怎么在Android中利用ScrollView实现一个放大回弹效果

public class ZoomInScrollView extends ScrollView {

  public ZoomInScrollView(Context context) {
    this(context, null);
  }

  public ZoomInScrollView(Context context, AttributeSet attrs) {
    this(context, attrs, 0);
  }

  public ZoomInScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
  }
}

@Override
protected void onFinishInflate() {
  super.onFinishInflate();
  // 设置不可过度滚动，否则上移后下拉会出现部分空白的情况
  setOverScrollMode(OVER_SCROLL_NEVER);
  View child = getChildAt(0);
  if (child != null && child instanceof ViewGroup) {
    // 获取默认第一个子View
    ViewGroup vg = (ViewGroup) getChildAt(0);
    if (vg.getChildAt(0) != null) {
      mHeaderView = vg.getChildAt(0);
    }
  }
}

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
  super.onSizeChanged(w, h, oldw, oldh);
  mHeaderWidth = mHeaderView.getMeasuredWidth();
  mHeaderHeight = mHeaderView.getMeasuredHeight();
}

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
  currentX = ev.getX();
  currentY = ev.getY();

  switch (ev.getAction()) {
    case MotionEvent.ACTION_MOVE:
      distanceX = currentX - lastX;
      distanceY = currentY - lastY;
      if (Math.abs(distanceX) < Math.abs(distanceY) && Math.abs(distanceY) > 12) {
        upDownSlide = true;
      }
      break;
  }

  lastX = currentX;
  lastY = currentY;

  if (upDownSlide && mHeaderView != null) {
    commOnTouchEvent(ev);
  }
  return super.dispatchTouchEvent(ev);
}

private void commOnTouchEvent(MotionEvent ev) {
  switch (ev.getAction()) {
    case MotionEvent.ACTION_UP:
      // 手指离开后头部恢复图片
      mIsPulling = false;
      replyView();
      clear();
      break;
    case MotionEvent.ACTION_MOVE:
      if (!mIsPulling) {
        // 第一次下拉
        if (getScrollY() == 0) {
          // 滚动到顶部时记录位置，否则正常返回
          mLastY = (int) ev.getY();
        } else {
          break;
        }
      }

      int distance = (int) ((ev.getY() - mLastY) * mScaleRatio);
      // 当前位置比记录位置要小时正常返回
      if (distance < 0) {
        break;
      }
      mIsPulling = true;
      setZoom(distance);
      break;
  }
}

private void setZoom(float s) {
  float scaleTimes = (float) ((mHeaderWidth + s) / (mHeaderWidth * 1.0));
  // 如超过最大放大倍数则直接返回
  if (scaleTimes > mScaleTimes) {
    return;
  }
  ViewGroup.LayoutParams layoutParams = mHeaderView.getLayoutParams();
  layoutParams.width = (int) (mHeaderWidth + s);
  layoutParams.height = (int) (mHeaderHeight * ((mHeaderWidth + s) / mHeaderWidth));
  // 设置控件水平居中
  ((MarginLayoutParams) layoutParams).setMargins(-(layoutParams.width - mHeaderWidth) / 2, 0, 0, 0);
  mHeaderView.setLayoutParams(layoutParams);
}

private void replyView() {
  final float distance = mHeaderView.getMeasuredWidth() - mHeaderWidth;
  // 设置动画
  ValueAnimator anim = ObjectAnimator.ofFloat(distance, 0.0F).setDuration((long) (distance * mReplyRatio));
  anim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
    @Override
    public void onAnimationUpdate(ValueAnimator animation) {
      setZoom((Float) animation.getAnimatedValue());
    }
  });
  anim.start();
}