剖析Qt 事件的产生、分发、接受、处理流程
Windows上Qt事件处理机制详解：
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. 用户的程序还可以自己定制事件。
2、谁来接受和处理事件：答案是QObject。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是如何利用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
#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QApplication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，确保在同一个线程
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 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);
...
}
// (续上文Section 7) Section 2-1：
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) {
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);
...
}
// 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(widget, &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 &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); ==〉Section 2-4
else
result = QApplication::sendEvent(receiver, event);
}
// 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;
bool returnValue;
QT_TRY {
//哇，终于来到大名鼎鼎的函数QCoreApplication::nofity()了 ==> Section 2-6
returnValue = notify(receiver, event);
} QT_CATCH (...) {
--threadData->loopLevel;
QT_RETHROW;
}
}
// 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 QApplication::notify(QObject *receiver, QEvent *e)
{
//代码很长，最主要的是一个大大的Switch,Case
..
switch ( e->type())
{
...
case QEvent::MouseButtonPress:
case QEvent::MouseButtonRelease:
case QEvent::MouseButtonDblClick:
case QEvent::MouseMove:
...
//让自己私有类(d是私有类的句柄）来进一步处理 ==> Section 2-7
res = d->notify_helper(w, w == receiver ? mouse : &me);
e->spont = false;
break;
}
...
}
// Section 2-7: $QTDIR\gui\kernel\qapplication.cpp
bool QApplicationPrivate::notify_helper(QObject *receiver, QEvent * e)
{
...
// 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, e))
return true;
// 递交事件给receiver => Section 2-8
bool consumed = receiver->event(e);
e->spont = false;
}
// 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::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是虚函数，QWidget的子类可以通过重载重新定义mousePress事件的行为
mousePressEvent((QMouseEvent*)event);
break;
} - main(int, char **)
- QApplication::exec()
- QCoreApplication::exec()
- QEventLoop::exec(ProcessEventsFlags )
- QEventLoop::processEvents(ProcessEventsFlags )
- QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags)
- 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)