android 拖拉和放大

public class MainActivity extends Activity {
    private ImageView imageView;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        imageView = (ImageView) this.findViewById(R.id.imageView);
        //为组件设置触摸监听器
        imageView.setOnTouchListener(new TouchListener());
    }

    private final class TouchListener implements OnTouchListener{
        private PointF startPoint = new PointF();
        private Matrix matrix = new Matrix();
        private Matrix currentMatrix = new Matrix();
        private int mode = 0;
        private static final int DRAG = 1;
        private static final int ZOOM = 2;
        private float startDis;//开始距离
        private PointF midPoint;//中间点

        public boolean onTouch(View v, MotionEvent event) {
            //event.getAction()返回的是一个int类型的数据，底8位代表事件， & MotionEvent.ACTION_MASK（255）返回底八位的数字
            switch (event.getAction() & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN://手指压下屏幕
               //设置模式为拖拉
                mode = DRAG;
                currentMatrix.set(imageView.getImageMatrix());//记录ImageView当前的移动位置
                startPoint.set(event.getX(), event.getY());
                break;

            case MotionEvent.ACTION_MOVE://手指在屏幕移动，该 事件会不断地触发
                if(mode == DRAG){
                    float dx = event.getX() - startPoint.x;//得到在x轴的移动距离
                    float dy = event.getY() - startPoint.y;//得到在y轴的移动距离
                    matrix.set(currentMatrix);//在没有进行移动之前的位置基础上进行移动
                    matrix.postTranslate(dx, dy);
                }else if(mode == ZOOM){//缩放
                    float endDis = distance(event);//结束距离
                    if(endDis > 10f){
                        float scale = endDis / startDis;//得到缩放倍数
                        matrix.set(currentMatrix);
                        matrix.postScale(scale, scale, midPoint.x, midPoint.y);
                    }
                }
                break;

            case MotionEvent.ACTION_UP://手指离开屏
            case MotionEvent.ACTION_POINTER_UP://有手指离开屏幕,但屏幕还有触点（手指）
                mode = 0;
                break;

            case MotionEvent.ACTION_POINTER_DOWN://当屏幕上还有触点（手指），再有一个手指压下屏幕
                mode = ZOOM;
                startDis = distance(event);
                if(startDis > 10f){
                    midPoint = mid(event);
                    currentMatrix.set(imageView.getImageMatrix());//记录ImageView当前的缩放倍数
                }
                break;
            }
            imageView.setImageMatrix(matrix);
            return true;
        }

    }
    /**
     * 计算两点之间的距离
     * @param event
     * @return
     */
    public static float distance(MotionEvent event) {
        float dx = event.getX(1) - event.getX(0);
        float dy = event.getY(1) - event.getY(0);
        return FloatMath.sqrt(dx*dx + dy*dy);
    }
    /**
     * 计算两点之间的中间点
     * @param event
     * @return
     */
    public static PointF mid(MotionEvent event){
        float midX = (event.getX(1) + event.getX(0)) / 2;
        float midY = (event.getY(1) + event.getY(0)) / 2;
        return new PointF(midX, midY);
    }
}