如何在Android界面中实现一个回弹效果

public class MyScrollView extends ScrollView {
  private View childView;
  public MyScrollView(Context context) {
    super(context);
  }
  public MyScrollView(Context context, AttributeSet attrs) {
    super(context, attrs);
  }
  public MyScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
  }
//  @Override
//  protected void onLayout(boolean changed, int l, int t, int r, int b) {
//    super.onLayout(changed, l, t, r, b);
//  }
  //获取子视图
  @Override
  protected void onFinishInflate() {
    super.onFinishInflate();
    if (getChildCount() > 0) {
      childView = getChildAt(0);
    }
  }
  private int lastY;//上一次y轴方向操作的坐标位置
  private Rect normal = new Rect();//用于记录临界状态的左、上、右、下
  private boolean isFinishAnimation = true;//是否动画结束
  private int lastX, downX, downY;
  //拦截:实现父视图对子视图的拦截
  //是否拦截成功，取决于方法的返回值。返回值true:拦截成功。反之，拦截失败
  @Override
  public boolean onInterceptTouchEvent(MotionEvent ev) {
    boolean isIntercept = false;
    int eventX = (int) ev.getX();
    int eventY = (int) ev.getY();
    switch (ev.getAction()) {
      case MotionEvent.ACTION_DOWN:
        lastX = downX = eventX;
        lastY = downY = eventY;
        break;
      case MotionEvent.ACTION_MOVE:
        //获取水平和垂直方向的移动距离
        int absX = Math.abs(eventX - downX);
        int absY = Math.abs(eventY - downY);
        if(absY > absX && absY >= UIUtils.dp2px(10)){
          isIntercept = true;//执行拦截
        }
        lastX = eventX;
        lastY = eventY;
        break;
    }
    return isIntercept;
  }
  @Override
  public boolean onTouchEvent(MotionEvent ev) {
    if (childView == null || !isFinishAnimation) {
      return super.onTouchEvent(ev);
    }
    int eventY = (int) ev.getY();//获取当前的y轴坐标
    switch (ev.getAction()) {
      case MotionEvent.ACTION_DOWN:
        lastY = eventY;
        break;
      case MotionEvent.ACTION_MOVE:
        int dy = eventY - lastY;//微小的移动量
        if (isNeedMove()) {
          if (normal.isEmpty()) {
            //记录了childView的临界状态的左、上、右、下
            normal.set(childView.getLeft(), childView.getTop(), childView.getRight(), childView.getBottom());
          }
          //重新布局
          childView.layout(childView.getLeft(), childView.getTop() + dy / 2, childView.getRight(), childView.getBottom() + dy / 2);
        }
        lastY = eventY;//重新赋值
        break;
      case MotionEvent.ACTION_UP:
        if (isNeedAnimation()) {
          //使用平移动画
          int translateY = childView.getBottom() - normal.bottom;
          TranslateAnimation translateAnimation = new TranslateAnimation(0, 0, 0, -translateY);
          translateAnimation.setDuration(200);
//        translateAnimation.setFillAfter(true);//停留在最终位置上
          translateAnimation.setAnimationListener(new Animation.AnimationListener() {
            @Override
            public void onAnimationStart(Animation animation) {
              isFinishAnimation = false;
            }
            @Override
            public void onAnimationEnd(Animation animation) {
              isFinishAnimation = true;
              childView.clearAnimation();//清除动画
              //重新布局
              childView.layout(normal.left, normal.top, normal.right, normal.bottom);
              //清除normal的数据
              normal.setEmpty();
            }
            @Override
            public void onAnimationRepeat(Animation animation) {
            }
          });
          //启动动画
          childView.startAnimation(translateAnimation);
        }
        break;
    }
    return super.onTouchEvent(ev);
  }
  //判断是否需要执行平移动画
  private boolean isNeedAnimation() {
    return !normal.isEmpty();
  }
  private boolean isNeedMove() {
    int childMeasuredHeight = childView.getMeasuredHeight();//获取子视图的高度
    int scrollViewMeasuredHeight = this.getMeasuredHeight();//获取布局的高度
    Log.e("TAG", "childMeasuredHeight = " + childMeasuredHeight);
    Log.e("TAG", "scrollViewMeasuredHeight = " + scrollViewMeasuredHeight);
    int dy = childMeasuredHeight - scrollViewMeasuredHeight;//dy >= 0
    int scrollY = this.getScrollY();//获取用户在y轴方向上的偏移量 （上 + 下 -）
    if (scrollY <= 0 || scrollY >= dy) {
      return true;//按照我们自定义的MyScrollView的方式处理
    }
    //其他处在临界范围内的，返回false。即表示，仍按照ScrollView的方式处理
    return false;
  }
}