图片拖拽缩小放大

public class Second extends AppCompatActivity implements View.OnTouchListener{

    private ImageView imageView;

    private Matrix matrix = new Matrix();
    private Matrix savedMatrix = new Matrix();

    private static final int NONE = 0;
    private static final int DRAG = 1;
    private static final int ZOOM = 2;
    private int mode = NONE;

    // 第一个按下的手指的点
    private PointF startPoint = new PointF();
    // 两个按下的手指的触摸点的中点
    private PointF midPoint = new PointF();
    // 初始的两个手指按下的触摸点的距离
    private float oriDis = 1f;

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.second);
        DisplayImageOptions options = new DisplayImageOptions.Builder()
                .cacheInMemory(true)
                .cacheOnDisk(true)
                .build();

        imageView = (ImageView) findViewById(R.id.tupian);
        Intent intent = getIntent();
        String url = intent.getStringExtra("url");
        ImageLoader.getInstance().displayImage(url,imageView,options);
        imageView.setOnTouchListener(Second.this);
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        ImageView view = (ImageView) v;


        // 可以处理处理多点触摸
//        http://blog.youkuaiyun.com/fobdddf/article/details/19479153
//        http://blog.sina.com.cn/s/blog_60e96a410100mjd2.html
//        https://my.oschina.net/qianjia/blog/109282
        switch (event.getAction() & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN:
                // 第一个手指按下事件
                matrix.set(view.getImageMatrix());
                savedMatrix.set(matrix);
                startPoint.set(event.getX(), event.getY());
                //拖动
                mode = DRAG;
                break;
            case MotionEvent.ACTION_POINTER_DOWN:
//            event.getX(012)
                // 第二个手指按下事件
                oriDis = distance(event);
                if (oriDis > 10f) {
                    savedMatrix.set(matrix);
                    midPoint = middle(event);
                    //标记成缩放状态
                    mode = ZOOM;
                }
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_POINTER_UP:
                // 手指放开事件
                mode = NONE;
                break;
            case MotionEvent.ACTION_MOVE:
                // 手指滑动事件
                if (mode == DRAG) {
                    // 是一个手指拖动
                    matrix.set(savedMatrix);
                    //移动图片
                    matrix.postTranslate(event.getX() - startPoint.x, event.getY()
                            - startPoint.y);
                } else if (mode == ZOOM) {
                    // 两个手指滑动
                    float newDist = distance(event);
                    if (newDist > 10f) {
                        matrix.set(savedMatrix);
                        float scale = newDist / oriDis;
                        //图片的缩放
                        matrix.postScale(scale, scale, midPoint.x, midPoint.y);
                    }
                }
                break;
        }


        // 设置ImageView的Matrix
        view.setImageMatrix(matrix);
        return true;
    }


    // 计算两个触摸点之间的距离
    private float distance(MotionEvent event) {
        //获取第0跟手指和第一根手指的坐标
        float x = event.getX(0) - event.getX(1);
        float y = event.getY(0) - event.getY(1);
        return (float) Math.sqrt(x * x + y * y);
    }


    // 计算两个触摸点的中点
    private PointF middle(MotionEvent event) {
        float x = event.getX(0) + event.getX(1);
        float y = event.getY(0) + event.getY(1);
        return new PointF(x / 2, y / 2);
    }
}