Android自定义scrollView实现顶部图片下拉放大

package com.example.myapplication.dropzoom; 
 
import android.animation.ObjectAnimator; 
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 Liujinhua on 2016/3/25. 
 * 下拉放大scrollView 
 */ 
public class DropZoomScrollView extends ScrollView implements View.OnTouchListener { 
 
  // 记录首次按下位置 
  private float mFirstPosition = 0; 
  // 是否正在放大 
  private Boolean mScaling = false; 
 
  private View dropZoomView; 
  private int dropZoomViewWidth; 
  private int dropZoomViewHeight; 
 
  public DropZoomScrollView(Context context) { 
    super(context); 
  } 
 
  public DropZoomScrollView(Context context, AttributeSet attrs) { 
    super(context, attrs); 
  } 
 
  public DropZoomScrollView(Context context, AttributeSet attrs, int defStyleAttr) { 
    super(context, attrs, defStyleAttr); 
  } 
 
  @Override 
  protected void onFinishInflate() { 
    super.onFinishInflate(); 
    init(); 
  } 
 
  @Override 
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 
    super.onMeasure(widthMeasureSpec, heightMeasureSpec); 
 
  } 
 
  private void init() { 
    setOverScrollMode(OVER_SCROLL_NEVER); 
    if (getChildAt(0) != null) { 
      ViewGroup vg = (ViewGroup) getChildAt(0); 
      if (vg.getChildAt(0) != null) { 
        dropZoomView = vg.getChildAt(0); 
        setOnTouchListener(this); 
 
      } 
    } 
  } 
 
  @Override 
  public boolean onTouch(View v, MotionEvent event) { 
    if (dropZoomViewWidth