如何在缩放后拖动图像？

Question

我遵循了一个关于如何缩放图像的简单教程。问题是我无法在缩放后拖动和缩放图像。我确实阅读了android Dragging和scalling并尝试了其他答案，但它不起作用。下面是放大图像的代码。有人可以在这里指导我吗？

public class MainActivity extends AppCompatActivity{

    ImageView imageView;
    Matrix matrix=new Matrix();
    Float scale=1f;
    ScaleGestureDetector SGD;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);


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

    }

    private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener{
        @Override
        public boolean onScale(ScaleGestureDetector detector) {
           scale= scale*detector.getScaleFactor();
            scale= Math.max(0.1f,Math.min(scale,3f));
            matrix.setScale(scale, scale);
            imageView.setImageMatrix(matrix);
            return true;
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        SGD.onTouchEvent(event);
                return true;
    }