QT事件循环机制源码学习

文章目录

• QT源码中事件梳理过程中调用函数如下：
• • [1.app.exec(); 开启应用程序的事件循环](#1.app.exec(); 开启应用程序的事件循环)
  • [2. int QCoreApplication::exec()](#2. int QCoreApplication::exec())
  • 3.使用QEventLoop开启事件循环
  • [4.bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)](#4.bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags))
  • 5.以鼠标点击为例，windows是如何将鼠标点击的系统消息转化到Qt可识别的事件类型
  • [6.bool QCoreApplication::notifyInternal2(QObject *receiver, QEvent *event)](#6.bool QCoreApplication::notifyInternal2(QObject *receiver, QEvent *event))
  • [7.bool QCoreApplication::notify(QObject *receiver, QEvent *event)](#7.bool QCoreApplication::notify(QObject *receiver, QEvent *event))
  • [8.bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)](#8.bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event))
  • 9.事件派发至目标对象QObject的子类，这里以QWidget为例

QT源码中事件梳理过程中调用函数如下：

1.app.exec(); 开启应用程序的事件循环

    QApplication app(argc, argv);
    return app.exec();

2. int QCoreApplication::exec()

3.使用QEventLoop开启事件循环

    QEventLoop eventLoop;
    self->d_func()->in_exec = true;
    self->d_func()->aboutToQuitEmitted = false;
    int returnCode = eventLoop.exec();

4.bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)

• QEventDispatcherWin32 是 Qt 框架中事件调度器的一个具体实现，用于 Windows 平台。它是QAbstractEventDispatcher 的子类，负责管理 Qt 应用程序的事件循环在 Windows 上的运行。linux平台是类QEventDispatcherUNIX
• 负责 Qt 应用程序在 Windows 上的主事件循环
• 处理 Windows 消息与 Qt 事件的转换和分发
• 是 QApplication::exec() 在 Windows 上的底层实现
• 通过打断点也可以看到开启事件循环后程序会持续进入这个函数QEventDispatcherWin32::processEvents

class Q_CORE_EXPORT QEventDispatcherWin32 : public QAbstractEventDispatcher
{
    Q_OBJECT
    Q_DECLARE_PRIVATE(QEventDispatcherWin32)
    
    ......

5.以鼠标点击为例，windows是如何将鼠标点击的系统消息转化到Qt可识别的事件类型

1.extern "C" LRESULT QT_WIN_CALLBACK qWindowsWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)

2.bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
                                  QtWindows::WindowsEventType et,
                                  WPARAM wParam, LPARAM lParam,
                                  LRESULT *result,
                                  QWindowsWindow **platformWindowPtr)


// 将操作系统打包的事件解包、翻译为QApplication可识别的事件
3.bool QWindowsMouseHandler::translateMouseEvent(QWindow *window, HWND hwnd,
                                               QtWindows::WindowsEventType et,
                                               MSG msg, LRESULT *result)

4.QWindowSystemInterface::handleMouseEvent(window, clientPosition, globalPosition, buttons, m_lastEventButton,
                                                         QEvent::MouseButtonRelease, keyModifiers, source);
                                                         
5.void QWidgetWindow::handleMouseEvent(QMouseEvent *event)

6.bool QApplicationPrivate::sendMouseEvent(QWidget *receiver, QMouseEvent *event,
                                         QWidget *alienWidget, QWidget *nativeWidget,
                                         QWidget **buttonDown, QPointer<QWidget> &lastMouseReceiver,
                                         bool spontaneous, bool onlyDispatchEnterLeave)
// 系统鼠标、键盘消息一般都是此接口  
7.QApplication::sendSpontaneousEvent(receiver, event);

8.bool QCoreApplication::sendSpontaneousEvent(QObject *receiver, QEvent *event)
{
    Q_TRACE(QCoreApplication_sendSpontaneousEvent, receiver, event, event->type());

    if (event)
        event->spont = true;
    return notifyInternal2(receiver, event);
}

6.bool QCoreApplication::notifyInternal2(QObject *receiver, QEvent *event)

7.bool QCoreApplication::notify(QObject *receiver, QEvent *event)

8.bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)

• 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。

bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)
{
    // Note: when adjusting the tracepoints in here
    // consider adjusting QApplicationPrivate::notify_helper too.
    Q_TRACE(QCoreApplication_notify_entry, receiver, event, event->type());
    bool consumed = false;
    bool filtered = false;
    Q_TRACE_EXIT(QCoreApplication_notify_exit, consumed, filtered);

    // send to all application event filters (only does anything in the main thread)
    if (QCoreApplication::self
            && receiver->d_func()->threadData.loadRelaxed()->thread.loadAcquire() == mainThread()
            && QCoreApplication::self->d_func()->sendThroughApplicationEventFilters(receiver, event)) {
        filtered = true;
        return filtered;
    }
    // 这个函数中会访问receiver(目标对象)的过滤器，判断是否需要过滤此事件，return true就是过滤掉 false则是不过滤会真正传递给receiver(目标对象)
    if (sendThroughObjectEventFilters(receiver, event)) {
        filtered = true;
        return filtered;
    }

 // receiver(目标对象)的事件处理函数被真正得到执行
    consumed = receiver->event(event);
    return consumed;
}

9.事件派发至目标对象QObject的子类，这里以QWidget为例

bool QWidget::event(QEvent *event)
{
    ...
 
    switch (event->type()) {
    case QEvent::MouseMove:
        mouseMoveEvent((QMouseEvent*)event);
        break;
 
    case QEvent::MouseButtonPress:
 #if 0
        resetInputContext();
 #endif
        mousePressEvent((QMouseEvent*)event);
        break;
        ...
}