Qt eventFilter無法過濾mouse事件

最近做了個測試發現Qt 的eventFilter無法過濾mouse事件，如mouseMove, mousePress, mouseRelease等，
原來是installEventFilter(QObject *)安裝不對。看Qt例子：

class MainWindow : public QMainWindow
 {
 public:
     MainWindow();

 protected:
     bool eventFilter(QObject *obj, QEvent *ev);

 private:
     QTextEdit *textEdit;
 };

 MainWindow::MainWindow()
 {
     textEdit = new QTextEdit;
     setCentralWidget(textEdit);

     textEdit->installEventFilter(this);
 }

 bool MainWindow::eventFilter(QObject *obj, QEvent *event)
 {
     if (obj == textEdit) {
         if (event->type() == QEvent::KeyPress) {
             QKeyEvent *keyEvent = static_cast<QKeyEvent*>(event);
             qDebug() << "Ate key press" << keyEvent->key();
             return true;
         } else {
             return false;
         }
     } else {
         // pass the event on to the parent class
         return QMainWindow::eventFilter(obj, event);
     }
 }

這裏eventFilter接受的是QEvent::KeyPress事件，打印event->type發現始終獲得不到mouse事件，
原來因爲QTextEdit是由QAbstractScollArea派生的，對於QAbstractScollArea，其過濾器要安裝在viewport上，
因此上面程序變換如下即可過濾mouse event：

class MainWindow : public QMainWindow
 {
 public:
     MainWindow();

 protected:
     bool eventFilter(QObject *obj, QEvent *ev);

 private:
     QTextEdit *textEdit;
 };

 MainWindow::MainWindow()
 {
     textEdit = new QTextEdit;
     setCentralWidget(textEdit);

     textEdit->installEventFilter(this);
     textEdit->viewPort->installEventFilter(this);
 } 
bool MainWindow::eventFilter(QObject *obj, QEvent *event) { if (obj == textEdit) 
 {
         if (event->type() == QEvent::KeyPress) 
         {
             QKeyEvent *keyEvent = static_cast<QKeyEvent*>(event);
             qDebug() << "Ate key press" << keyEvent->key();
             return true;
         }
        else 
        {
            return false;
        }
     }
     else if (obj == textEdit->viewPort())
     {
          // To do.....
         if (event->type() == QEvent::mouseMove) 
         {
              return true;
         }
     } 
    else 
    {
        // pass the event on to the parent class
        return QMainWindow::eventFilter(obj, event);
    }
 }