Qt 事件处理机制 （上篇）

　　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，QTDIRsrccorelibkernelqeventdispatcher_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，QTDIRsrccorelibkernelqeventdispatcher_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QTDIRsrcguikernelqapplication_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QTDIRsrcguikernelqapplication_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！