自定义LayoutManager

1、Recyclerview回收机制：
自定义LayoutManager时需要将不需要的View进行回收，在需要使用View时通过getViewForPosition获取View，这个View可能是之前回收的View，也可能是新new出来的，具体操作由recyclerView的recycler实现。
recyclerView内部有两个缓存：scrap和recycler。scrap缓存的View是无需重新绑定数据，可以直接使用。recycler缓存的View则需要重新绑定数据。
2、自定义LayoutManager
（1）继承RecyclerView.LayoutManager,重写四个方法

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

接下来重写onLayoutChildren对子View进行布局,在此方法里不要layout出所有的View，要将不在屏幕内的View回收

@Override    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {        super.onLayoutChildren(recycler, state);        if (getItemCount() < 0) {            return;        }        if (state.isPreLayout()) {            return;        }        //缓存所有的View到scrap中        detachAndScrapAttachedViews(recycler);        int offSetY = 0;        totalHeight = 0;        for (int i = 0; i < getItemCount(); i++) {            //取出缓存的View            View view = recycler.getViewForPosition(i);            //把View加到recyclerview中            addView(view);            measureChildWithMargins(view, 0, 0);            int width = getDecoratedMeasuredWidth(view);            int height = getDecoratedMeasuredHeight(view);            offSetY += height;            totalHeight += height;            Rect frame = allItemFrames.get(i);            if (frame == null) {                frame = new Rect();            }            //保存每个View的边界rect            frame.set(0, offSetY, width, offSetY + height);            allItemFrames.put(i, frame);        }        recycleAndFillItems(recycler, state);    }

private void recycleAndFillItems(RecyclerView.Recycler recycler, RecyclerView.State state) {        if (state.isPreLayout()) {            return;        }        //当前屏幕显示的区域        Rect displayFrame = new Rect(0, verticalScrollOffset, getHorizontalSpace(), verticalScrollOffset + getVerticalSpace());        //将滑出屏幕的View进行回收        Rect childFrame = new Rect();        for (int i = 0; i < getItemCount(); i++) {            View child = getChildAt(i);            if (child != null) {                childFrame.left = getDecoratedLeft(child);                childFrame.top = getDecoratedTop(child);                childFrame.right = getDecoratedRight(child);                childFrame.bottom = getDecoratedBottom(child);                if (!Rect.intersects(displayFrame, childFrame)) {                    removeAndRecycleView(child, recycler);                }            }        }        //重新布局在屏幕上的View        for (int i = 0; i < getItemCount(); i++) {            if (Rect.intersects(displayFrame, allItemFrames.get(i))) {                View scrap = recycler.getViewForPosition(i);                measureChildWithMargins(scrap, 0, 0);                //scrap.setTranslationX(getWidth()/2);                addView(scrap);                Rect frame = allItemFrames.get(i);                layoutDecorated(scrap, frame.left, frame.top - verticalScrollOffset, frame.right, frame.bottom - verticalScrollOffset);            }        }    }

然后实现垂直滚动：

@Override    public boolean canScrollVertically() {        return true;    }    @Override    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {        detachAndScrapAttachedViews(recycler);        int realScrollDistance = dy;        //边界修复        if (verticalScrollOffset + dy < 0) {//上边界            realScrollDistance = -verticalScrollOffset;        } else if (verticalScrollOffset + dy > totalHeight - getVerticalSpace()) {//下边界            realScrollDistance = totalHeight - getVerticalSpace() - verticalScrollOffset;        }        verticalScrollOffset += realScrollDistance;        //移动子View        offsetChildrenVertical(-realScrollDistance);        //填充数据        recycleAndFillItems(recycler, state);        return realScrollDistance;    }

代码地址：自定义LayoutManager