android ScrollView实现下拉放大头部图片

public class HeadZoomScrollView extends ScrollView {

 public HeadZoomScrollView(Context context) {
  super(context);
 }

 public HeadZoomScrollView(Context context, AttributeSet attrs) {
  super(context, attrs);
 }

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

 // 用于记录下拉位置
 private float y = 0f;
 // zoomView原本的宽高
 private int zoomView;
 private int zoomView;

 // 是否正在放大
 private boolean mScaling = false;

 // 放大的view，默认为第一个子view
 private View zoomView;
 public void setZoomView(View zoomView) {
  this.zoomView = zoomView;
 }

 // 滑动放大系数，系数越大，滑动时放大程度越大
 private float mScaleRatio = 0.4f;
 public void setmScaleRatio(float mScaleRatio) {
  this.mScaleRatio = mScaleRatio;
 }

 // 最大的放大倍数
 private float mScaleTimes = 2f;
 public void setmScaleTimes(int mScaleTimes) {
  this.mScaleTimes = mScaleTimes;
 }

 // 回弹时间系数，系数越小，回弹越快
 private float mReplyRatio = 0.5f;
 public void setmReplyRatio(float mReplyRatio) {
  this.mReplyRatio = mReplyRatio;
 }

 @Override
 protected void onFinishInflate() {
  super.onFinishInflate();
//  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);
//  获得默认第一个view
  if (getChildAt(0) != null && getChildAt(0) instanceof ViewGroup && zoomView == null) {
   ViewGroup vg = (ViewGroup) getChildAt(0);
   if (vg.getChildCount() > 0) {
    zoomView = vg.getChildAt(0);
   }
  }
 }

 @Override
 public boolean onTouchEvent(MotionEvent ev) {
  if (zoomViewWidth <= 0 || zoomViewHeight <=0) {
   zoomViewWidth = zoomView.getMeasuredWidth();
   zoomViewHeight = zoomView.getMeasuredHeight();
  }
  if (zoomView == null || zoomViewWidth <= 0 || zoomViewHeight <= 0) {
   return super.onTouchEvent(ev);
  }
  switch (ev.getAction()) {
   case MotionEvent.ACTION_MOVE:
    if (!mScaling) {
     if (getScrollY() == 0) {
      y = ev.getY();//滑动到顶部时，记录位置
     } else {
      break;
     }
    }
    int distance = (int) ((ev.getY() - y)*mScaleRatio);
    if (distance < 0) break;//若往下滑动
    mScaling = true;
    setZoom(distance);
    return true;
   case MotionEvent.ACTION_UP:
    mScaling = false;
    replyView();
    break;
  }
  return super.onTouchEvent(ev);
 }

 /**放大view*/
 private void setZoom(float s) {
  float scaleTimes = (float) ((zoomViewWidth+s)/(zoomViewWidth*1.0));
//  如超过最大放大倍数，直接返回
  if (scaleTimes > mScaleTimes) return;

  ViewGroup.LayoutParams layoutParams = zoomView.getLayoutParams();
  layoutParams.width = (int) (zoomViewWidth + s);
  layoutParams.height = (int)(zoomViewHeight*((zoomViewWidth+s)/zoomViewWidth));
//  设置控件水平居中
  ((MarginLayoutParams) layoutParams).setMargins(-(layoutParams.width - zoomViewWidth) / 2, 0, 0, 0);
  zoomView.setLayoutParams(layoutParams);
 }

 /**回弹*/
 private void replyView() {
  final float distance = zoomView.getMeasuredWidth() - zoomViewWidth;
  // 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();
 }

 @Override
 protected void onScrollChanged(int l, int t, int oldl, int oldt) {
  super.onScrollChanged(l, t, oldl, oldt);
  if (onScrollListener!=null) onScrollListener.onScroll(l,t,oldl,oldt);
 }

 private OnScrollListener onScrollListener;
 public void setOnScrollListener(OnScrollListener onScrollListener) {
  this.onScrollListener = onScrollListener;
 }

 /**滑动监听*/
 public interface OnScrollListener{
  void onScroll(int scrollX,int scrollY,int oldScrollX, int oldScrollY);
 }


}