需求：RecyclerView中實(shí)現(xiàn)往上滑動(dòng)背景顏色或者圖片變淡直至白色或者透明，往下滑背景顏色或者圖片從白色逐漸變成正常顏色

device-2020-03-27-154834.png

下滑的時(shí)候

device-2020-03-27-154853.png

常見思路做法：
int totalScrollY;

recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
@Override
public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
}

        @Override
        public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);
            totalScrollY += dy;
            if (totalScrollY <= 0) {
                bgView.getBackground().mutate().setAlpha(255);
            } else if (totalScrollY <= 120) {
                bgView.getBackground().mutate().setAlpha((int) (255f - 255.0f / 120 * totalScrollY));
            } else {
                bgView.getBackground().mutate().setAlpha(0);
            }
        }
    });

利用onScrolled里面的dy值來進(jìn)行RecyclerView在y方向上面的滑動(dòng)距離統(tǒng)計(jì)，然后根據(jù)滑動(dòng)距離進(jìn)行view的變化。

大部分的情況下該種做法是沒有問題。不過在很極端的情況下會(huì)出現(xiàn)totalScrollY的值不準(zhǔn)確的問題。

換種做法，totalScrollY不是通過累加dy來計(jì)算(注意：下面這種做法只適用于item的高度是固定統(tǒng)一的情況)

private int getScrollYDistance() {
try {
GridLayoutManager layoutManager = (GridLayoutManager) iRecyclerView.getLayoutManager();
if (layoutManager != null) {
int position = layoutManager.findFirstVisibleItemPosition();
View firstVisibleChildView = layoutManager.findViewByPosition(position);
if (firstVisibleChildView != null) {
int itemHeight = firstVisibleChildView.getHeight();
return position* itemHeight - firstVisibleChildView.getTop();
}
}
} catch (Exception e) {
e.printStackTrace();
}
return 0;
}