Qt 事件處理機制

時間 2019-11-08

標籤事件處理機制简体版

原文原文鏈接

原文地址：http://mobile.51cto.com/symbian-272812.htmc++

在Qt中，事件被封裝成一個個對象，全部的事件均繼承自抽象類QEvent. 接下來依次談談Qt中有誰來產生、分發、接受和處理事件。windows

本篇來介紹Qt 事件處理機制。深刻了解事件處理系統對於每一個學習Qt人來講很是重要，能夠說，Qt是以事件驅動的UI工具集。你們熟知Signals/Slots在多線程的實現也依賴於Qt的事件處理機制多線程

在Qt中，事件被封裝成一個個對象，全部的事件均繼承自抽象類QEvent. 接下來依次談談Qt中有誰來產生、分發、接受和處理事件：app

一、誰來產生事件：最容易想到的是咱們的輸入設備，好比鍵盤、鼠標產生的框架

keyPressEvent，keyReleaseEvent，mousePressEvent，mouseReleaseEvent事件（他們被封裝成QMouseEvent和QKeyEvent），這些事件來自於底層的操做系統，它們以異步的形式通知Qt事件處理系統，後文會仔細道來。固然Qt本身也會產生不少事件，好比QObject::startTimer()會觸發QTimerEvent.
用戶的程序可還以本身定製事件異步

二、誰來接受和處理事件：答案是QObject。在Qt的內省機制剖析一文已經介紹QObject
類是整個Qt對象模型的心臟，事件處理機制是QObject三大職責（內存管理、內省(intropection)與事件處理制）之一。任何一個想要接受並處理事件的對象均須繼承自QObject，能夠選擇重載QObject::event()函數或事件的處理權轉給父類。socket

三、誰來負責分發事件：對於non-GUI的Qt程序，是由QCoreApplication負責將QEvent分發給QObject的子類-Receiver.
對於Qt GUI程序，由QApplication來負責函數

接下來，將經過對代碼的解析來看看QT是利用event loop從事件隊列中獲取用戶輸入事件，又是如何將事件轉義成QEvents，並分發給相應的QObject處理。工具

section1oop

1 #include <QApplication>     
2 #include "widget.h"    
3 int main(int argc, char *argv[])     
4 {         
5         QApplication app(argc, argv); 
6         Widget window;  // Widget 繼承自QWidget
7         window.show();
8         return app.exec(); // 進入Qpplication事件循環，見section 2
9 }

section2

1 int QApplication::exec()
2 {
3 #ifndef QT_NO_ACCESSIBILITY
4     QAccessible::setRootObject(qApp);
5 #endif    //簡單的交給QCoreApplication來處理事件循環=〉section 3
6     return QCoreApplication::exec();
7 }

section3

 1 int QCoreApplication::exec()
 2 {
 3     if (!QCoreApplicationPrivate::checkInstance("exec"))
 4         return -1;
 5     //獲得當前Thread數據  
 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     }
       //檢查event loop是否已經建立 
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 處理事件隊列循環 ==> Section 4
20     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);  //訪問QEventloop私有類實例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 loop
20     QCoreApplication *app = QCoreApplication::instance();
21     if (app && app->thread() == thread())
22         QCoreApplication::removePostedEvents(app, QEvent::Quit);
23     //這裏的實現代碼很多，最爲重要的是如下幾行 
24 #if defined(QT_NO_EXCEPTIONS)
25     while (!d->exit)
26         processEvents(flags | WaitForMoreEvents | EventLoopExec);
27 #else
28     try {
29         while (!d->exit)  //只要沒有碰見exit，循環派發事件 
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 below
37         locker.relock();
38         QEventLoop *eventLoop = d->threadData->eventLoops.pop();
39         Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");
40         Q_UNUSED(eventLoop); // --release warning
41         d->inExec = false;
42         --d->threadData->loopLevel;
43 
44         throw;
45     }
46 #endif
47 
48     // copied above
49     locker.relock();
50     QEventLoop *eventLoop = d->threadData->eventLoops.pop();
51     Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");
52     Q_UNUSED(eventLoop); // --release warning
53     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);  //將事件派發給與平臺相關的QAbstractEventDispatcher子類 =>Section 6
9 }

// Section 6，QTDIR\src\corelib\kernel\qeventdispatcher_win.cpp
// 這段代碼是完成與windows平臺相關的windows c++。以跨平臺著稱的Qt同時也提供了對Symiban,Unix等平臺的消息派發支持
// 其事現分別封裝在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(); //從處理用戶輸入隊列中取出一條事件，處理隊列裏面的用戶輸入事件
 30             } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {
 31                 // process queued socket events
 32                 haveMessage = true;
 33                 msg = d->queuedSocketEvents.takeFirst();  // 從處理socket隊列中取出一條事件,處理隊列裏面的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);  // 用戶輸入事件入隊列，待之後處理 
 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 事件入隊列，待之後處理   
 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);   //將事件打包成message調用Windows API派發出去
102                     DispatchMessage(&msg);    //分發一個消息給窗口程序。消息被分發到回調函數，將消息傳遞給windows系統，windows處理完畢，會調用回調函數 => 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 break
108                 break;
109             }
110             retVal = true;
111         }
112 
113         // still nothing - wait for message or signalled objects
114         canWait = (!retVal
115                    && !d->interrupt
116                    && (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 events
135         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窗口回調函數定義在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         //將消息從新封裝成QEvent的子類QMouseEvent ==> Section 8
5          result = widget->translateMouseEvent(msg);
6          ...     
7 }

從Section 1~Section7， Qt進入QApplication的event loop，通過層層委任，最終QEventloop的processEvent將經過與平臺相關的QAbstractEventDispatcher的子類QEventDispatcherWin32得到用戶的用戶輸入事件，並將其打包成message後，經過標準Windows API ，把消息傳遞給了Windows OS，Windows OS獲得通知後回調QtWndProc, 至此事件的分發與處理完成了一半的路程。

在下文中，咱們將進一步討論當咱們收到來在Windows的回調後，事件又是怎麼一步步打包成QEvent並經過QApplication分發給最終事件的接受和處理者QObject::event

事件的產生、分發、接受和處理，並以視窗系統鼠標點擊QWidget爲例，對代碼進行了剖析，向你們分析了Qt框架如何經過Event
Loop處理進入處理消息隊列循環，如何一步一步委派給平臺相關的函數獲取、打包用戶輸入事件交給視窗系統處理，函數調用棧以下：

1 main(int, char **)   
2 QApplication::exec()   
3 QCoreApplication::exec()   
4 QEventLoop::exec(ProcessEventsFlags )   
5 QEventLoop::processEvents(ProcessEventsFlags )
6 QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags)

本文將介紹Qt app在視窗系統回調後，事件又是怎麼一步步經過QApplication分發給最終事件的接受和處理者QWidget::event, （QWidget繼承Object,重載其虛函數event)，如下全部的討論都將嵌入在源碼之中。

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     //檢查message是否屬於Qt可轉義的鼠標事件
 5     if (qt_is_translatable_mouse_event(message)) {
 6         if (QApplication::activePopupWidget() != 0) { // in popup mode
 7             POINT curPos = msg.pt;
 8             //取得鼠標點擊座標所在的QWidget指針，它指向咱們在main建立的widget實例
 9             QWidget* w = QApplication::widgetAt(curPos.x, curPos.y);
10             if (w)
11                 widget = (QETWidget*)w;
12         }
13 
14         if (!qt_tabletChokeMouse) {
15             //對，就在這裏。Windows的回調函數將鼠標事件分發回給了Qt Widget
16             // => Section 2-2
17             result = widget->translateMouseEvent(msg);        // mouse event
18         ...
19 }

// Section 2-2 $QTDIR\src\gui\kernel\qapplication_win.cpp
//該函數所在與Windows平臺相關，主要職責就是把已windows格式打包的鼠標事件解包、翻譯成QApplication可識別的QMouseEvent,QWidget.

1 bool QETWidget::translateMouseEvent(const MSG &msg)     
2 {
3           //.. 這裏很長的代碼給以忽略    
4           // 讓咱們看一下sendMouseEvent的聲明
5           // widget是事件的接受者； e是封裝好的QMouseEvent
6           // ==> 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     //至此與平臺相關代碼處理完畢
 8     //MouseEvent默認的發送方式是spontaneous, 因此將執行
 9     //sendSpontaneousEvent。 sendSpontaneousEvent() 與 sendEvent的代碼實現幾乎相同
10     //除了將QEvent的屬性spontaneous標記不一樣。 這裏是解釋什麼spontaneous事件：若是事件由應用程序以外產生的，好比一個系統事件。
11      //顯然MousePress事件是由視窗系統產生的一個的事件（詳見上文Section 1~ Section 7),所以它是   spontaneous事件 
12     if (spontaneous)
13         result = QApplication::sendSpontaneousEvent(receiver, event);
14     else
15         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       //將event標記爲自發事件
4      //進一步調用 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     // 幾行代碼對於Qt Jambi (QT Java綁定版本） 和QSA (QT Script for Application)的支持
 4     
 5     ...
 6     
 7     // 如下代碼主要意圖爲Qt強制事件只可以發送給當前線程裏的對象，也就是說receiver->d_func()->threadData應該等於QThreadData::current()。
 8     //注意，跨線程的事件須要藉助Event Loop來派發
 9     QObjectPrivate *d = receiver->d_func();
10     QThreadData *threadData = d->threadData;
11     ++threadData->loopLevel;
12 
13     //哇，終於來到大名鼎鼎的函數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和它的重載函數QApplication::notify在Qt的派發過程當中起到核心的做用，Qt的官方文檔時這樣說的：
//任何線程的任何對象的全部事件在發送時都會調用notify函數。

 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_DEBUG
14     d->checkReceiverThread(receiver);
15 #endif
16 
17     return receiver->isWidgetType() ? false : d->notify_helper(receiver, event);
18 }

notify 調用 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      // 向事件過濾器發送該事件，這裏介紹一下Event Filters. 事件過濾器是一個接受即將發送給目標對象全部事件的對象。 
 7     //如代碼所示它開始處理事件在目標對象行動以前。過濾器的QObject::eventFilter（）實現被調用，能接受或者丟棄過濾
 8     //容許或者拒絕事件的更進一步的處理。若是全部的事件過濾器容許更進一步的事件處理，事件將被髮送到目標對象自己。
 9     //若是他們中的一箇中止處理，目標和任何後來的事件過濾器不能看到任何事件。
10     if (sendThroughObjectEventFilters(receiver, event))
11         return true;
12     // deliver the event
13     // 遞交事件給receiver  => Section 2-8 
14     return receiver->event(event);
15 }

// Section 2-8 $QTDIR\gui\kernel\qwidget.cpp

// QApplication經過notify及其私有類notify_helper,將事件最終派發給了QObject的子類- 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 is
12         // a responsibility of input method. reset() will be
13         // called by mouseHandler() of input method if necessary
14         // via mousePressEvent() of text widgets.
15 #if 0
16         resetInputContext();
17 #endif
18         mousePressEvent((QMouseEvent*)event);
19         break;
20 
21         ...
22 
23 }

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