9.视差特效、回弹动画、overScrollBy

视差特效 
* 应用场景: 微信朋友圈, QQ空间, 微博个人展示，向下拉出，松开回弹
* 功能实现:
> 1. 重写overScrollBy

> 2. 松手之后执行动画, 类型估值器

. 

activity_main

1. <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
       xmlns:tools="http://schemas.android.com/tools"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       tools:context=".MainActivity" >
       <com.itheima.parallaxdemo.ui.MyListView
           android:id="@+id/lv"
           android:layout_width="match_parent"
           android:layout_height="match_parent" />
   </RelativeLayout>

view_header

1. <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:orientation="vertical" >
       <ImageView
           android:id="@+id/iv"
           android:layout_width="match_parent"
           android:layout_height="160dp"
           android:scaleType="centerCrop"
           android:src="@drawable/parallax_img" />
   </LinearLayout>

MyListView

1. /**
    * 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
   		drawableHeight = mImage.getDrawable().getIntrinsicHeight(); // 488,图片的高，而不是显示的高
   
   		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){
   				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);//不起作用，写几都行
   		mValueAnim.addUpdateListener(new 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);//固值器
   				mImage.getLayoutParams().height = newHeight;//设置imageview高度
   				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));
       }
   
   }

ResetAnimation

1. 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;//设置imageview高
   		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));
       }
   
   }
   

2. 　　MainActivity

3. 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));
   
   	}
   }