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 <= 0 || dropZoomViewHeight <= 0) {       dropZoomViewWidth = dropZoomView.getMeasuredWidth();       dropZoomViewHeight = dropZoomView.getMeasuredHeight();     }     switch (event.getAction()) {       case MotionEvent.ACTION_UP:         //手指离开后恢复图片         mScaling = false;         replyImage();         break;       case MotionEvent.ACTION_MOVE:         if (!mScaling) {           if (getScrollY() == 0) {             mFirstPosition = event.getY();// 滚动到顶部时记录位置，否则正常返回           } else {             break;           }         }         int distance = (int) ((event.getY() - mFirstPosition) * 0.6); // 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         mScaling = true;         setZoom(1 + distance);         return true; // 返回true表示已经完成触摸事件，不再处理     }     return false;   }    // 回弹动画 (使用了属性动画)   public void replyImage() {     final float distance = dropZoomView.getMeasuredWidth() - dropZoomViewWidth;      // 设置动画     ValueAnimator anim = ObjectAnimator.ofFloat(0.0F, 1.0F).setDuration((long) (distance * 0.7));      anim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {       @Override       public void onAnimationUpdate(ValueAnimator animation) {         float cVal = (Float) animation.getAnimatedValue();         setZoom(distance - ((distance) * cVal));       }     });     anim.start();    }    //缩放   public void setZoom(float s) {     if (dropZoomViewHeight <= 0 || dropZoomViewWidth <= 0) {       return;     }     ViewGroup.LayoutParams lp = dropZoomView.getLayoutParams();     lp.width = (int) (dropZoomViewWidth + s);     lp.height = (int) (dropZoomViewHeight * ((dropZoomViewWidth + s) / dropZoomViewWidth));     dropZoomView.setLayoutParams(lp);   } } 

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"   android:layout_width="match_parent"   android:layout_height="match_parent"   android:orientation="vertical">    <com.example.myapplication.dropzoom.DropZoomScrollView     android:layout_width="match_parent"     android:layout_height="match_parent">      <LinearLayout       android:layout_width="match_parent"       android:layout_height="wrap_content"       android:orientation="vertical">         <ImageView         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_gravity="center"         android:src="@drawable/bg" />        <ImageView         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_gravity="center"         android:src="@drawable/home_bg" />        <ImageView         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_gravity="center"         android:src="@drawable/home_bg" />        <ImageView         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_gravity="center"         android:src="@drawable/home_bg" />        <ImageView         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_gravity="center"         android:src="@drawable/home_bg" />     </LinearLayout>   </com.example.myapplication.dropzoom.DropZoomScrollView> </LinearLayout>