3.模仿qq空间背景下拉动画

视差特效 

1.自定义listview  在里面增加HeadView

public class MylistView extends ListView {

	private int drawableHeight;
	private int height;
	private ImageView imageview;

	public MylistView(Context context) {
		super(context);
	}

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

	public MylistView(Context context, AttributeSet attrs, int defStyle) {
		super(context, attrs, defStyle);
	}
	
	public void setParallaxImage(ImageView imageview) {

		this.imageview = imageview;
		drawableHeight = imageview.getDrawable().getIntrinsicHeight();//1464图片本身的高
		height = imageview.getHeight();//480图片表现出来的高
		Log.d("tag", "height: " + height + " 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
		
		
		// 手指拉动 并且 是下拉  实现放大效果
		if (isTouchEvent && deltaY < 0) {
			//展示出来的高度小于图片的高度  放大到图片的高度
			if (imageview.getHeight() <= drawableHeight) {		//让拉三个像素,移动一个像素
				int newHeight = (int) (imageview.getHeight() + Math.abs(deltaY/3.0f));
				imageview.getLayoutParams().height = newHeight;
				imageview.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:
			final int startHeight = imageview.getHeight();//获取的新的高度
			final int endHeight = height;//最后展示出来的高度
			//手指抬起的时候,执行动画
			valueAnimator(startHeight, endHeight);
			break;

		default:
			break;
		}
		return super.onTouchEvent(ev);
	}
	
	//方式一
	private void valueAnimator(final int startHeight, final int endHeight) {
		//动画从开始到结束,执行的监听
		ValueAnimator ofInt = ValueAnimator.ofInt(1);
		ofInt.addUpdateListener(new AnimatorUpdateListener() {
			@Override
			public void onAnimationUpdate(ValueAnimator animation) {
				float animatedFraction = animation.getAnimatedFraction();//0--1
				Integer newHeight = evaluate(animatedFraction, startHeight, endHeight);

				imageview.getLayoutParams().height = newHeight;
				imageview.requestLayout();
			}
			
		});			//来回插值器			张力,越大就越难拉
		ofInt.setInterpolator(new OvershootInterpolator(4));
		ofInt.setDuration(500);
		ofInt.start();
	}
	 public Integer evaluate(float fraction, Integer startValue, Integer endValue) {
	        int startInt = startValue;
	        return (int)(startInt + fraction * (endValue - startInt));
	 }

}

2.MainActivity 获取listview中的孩子imageView的宽高,因为这个时候,还没有绘制完成

public class MainActivity extends Activity {

	private MylistView mylistView;
	private ImageView imageview;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		mylistView = (MylistView) findViewById(R.id.lv);
		mylistView.setOverScrollMode(View.OVER_SCROLL_NEVER);//没有那个拉下来的默认特效
		View inflate = View.inflate(MainActivity.this, R.layout.view_header, null);
		imageview = (ImageView) inflate.findViewById(R.id.iv);
		mylistView.addHeaderView(inflate);
		 
		//当加载布局完之后就会调用这个方法,就可以拿到imageview的测量高度了
		mylistView.getViewTreeObserver().addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
			
			@Override
			public void onGlobalLayout() {
				mylistView.setParallaxImage(imageview);//当布局加载完毕之后,调用这个方法,就可以得到布局中的imageview了
				mylistView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
			}
		});
		
		
		mylistView.setAdapter(new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_list_item_1, Cheeses.NAMES));
	}
}