RecyclerView stick头 通过ItemDecoration来实现

ItemDecoration

当我们调用 RecyclerView 的 addItemDecoration 添加 decoration 的时候，RecyclerView 就会调用该类的 onDraw 方法去绘制分割线，也就是说分割线是绘制出来的；

RecyclerView.ItemDecoration 类是抽象类；

有三个重要的方法
getItemOffsets 当item在界面可见时会被调用
onDraw 默认会调用一次，每次触摸移动都会一直在调用
onDrawOver 默认会调用一次，每次触摸移动都会一直在调用

onDraw

绘制Item的分割线和默认stick头的样式

1. 遍历当前界面的所有条目
2. 判断如果是header条目，则draw Stick头，否则draw一个线。

public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
{
        if (parent.getAdapter() instanceof StickFragment.NBAStarAdapter) {
            StickFragment.NBAStarAdapter adapter =(StickFragment.NBAStarAdapter) parent.getAdapter();
(parent.getLayoutManager())).findFirstVisibleItemPosition()
            ));
            int count = parent.getChildCount();//获取可见范围内Item的总数
            for (int i = 0; i < count; i++) {
                View view = parent.getChildAt(i);
                int position = parent.getChildLayoutPosition(view);
                boolean isHeader = adapter.isGroupHeader(position);
                int left = parent.getPaddingLeft();
                int right = parent.getWidth() - parent.getPaddingRight();
                if (isHeader) {
                    c.drawRect(left, view.getTop() - mItemHeaderHeight, right, view.getTop(), mItemHeaderPaint);
                    mTextPaint.getTextBounds(adapter.getGroupName(position), 0, adapter.getGroupName(position).length(), mTextRect);
                    c.drawText(adapter.getGroupName(position), left + mTextPaddingLeft, (view.getTop() - mItemHeaderHeight) + mItemHeaderHeight / 2 + mTextRect.height() / 2, mTextPaint);
                } else {
                    c.drawRect(left, view.getTop() - 1, right, view.getTop(), mLinePaint);
                }
            }
        }
    }

onDrawOver

绘制stick头在滑动时的样式

1. 如果当前没有走到stick的条目样式时，直接return
2. 如果下一个条目是header类型，则需要把前一个stick条目给缩小（移除掉屏幕）。
3. 下一个条目不header类型，则保持stick头的样式一直在屏幕最上面一直绘制成最新样式

@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
    if (parent.getAdapter() instanceof StickFragment.NBAStarAdapter) {
        StickFragment.NBAStarAdapter adapter = (StickFragment.NBAStarAdapter) parent.getAdapter();
        int position = ((LinearLayoutManager) (parent.getLayoutManager())).findFirstVisibleItemPosition();
        View view = parent.findViewHolderForAdapterPosition(position).itemView;
        boolean isHeader = adapter.isGroupHeader(position + 1);
        int top = parent.getPaddingTop();
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();
        if (adapter.getGroupName(position).isNullOrEmpty()) {
            return
        }
        if (isHeader) {
            int bottom = Math.min(mItemHeaderHeight, view.getBottom());
            c.drawRect(left, top + view.getTop() - mItemHeaderHeight, right, top + bottom, mItemHeaderPaint);
            mTextPaint.getTextBounds(adapter.getGroupName(position), 0, adapter.getGroupName(position).length(), mTextRect);
            c.drawText(adapter.getGroupName(position), left + mTextPaddingLeft, top + mItemHeaderHeight / 2 + mTextRect.height() / 2 - (mItemHeaderHeight - bottom), mTextPaint);
        } else {
            c.drawRect(left, top, right, top + mItemHeaderHeight, mItemHeaderPaint);
            mTextPaint.getTextBounds(adapter.getGroupName(position), 0, adapter.getGroupName(position).length(), mTextRect);
            c.drawText(adapter.getGroupName(position), left + mTextPaddingLeft, top + mItemHeaderHeight / 2 + mTextRect.height() / 2, mTextPaint);
        }
        c.save();
    }

}