Android Studio如何在SurfaceView中实现OnFling

Question

我正在尝试通过实现 GestureDetector.OnGestureListener

来使用onFling

我有此代码：

GestureDetector gDetector;
gDetector = new GestureDetector( context, this );

 @Override
    public boolean onDown(MotionEvent arg0) {
        return true;
    }
    @Override
    public boolean onFling(MotionEvent start, MotionEvent finish, float xVelocity, float yVelocity) {
        if (start.getRawY() < finish.getRawY()) {
            Log.d( TAG, "Swiped down!" );
        } else {
            Log.d( TAG, "Swiped up!" );
        }
        return true;
    }
    @Override
    public void onLongPress(MotionEvent arg0) {
    }
    @Override
    public boolean onScroll(MotionEvent arg0, MotionEvent arg1, float arg2, float arg3) {
        return false;
    }
    @Override
    public void onShowPress(MotionEvent arg0) {
    }
    @Override
    public boolean onSingleTapUp(MotionEvent arg0) {
        Log.d( TAG, "Just tapped at " + arg0.getRawX() + "," + arg0.getRawY() + "!" );
        return false;
    }

    @Override
    public boolean onTouchEvent(MotionEvent motionEvent) {
        return gDetector.onTouchEvent(motionEvent);

不幸的是，从未击中过onFling，但是却击中了onTouchEvent。

我的问题很简单：为什么？

谢谢您的帮助！