Android 解决 Reclyeview复用问题 recyclerview复用需要注意的问题

学习启舰的自定义控件三部曲视图篇（七）——RecyclerView系列之四实现回收复用，记录一下自己的理解，方便以后复习查看。

回收复用主要有两部分：（直接copy）

第一：在onLayoutChildren初始布局时：

使用 detachAndScrapAttachedViews(recycler)将所有的可见HolderView剥离
一屏中能放几个item就获取几个HolderView，撑满初始化的一屏即可，不要多创建

第二：在scrollVerticallyBy滑动时：

先判断在滚动dy距离后，哪些holderView需要回收，如果需要回收就调用removeAndRecycleView(child, recycler)先将它回收。
然后向系统获取HolderView对象来填充滚动出来的空白区域。

主要代码：

public class CustomLayoutManager extends RecyclerView.LayoutManager {

    private int mTotalHeight;  //recycler的高度
    private int mItemWidth, mItemHeight; //item的高度与宽度，假设item的高度宽度相等
    private int mSumDy = 0; //累计滑动的距离
    
    private SparseArray<Rect> mItemRects = new SparseArray<>(); //保存每个item的位置矩形
    
    /**
    *必须实现的方法
    */
    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return new RecyclerView.LayoutParams(RecyclerView.LayoutParams.WRAP_CONTENT, RecyclerView.LayoutParams.WRAP_CONTENT);
    }

/**
*计算RecyclerView在竖直方向的显示高度
*/
    private int getVerticalSpace() {
        return getHeight() - getPaddingTop() - getPaddingBottom();
    }

/**
*对RecyclerView中的holderView进行布局
*
*/
    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        if (getItemCount() == 0) { //没有item
            detachAndScrapAttachedViews(recycler);
            return;
        }
//1. 将所有可见的holderView 从RecyclerView上剥离
        detachAndScrapAttachedViews(recycler);
        
//2. 计算整个RecyclerView上可以显示的holderView的个数，此处假定每个holderView的高度相同
        View childView = recycler.getViewForPosition(0);
        measureChildWithMargins(childView, 0, 0);
        mItemWidth = getDecoratedMeasuredWidth(childView);
        mItemHeight = getDecoratedMeasuredHeight(childView);
        int visibleCount = getVerticalSpace() / mItemHeight; //为考虑除不完，还需要考虑除不完的情况

//3. 计算每个holderView的位置矩形，并将其保存在mItemRects中
        int offsetY = 0;
        for (int i = 0; i < getItemCount(); i++) {
            Rect rect = new Rect(0, offsetY, mItemWidth, offsetY + mItemHeight);
            mItemRects.put(i, rect);
            offsetY += mItemHeight;
        }
//4. 将在RecyclerView中显示的holderView加入并设定布局参数
        for (int i = 0; i < visibleCount; i++) {
            Rect rect = mItemRects.get(i);
            View view = recycler.getViewForPosition(i);
            addView(view);
            measureChildWithMargins(view, 0, 0);
            layoutDecorated(view, rect.left, rect.top, rect.right, rect.bottom);
        }
        //计算整个RecyclerView展开的高度
        mTotalHeight = Math.max(offsetY, getVerticalSpace());
    }

//如果需要滑动，需要实现方法并返回true
    @Override
    public boolean canScrollVertically() {
        return true;
    }

/**
*实现滑动
*/
    @Override
    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
        int travel = dy;
        //1. 实现滑动到顶与到底的情况
        if (mSumDy + dy < 0) { //向下滑动到顶时： 累计滑动的距离 + 此次滑动的距离 < 0
            travel = -mSumDy;
        } else if (mSumDy + dy > mTotalHeight - getVerticalSpace()) { 
        //向上滑动到底时： 累计滑动的距离 + 此次滑动距离 > 真个RecyclerView展开后的距离 - RecyclerView的竖直空间
            travel = mTotalHeight - getVerticalSpace() - mSumDy;
        }

        2.搞定回收
        for (int i = getChildCount() - 1; i>=0; i--) {
            View child = getChildAt(i); //获得显示的holderView最底下的holderView
            
            if (travel > 0) { //向上滑动，上面的item需要回收
                if (getDecoratedBottom(child) - travel < 0){ //表示child已经移出了RecyclerView显示区域，需要回收， holderView的Bottom - 滑动的距离 < 0，表示已经滑动了RecyclerView显示区域
                    removeAndRecycleView(child, recycler); //将其移出并回收
                    continue;
                }
            }else if (travel < 0) { //向下滑动， 下面的item需要回收
                //item的顶部 + 向下滑动的绝对值 > RecyclerView的高度 - 底部的padding ，表示item已经移出RecyclerView的底部
                if (getDecoratedTop(child) - travel > getHeight() - getPaddingBottom()) {
                    removeAndRecycleView(child, recycler);//将其移出并回收
                    continue;
                }
            }

        }

        // 3. 搞定复用
        Rect visibleRect = getVisibleArea(travel); //计算RecyclerView中在屏幕可见的item区域
        if (travel >= 0) { //向上滑动
            View lastView = getChildAt(getChildCount() - 1); //获取最下面的item
            int minPos = getPosition(lastView) + 1; // 找到最下面item在RecyclerView中的位置position
            for (int i = minPos; i <= getItemCount() - 1; i++) { //遍历下面不可见的item
                Rect rect = mItemRects.get(i);
                if (Rect.intersects(visibleRect, rect)) { //如果item的rect在可见visibleRect中，就加上去并设定布局参数
                    View child = recycler.getViewForPosition(i);
                    addView(child); 
                    measureChildWithMargins(child, 0, 0);
                    layoutDecorated(child, rect.left, rect.top - mSumDy, rect.right, rect.bottom - mSumDy);
                }else {
                    break;
                }
            }
        }else { //向下滑动
            View fistView = getChildAt(0);
            //即将出现的item的最大position， 获得在RecyclerView显示的第一个holderView的position
            int maxPos = getPosition(fistView) - 1; 
            for (int i = maxPos; i >= 0; i--) {
                Rect rect = mItemRects.get(i);
                if (Rect.intersects(visibleRect, rect)){
                    View child = recycler.getViewForPosition(i);
                    addView(child, 0);
                    measureChildWithMargins(child, 0, 0);
                    layoutDecorated(child, rect.left, rect.top - mSumDy, rect.right, rect.bottom - mSumDy);
                }else {
                    break;
                }
            }
        }
        mSumDy += travel;
        offsetChildrenVertical(-travel);
        return travel;
    }

/**
*获得RecyclerView的现实区域
*/
    private Rect getVisibleArea(int travel) {
        Rect result = new Rect(getPaddingLeft(), getPaddingTop() + mSumDy + travel, getWidth() + getPaddingRight(), getVerticalSpace() + mSumDy + travel);
        return result;
    }
}