自定义view实战笔记--视差特效

// 把当前的头布局的高度currentHeight恢复到初始高度orignalHeight
final int currentHeight = iv_header.getHeight();

// 300 -> 160
ValueAnimator animator = ValueAnimator.ofInt(currentHeight, orignalHeight);
// 动画更新的监听
animator.addUpdateListener(new AnimatorUpdateListener() {

@Override
public void onAnimationUpdate(ValueAnimator animation) {
// 0.0 -> 1.0
// 获取动画执行过程中的分度值
float fraction = animation.getAnimatedFraction();
//                  Integer evaluate = evaluate(fraction, currentHeight, orignalHeight);

// 获取中间的值
Integer animatedValue = (Integer) animation.getAnimatedValue();
System.out.println("fraction: " + fraction + " animatedValue: " + animatedValue);

// evaluate == animatedValue

// 让新的高度值生效
iv_header.getLayoutParams().height = animatedValue;
iv_header.requestLayout();

}
});
animator.setInterpolator(new OvershootInterpolator(2));
animator.setDuration(500);
animator.start();

/**
* 滑动到ListView两端才会被调用
*/
@Override
protected boolean overScrollBy(int deltaX, int deltaY, int scrollX,
int scrollY, int scrollRangeX, int scrollRangeY,
int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) {
// deltaY 竖直方向滑动的瞬时变化量, 顶部下拉为- , 底部上拉为+
// scrollY 竖直方向的滑动超出的距离, 顶部为-, 底部为+
// scrollRangeY 竖直方向滑动的范围
// maxOverScrollY 竖直方向最大的滑动位置
// isTouchEvent 是否是用户触摸拉动 , true表示用户手指触摸拉动, false 是惯性

System.out.println("deltaY: " + deltaY + " scrollY: " + scrollY
+ " scrollRangeY: " + scrollRangeY + " maxOverScrollY: " + maxOverScrollY
+ " isTouchEvent: " + isTouchEvent);

// 顶部下拉, 用户触摸操作
if(deltaY < 0 && isTouchEvent){
// deltaY的绝对值, 累加给Header
int newHeight = iv_header.getHeight() + Math.abs(deltaY / 3);
if(newHeight <= drawableHeight){
System.out.println("newHeight: " + newHeight);
// 让新的值生效
iv_header.getLayoutParams().height = newHeight;
iv_header.requestLayout();
}

}

return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX,
scrollRangeY, maxOverScrollX, maxOverScrollY, isTouchEvent);
}

ivHeader.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {

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@Override
public void onGlobalLayout() {
// 图片的原始高度
drawableHeight = ivHeader.getDrawable().getIntrinsicHeight();
//Imageview的初始高度,用来做动画
originalHeight = ivHeader.getHeight();
ivHeader.getViewTreeObserver().removeGlobalOnLayoutListener(this);
}
});