Android RecyclerView 动画展开item显示详情

stackoverflow上看到这个问题，答主给了个demo
http://stackoverflow.com/questions/27446051/recyclerview-animate-item-resize
看懂了之后发个博，记录一下，刚开始看别人代码好难受，就这么3个文件看了一晚上。。

效果如下

res文件
main_activity文件就是一个recyclerview
main_item是两个textview 一个标题一个详细信息

MainActivity就是加载了一个RecyclerView

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity);

        final RecyclerView rv = (RecyclerView) findViewById(R.id.rv);

        final LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        rv.setLayoutManager(layoutManager);

        final MainAdapter adapter = new MainAdapter();
        rv.setAdapter(adapter);
    }

}

MainAdapter中new了一个keepOne对象，点进去看这个类，有两个方法：bind和toggle，其中的bind是在MainAdapter中的onBindViewHolder（）方法中调用，而toggle是响应viewholder的点击事件

public static class KeepOneH<VH extends RecyclerView.ViewHolder & Expandable> {
//    opened为-1表示所有item是关闭状态，open为pos值的表示pos位置的item为展开的状态
        private int _opened = -1;
        public void bind(VH holder, int pos) {
            if (pos == _opened)
//                3
//            直接显示expandView 无动画
                ExpandableViewHoldersUtil.openH(holder, holder.getExpandView(), false);
            else
//            直接关闭expandView 无动画
                ExpandableViewHoldersUtil.closeH(holder, holder.getExpandView(), false);
        }

@SuppressWarnings("unchecked")
//        响应点击事件的方法
        public void toggle(VH holder) {
//            如果点击的就是开着的item，就关闭该item并把opened置-1
//            ???TODO
            if (_opened == holder.getPosition()) {
                _opened = -1;
//                关闭expandView 有动画
                ExpandableViewHoldersUtil.closeH(holder, holder.getExpandView(), true);
            }
//            如果点击其他本来关闭着的item，则把opened值换成当前pos，把之前开的item给关掉
            else {
                int previous = _opened;
                _opened = holder.getPosition();
//                展开expandView 有动画
                ExpandableViewHoldersUtil.openH(holder, holder.getExpandView(), true);

//                用动画关闭之前的item
                final VH oldHolder = (VH) ((RecyclerView) holder.itemView.getParent()).findViewHolderForPosition(previous);
                if (oldHolder != null)
                    ExpandableViewHoldersUtil.closeH(oldHolder, oldHolder.getExpandView(), true);
            }
        }
    }

点进openH和closeH方法进去看

//    4
    public static void openH(final RecyclerView.ViewHolder holder, final View expandView, final boolean animate) {
//        animate参数为true，则有动画效果
        if (animate) {
            expandView.setVisibility(View.VISIBLE);
//            5
//            改变高度的动画，具体操作点进去看
            final Animator animator = ViewHolderAnimator.ofItemViewHeight(holder);
//            扩展的动画结束后透明度动画开始
            animator.addListener(new AnimatorListenerAdapter() {
                @Override public void onAnimationEnd(Animator animation) {
                    final ObjectAnimator alphaAnimator = ObjectAnimator.ofFloat(expandView, View.ALPHA, 1);
                    alphaAnimator.addListener(new ViewHolderAnimator.ViewHolderAnimatorListener(holder));
                    alphaAnimator.start();
                }
            });
            animator.start();
        }
//        animate参数为false，则直接设置为可见
        else {
            expandView.setVisibility(View.VISIBLE);
            expandView.setAlpha(1);
        }
    }

openH方法接收3个参数，
第一个是viewholder.
第二个是展开部分的view，由holder.getExpandView()方法获取。这里定义了一个接口

public static interface Expandable {
        public View getExpandView();
    }

在MainAdapter中传入infos这个Textview

@Override
        public View getExpandView() {
            return infos;
        }

第三个是一个标记，true时有动画，false时直接设置其展开或者是关闭的状态。所以在bind()方法中调用的openH()都是false，而toggle()中调用的设置为true。

openH方法中 具体动画的操作为ViewHolderAnimator.ofItemViewHeight(holder)

public static Animator ofItemViewHeight(RecyclerView.ViewHolder holder) {
        View parent = (View) holder.itemView.getParent();
        if (parent == null)
            throw new IllegalStateException("Cannot animate the layout of a view that has no parent");

//        测量扩展动画的起始高度和结束高度
        int start = holder.itemView.getMeasuredHeight();
        holder.itemView.measure(View.MeasureSpec.makeMeasureSpec(parent.getMeasuredWidth(), View.MeasureSpec.AT_MOST), View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED));
        int end = holder.itemView.getMeasuredHeight();
//  6
        final Animator animator = LayoutAnimator.ofHeight(holder.itemView, start, end);
//        设定该item在动画开始结束和取消时能否被recycle
        animator.addListener(new ViewHolderAnimatorListener(holder));
//        设定结束时这个item的宽高
        animator.addListener(new LayoutParamsAnimatorListener(holder.itemView, ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT));

        return animator;
    }

可以看出 具体展开的动画在LayoutAnimator.ofHeight(holder.itemView, start, end);中，ViewHolderAnimator只是测量参数，设定监听两个监听事件
1设定在动画开始结束和取消状态下是否可以被回收

public ViewHolderAnimatorListener(RecyclerView.ViewHolder holder) {
            _holder = holder;
        }

        @Override
        public void onAnimationStart(Animator animation) {
            _holder.setIsRecyclable(false);
        }

        @Override
        public void onAnimationEnd(Animator animation) {
            _holder.setIsRecyclable(true);
        }

        @Override
        public void onAnimationCancel(Animator animation) {
            _holder.setIsRecyclable(true);
        }
    }

2.设定在动画结束后view的高和宽分别为warp_content,match_parent.

public static class LayoutParamsAnimatorListener extends AnimatorListenerAdapter {
        private final View _view;
        private final int _paramsWidth;
        private final int _paramsHeight;

        public LayoutParamsAnimatorListener(View view, int paramsWidth, int paramsHeight) {
            _view = view;
            _paramsWidth = paramsWidth;
            _paramsHeight = paramsHeight;
        }

        @Override
        public void onAnimationEnd(Animator animation) {
            final ViewGroup.LayoutParams params = _view.getLayoutParams();
            params.width = _paramsWidth;
            params.height = _paramsHeight;
            _view.setLayoutParams(params);
        }
    }

再深入一层看展开的动画

public class LayoutAnimator {

    public static class LayoutHeightUpdateListener implements ValueAnimator.AnimatorUpdateListener {

        private final View _view;

        public LayoutHeightUpdateListener(View view) {
            _view = view;
        }

        @Override
        public void onAnimationUpdate(ValueAnimator animation) {
            final ViewGroup.LayoutParams lp = _view.getLayoutParams();
            lp.height = (int) animation.getAnimatedValue();
            _view.setLayoutParams(lp);
        }

    }

    public static Animator ofHeight(View view, int start, int end) {
        final ValueAnimator animator = ValueAnimator.ofInt(start, end);
        animator.addUpdateListener(new LayoutHeightUpdateListener(view));
        return animator;
    }
}

是用ValueAnimator.ofInt生成一系列高度值，然后监听动画的变化，不断设定view的高度值

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