包成message调用Windows API派发出去 //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 DispatchMessage(&msg); } } } } while (canWait); ... 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); ... } // 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); ... }
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。
小结：Qt 事件处理机制 （上篇）的内容介绍完了，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.请继续看Qt 事件处理机制 （下篇）。最后希望本文能帮你解决问题！