Android实现下拉放大图片松手自动反弹效果

import android.animation.ValueAnimator; 
import android.content.Context; 
import android.util.AttributeSet; 
import android.view.MotionEvent; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.ScrollView; 
 
/** 
 * Created by BAIPEI on 2017/11/21. 
 */ 
 
public class HeadZoomScrollView extends ScrollView { 
  private View mZoomView; 
  private int mZoomViewWidth; 
  private int mZoomViewHeight; 
 
  private float firstPosition;//记录第一次按下的位置 
  private boolean isScrolling;//是否正在缩放 
  private float mScrollRate = 0.3f;//缩放系数，缩放系数越大，变化的越大 
  private float mReplyRate = 0.5f;//回调系数，越大，回调越慢 
 
  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); 
  } 
 
  public void setmZoomView(View mZoomView) { 
    this.mZoomView = mZoomView; 
  } 
 
  public void setmScrollRate(float mScrollRate) { 
    this.mScrollRate = mScrollRate; 
  } 
 
  public void setmReplyRate(float mReplyRate) { 
    this.mReplyRate = mReplyRate; 
  } 
 
  @Override 
  protected void onFinishInflate() { 
    super.onFinishInflate(); 
    init(); 
  } 
 
  private void init() { 
    setOverScrollMode(OVER_SCROLL_NEVER); 
    if (getChildAt(0) != null) { 
      ViewGroup vg = (ViewGroup) getChildAt(0); 
      if (vg.getChildAt(0) != null) { 
        mZoomView = vg.getChildAt(0); 
      } 
    } 
  } 
 
  @Override 
  public boolean onTouchEvent(MotionEvent ev) { 
    if (mZoomViewWidth <= 0 || mZoomViewHeight <= 0) { 
      mZoomViewWidth = mZoomView.getMeasuredWidth(); 
      mZoomViewHeight = mZoomView.getMeasuredHeight(); 
    } 
    switch (ev.getAction()) { 
      case MotionEvent.ACTION_UP: 
        //手指离开后恢复图片 
        isScrolling = false; 
        replyImage(); 
        break; 
      case MotionEvent.ACTION_MOVE: 
        if (!isScrolling) { 
          if (getScrollY() == 0) { 
            firstPosition = ev.getY();// 滚动到顶部时记录位置，否则正常返回 
          } else { 
            break; 
          } 
        } 
        int distance = (int) ((ev.getY() - firstPosition) * mScrollRate); // 滚动距离乘以一个系数 
        if (distance <0) { // 当前位置比记录位置要小，正常返回 
          break; 
        } 
 
        // 处理放大 
        isScrolling = true; 
        setZoom(distance); 
        return true; // 返回true表示已经完成触摸事件，不再处理 
    } 
    return true; 
  } 
 
  //回弹动画 
  private void replyImage() { 
    float distance = mZoomView.getMeasuredWidth() - mZoomViewWidth; 
    ValueAnimator valueAnimator = ValueAnimator.ofFloat(distance, 0f).setDuration((long) (distance * mReplyRate)); 
    valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { 
      @Override 
      public void onAnimationUpdate(ValueAnimator animation) { 
        setZoom((Float) animation.getAnimatedValue()); 
      } 
    }); 
    valueAnimator.start(); 
  } 
 
  public void setZoom(float zoom) { 
    if (mZoomViewWidth <= 0 || mZoomViewHeight <= 0) { 
      return; 
    } 
    ViewGroup.LayoutParams lp = mZoomView.getLayoutParams(); 
    lp.width = (int) (mZoomViewWidth + zoom); 
    lp.height = (int) (mZoomViewHeight * ((mZoomViewWidth + zoom) / mZoomViewWidth)); 
    ((MarginLayoutParams) lp).setMargins(-(lp.width - mZoomViewWidth) / 2, 0, 0, 0); 
    mZoomView.setLayoutParams(lp); 
  } 
 
}