LayoutManager实现的右滑列表

在逛Dribbble时无意间看到这样的效果，看着挺cool的，就自己来实现一番。

Dribble.gif

实现的效果：

效果	效果
SkidRightLayoutManager1.gif	SkidRightLayoutManager2.gif

实现思路是自定义LayoutManager来实现Item中的布局，通过RecyclerView来展示。

1.必须实现的方法

  @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return new RecyclerView.LayoutParams(RecyclerView.LayoutParams.WRAP_CONTENT,
                RecyclerView.LayoutParams.WRAP_CONTENT);
    }

2.通过重写onLayoutChildren方法来实现初始化的布局以及初始化一些需要的数据

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        if (state.getItemCount() == 0 || state.isPreLayout()) return;
        //首先将所有的Item回收到缓存中
        removeAndRecycleAllViews(recycler);
        //第一进来，声明Item的宽高，并标识Item的数目不为0.
        if (!mHasChild) {
            mItemViewHeight = getVerticalSpace();
            mItemViewWidth = (int) (mItemViewHeight / mItemHeightWidthRatio);
            mHasChild = true;
        }
        mItemCount = getItemCount();
        mScrollOffset = makeScrollOffsetWithinRange(mScrollOffset);
        //对Item进行布局
        fill(recycler);
    }

3.Item的布局以及缩放的关键就是在fill(recycler)函数和fillChild（）函数中，同时对Item进行回收处理

 public void fill(RecyclerView.Recycler recycler) {
        //获取最右边Item的位置
        int bottomItemPosition = (int) Math.floor(mScrollOffset / mItemViewWidth);
        //获取最右边的Item的可见宽度
        int bottomItemVisibleSize = mScrollOffset % mItemViewWidth;
        //获取最右边Item可见宽度相对于Item宽度的比例
        final float offsetPercent = bottomItemVisibleSize * 1.0f / mItemViewWidth;
        final int space = getHorizontalSpace();
        //用于存储要显示的Item的信息，从最右边的Item开始添加
        ArrayList layoutInfos = new ArrayList<>();
        for (int i = bottomItemPosition - 1, j = 1, remainSpace = space - mItemViewWidth;
             i >= 0; i--, j++) {
            //左右两个Item间最大偏移量
            double maxOffset = (getHorizontalSpace() - mItemViewWidth) / 2 * Math.pow(mScale, j);
            //Item左边的位置
            int start = (int) (remainSpace - offsetPercent * maxOffset);
            ItemViewInfo info = new ItemViewInfo(start,
                    (float) (Math.pow(mScale, j - 1) * (1 - offsetPercent * (1 - mScale))),
                    offsetPercent,
                    start * 1.0f / space
            );
            layoutInfos.add(0, info);

            remainSpace -= maxOffset;
            if (remainSpace <= 0) {
                info.setTop((int) (remainSpace + maxOffset));
                info.setPositionOffset(0);
                info.setLayoutPercent(info.getTop() / space);
                info.setScaleXY( (float) Math.pow(mScale, j - 1));
                break;
            }
        }
     
        if (bottomItemPosition < mItemCount) {
            final int start = space - bottomItemVisibleSize;
            layoutInfos.add(new ItemViewInfo(start, 1.0f,
                    bottomItemVisibleSize * 1.0f / mItemViewWidth, start * 1.0f / space).
                    setIsBottom());
        } else {
            bottomItemPosition -= 1;
        }
         //Item回收处理的逻辑
        int layoutCount = layoutInfos.size();
        final int startPos = bottomItemPosition - (layoutCount - 1);
        final int endPos = bottomItemPosition;
        final int childCount = getChildCount();
        for (int i = childCount - 1; i >= 0; i--) {
            View childView = getChildAt(i);
            int pos = convert2LayoutPosition(getPosition(childView));
            if (pos > endPos || pos < startPos) {
                removeAndRecycleView(childView, recycler);
            }
        }
        detachAndScrapAttachedViews(recycler);

        for (int i = 0; i < layoutCount; i++) {
            //将Item进行排列
            fillChild(recycler.getViewForPosition(convert2AdapterPosition(startPos + i)), layoutInfos.get(i));
        }
    }

    private void fillChild(View view, ItemViewInfo layoutInfo) {
        addView(view);
        measureChildWithExactlySize(view);
        //计算缩放比例
        final int scaleFix = (int) (mItemViewWidth * (1 - layoutInfo.getScaleXY()) / 2);

        int top = (int) getPaddingTop();
        //排列Item
        layoutDecoratedWithMargins(view, layoutInfo.getTop() - scaleFix, top
                , layoutInfo.getTop() + mItemViewWidth - scaleFix, top + mItemViewHeight);
        //对Item进行缩放
        ViewCompat.setScaleX(view, layoutInfo.getScaleXY());
        ViewCompat.setScaleY(view, layoutInfo.getScaleXY());
    }

4.设置列表的滑动方向

 @Override
    public boolean canScrollHorizontally() {
        return true;
    }

5.滑动处理逻辑，滑动的同时也要对Item进行位置排列

    @Override
    public int scrollHorizontallyBy(int dx, RecyclerView.Recycler recycler, RecyclerView.State state) {
        int pendingScrollOffset = mScrollOffset + dx;
        mScrollOffset = makeScrollOffsetWithinRange(pendingScrollOffset);
        fill(recycler);
        return mScrollOffset - pendingScrollOffset + dx;
    }

主要逻辑已将完成，一句代码实现cool~~~列表

 mSkidRightLayoutManager = new SkidRightLayoutManager(1.5f, 0.85f);
        mRecyclerView.setLayoutManager(mSkidRightLayoutManager);

详细代码请看github源码哦