Android 实现 我的首页背景 图实现 下拉 视差动画展示 全图，松手回弹

要点：
1.  ImageView 设置： android:scaleType="centerCrop"    中间压缩，两边展示，只显示控件高度
2.  监听 ListView 的overScrollBy方法 下拉滑动

 deltaY： 瞬时变化变量 ListView 的
int newHeight = (int) (mImage.getHeight() + Math.abs(deltaY / 3.0f));
mImage.getLayoutParams().height = newHeight;   // 下拉视差动画,展示全图
    mImage.requestLayout();

3. MotionEvent.ACTION_UP  松手
 int startHeight = mImage.getHeight();    // 已经下拉高度

从 startHeight -> mOriginalHeight  通过估值器 计算出来，在规定时间内，【图片从给一个高度到另外一个高度】
动态设置  图片高度

mOriginalHeight = mImage.getHeight();  // 显示高度
drawableHeight = mImage.getDrawable().getIntrinsicHeight();   // 图片原始高度滑动

核心代码 MyListView

package com.itheima.parallaxdemo.ui;

import android.animation.ValueAnimator;
import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.animation.OvershootInterpolator;
import android.view.animation.TranslateAnimation;
import android.widget.ImageView;
import android.widget.ListView;


/**
 * 视差特效ListView
 * overScrollBy
 * @author poplar
 *
 */
public class MyListView extends ListView {

	private static final String TAG = "TAG";
	private int mOriginalHeight;
	private int drawableHeight;
	private ImageView mImage;

	public MyListView(Context context, AttributeSet attrs, int defStyle) {
		super(context, attrs, defStyle);
	}

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

	public MyListView(Context context) {
		super(context);
	}
	
	/**
	 * 设置ImageView图片, 拿到引用
	 * @param mImage
	 */
	public void setParallaxImage(ImageView mImage) {
		this.mImage = mImage;
		mOriginalHeight = mImage.getHeight(); // 160,当前显示图片高度
		drawableHeight = mImage.getDrawable().getIntrinsicHeight(); // 488,图片原始高度
		
		Log.d(TAG, "height: " + mOriginalHeight + " drawableHeight: " + drawableHeight);
	}
	
	@Override
	protected boolean overScrollBy(int deltaX, int deltaY, 
			int scrollX, int scrollY, int scrollRangeX, int scrollRangeY,
			int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) {
		// deltaY : 竖直方向的瞬时偏移量 / 变化量 dx   顶部到头下拉为-, 底部到头上拉为+
		// scrollY : 竖直方向的偏移量 / 变化量
		// scrollRangeY : 竖直方向滑动的范围
		// maxOverScrollY : 竖直方向最大滑动范围
		// isTouchEvent : 是否是手指触摸滑动, true为手指, false为惯性
		
		Log.d(TAG, "deltaY: " +deltaY + " scrollY: " + scrollY + " scrollRangeY: " + scrollRangeY
				+ " maxOverScrollY: " + maxOverScrollY + " isTouchEvent: " + isTouchEvent);
		
		// 手指拉动 并且 是下拉
		if(isTouchEvent && deltaY < 0){
			// 把拉动的瞬时变化量的绝对值交给Header, 就可以实现放大效果
			if(mImage.getHeight() <= drawableHeight){
				// 除以3避免 滑动过大
				int newHeight = (int) (mImage.getHeight() + Math.abs(deltaY / 3.0f));
			
				// 高度不超出图片最大高度时,才让其生效
				mImage.getLayoutParams().height = newHeight;
				mImage.requestLayout();
			}
		}
		
		return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX,
				scrollRangeY, maxOverScrollX, maxOverScrollY, isTouchEvent);
	}
	
	@Override
	public boolean onTouchEvent(MotionEvent ev) {
		
		switch (ev.getAction()) {
			case MotionEvent.ACTION_UP:
				// 执行回弹动画, 方式一: 属性动画\值动画
				// 从当前高度mImage.getHeight(), 执行动画到原始高度mOriginalHeight
				final int startHeight = mImage.getHeight();
				final int endHeight = mOriginalHeight;
				
//				valueAnimator(startHeight, endHeight);

				// 执行回弹动画, 方式二: 自定义Animation
				ResetAnimation animation = new ResetAnimation(mImage, startHeight, endHeight);
				startAnimation(animation);
				
				break;
		}
		return super.onTouchEvent(ev);
	}

	private void valueAnimator(final int startHeight, final int endHeight) {
		ValueAnimator mValueAnim = ValueAnimator.ofInt(1);
		mValueAnim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
			
			@Override
			public void onAnimationUpdate(ValueAnimator mAnim) {
				float fraction = mAnim.getAnimatedFraction();
				// percent 0.0 -> 1.0
				Log.d(TAG, "fraction: " +fraction);
				Integer newHeight = evaluate(fraction, startHeight, endHeight);

				mImage.getLayoutParams().height = newHeight;
				mImage.requestLayout();
			}
		});
		
		mValueAnim.setInterpolator(new OvershootInterpolator());
		mValueAnim.setDuration(500);
		mValueAnim.start();
	}
	
    public Integer evaluate(float fraction, Integer startValue, Integer endValue) {
        int startInt = startValue;
        return (int)(startInt + fraction * (endValue - startInt));
    }	

}

通过 Animation 继承 ResetAnimation

package com.itheima.parallaxdemo.ui;

import android.view.animation.Animation;
import android.view.animation.OvershootInterpolator;
import android.view.animation.Transformation;
import android.widget.ImageView;

public class ResetAnimation extends Animation {

	private final ImageView mImage;
	private final int startHeight;
	private final int endHeight;

	public ResetAnimation(ImageView mImage, int startHeight, int endHeight) {
		this.mImage = mImage;
		this.startHeight = startHeight;
		this.endHeight = endHeight;
		
		setInterpolator(new OvershootInterpolator());
		//设置动画执行时长
		setDuration(500);
	}

	@Override
	protected void applyTransformation(float interpolatedTime, Transformation t) {
		// interpolatedTime 0.0f -> 1.0f
		

		Integer newHeight = evaluate(interpolatedTime, startHeight, endHeight);

		mImage.getLayoutParams().height = newHeight;
		mImage.requestLayout();
		
		super.applyTransformation(interpolatedTime, t);
	}
	
	
    /**
     * 类型估值器
     * @param fraction
     * @param startValue
     * @param endValue
     * @return
     */
    public Integer evaluate(float fraction, Integer startValue, Integer endValue) {
        int startInt = startValue;
        return (int)(startInt + fraction * (endValue - startInt));
    }
	
	
	
}

MainActivity 代码实现：

package com.itheima.parallaxdemo;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewTreeObserver.OnGlobalLayoutListener;
import android.widget.ArrayAdapter;
import android.widget.ImageView;

import com.itheima.parallaxdemo.ui.MyListView;
import com.itheima.parallaxdemo.util.Cheeses;

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		
		final MyListView mListView = (MyListView) findViewById(R.id.lv);
		mListView.setOverScrollMode(View.OVER_SCROLL_NEVER);

		// 加Header
		final View mHeaderView = View.inflate(MainActivity.this, R.layout.view_header, null);
		final ImageView mImage = (ImageView) mHeaderView.findViewById(R.id.iv);
		mListView.addHeaderView(mHeaderView);
		
		mHeaderView.getViewTreeObserver().addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
			
			@Override
			public void onGlobalLayout() {
				// 当布局填充结束之后, 此方法会被调用

				mListView.setParallaxImage(mImage);
				
				mHeaderView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
			}
		});
		
		// 填充数据
		mListView.setAdapter(new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_list_item_1, Cheeses.NAMES));
		
	}

}

  上图效果：