Android ImageView缩放和平移

我是安卓系统的学生，我想得到一些帮助。我很难在ImageView上进行收缩缩放和拖动/平移。我做了一些研究，但我感到困惑。我可以将两个事件分开进行，但我尝试将它们一起使用，而收缩缩放不起作用。任何帮助我都会感激的。这是我的代码：

...     
private Matrix matrix = new Matrix();
private float scale = 1f;
private ScaleGestureDetector SGD;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    imageView =(ImageView)findViewById(R.id.imageView);
    SGD = new ScaleGestureDetector(this,new ScaleListener());

    imageView.setOnTouchListener(new View.OnTouchListener() {
        @Override
        public boolean onTouch(View v, MotionEvent event) {
            float currentX, currentY;

            switch (event.getAction()){
                case MotionEvent.ACTION_DOWN:
                    preX = event.getX();
                    preY = event.getY();
                    break;
                case MotionEvent.ACTION_MOVE:
                    currentX = event.getX();
                    currentY = event.getY();
                    imageView.scrollBy((int) (preX - currentX), (int) (preY - currentY));
                    preX = currentX;
                    preY = currentY;
                    break;
                case MotionEvent.ACTION_UP:
                    currentX = event.getX();
                    currentY = event.getY();
                    imageView.scrollBy((int) (preX - currentX), (int) (preY - currentY));
                    break;
            }
            return true;
        }
    });


}


public boolean onTouchEvent(MotionEvent ev) {
    SGD.onTouchEvent(ev);
    return true;
}


private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
    @Override
    public boolean onScale(ScaleGestureDetector detector) {
        scale *= detector.getScaleFactor();
        scale = Math.max(0.1f, Math.min(scale, 5.0f));

        matrix.setScale(scale, scale);
        imageView.setImageMatrix(matrix);

        return true;
    }
 }
}

也许你可以参考，也许你可以参考。