9Qt 事件处理机制
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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QWidget
7 window.show();
8 return app.exec(); // 进入Qpplication事件循环，见section 2
9 }
section2
1 int QApplication::exec()
2 {
3 #ifndef QT_NO_ACCESSIBILITY
4 QAccessible::setRootObject(qApp);
5 #endif //简单的交给QCoreApplication来处理事件循环=〉section 3
6 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 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); //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 loop
20 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 #else
28 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 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); //将事件派发给与平台相关的QAbstractEventDispatcher子类 =>Section 6
9 }
- // 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 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 8
5 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 Widget
16 // => Section 2-2
17 result = widget->translateMouseEvent(msg); // mouse event
18 ...
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 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 //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 = d->threadData;
11 ++threadData->loopLevel;
12
13 //哇，终于来到大名鼎鼎的函数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_DEBUG
14 d->checkReceiverThread(receiver);
15 #endif
16
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 event
13 // 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 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 }
转自：http://www.cnblogs.com/lfsblack/p/5338668.html
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