QT里关于eventFilter例子为何不能执行，请指教

用的qt4.8,qt creater2.4.1
源码如下:
#include <QMainWindow>

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);
}
}