RecyclerView使用ItemDecoration实现吸顶效果

写此博客目的仅仅用于帮助自己做笔记
吸顶效果在很多APP都可以看见，现在可以借助RecyclerView的ItemDecoration 来简单实现一个吸顶效果直接上效果图：

1. 首先实现一个Model实体，该Model需要包含区分不同组的字段，例如下面这个实体用来区分的便是mTitle字段，当然这个字段是随便取什么的，只要定好就可以。

public class Model {
    private String mTitle;
    private String mContent;
    private int mRsId;

    public Model(String title, String content, int rsId) {
        mTitle = title;
        mContent = content;
        mRsId = rsId;
    }

    public String getTitle() {
        return mTitle;
    }

    public void setTitle(String title) {
        mTitle = title;
    }

    public String getContent() {
        return mContent;
    }

    public void setContent(String content) {
        mContent = content;
    }

    public int getRsId() {
        return mRsId;
    }

    public void setRsId(int rsId) {
        mRsId = rsId;
    }
}

1. 创建一个RecyclerView并且实现实现适配器，先实现如下效果：
   

2. 适配器代码如下：

public class Adapter extends RecyclerView.Adapter<Adapter.ViewHolder> {

    private List mModelList;
    private LayoutInflater mInflater;

    public Adapter(Context context) {
        mInflater = LayoutInflater.from(context);

    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = mInflater.inflate(R.layout.item_l, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        Model model = mModelList.get(position);
        holder.mTextView.setText(model.getContent());
        holder.mImageView.setImageResource(model.getRsId());
    }

    @Override
    public int getItemCount() {
        return mModelList.size();
    }

    static class ViewHolder extends RecyclerView.ViewHolder {

        private TextView mTextView;
        private ImageView mImageView;

        public ViewHolder(View itemView) {
            super(itemView);
            mTextView = itemView.findViewById(R.id.txt);
            mImageView = itemView.findViewById(R.id.img);
        }
    }
}

编写一个接口用来对数据判断

public interface ISticky {
    //判断是否为同类别的第一个位置
    boolean isFirstPosition(int pos);
    //获取标题
    String getGroupTitle(int pos);
}

1. 最重要的一步来了 就是自定义ItemDecoration，代码如下，详细注释我已经写在里面了

public class StickyItemDecoration extends RecyclerView.ItemDecoration {

    private ISticky mISticky;
    //矩形高度
    private int mRectHeight;
    //文字TextSize
    private int mTextPaintSize;
    private Paint mTxtPaint;
    private Paint mRectPaint;
    //分割线画笔
    private Paint mDividerPaint;

    public StickyItemDecoration(Context context,ISticky iSticky) {
        mISticky=iSticky;
        mRectHeight= (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,30,
                context.getResources().getDisplayMetrics());

        mTextPaintSize=(int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,17,
                context.getResources().getDisplayMetrics());
        mTxtPaint=new Paint(Paint.ANTI_ALIAS_FLAG);
        mTxtPaint.setColor(Color.BLACK);
        mTxtPaint.setTextSize(mTextPaintSize);

        mRectPaint=new Paint(Paint.ANTI_ALIAS_FLAG);
        mRectPaint.setStyle(Paint.Style.FILL);
        mRectPaint.setColor(Color.parseColor("#DDDDDD"));

        mDividerPaint=new Paint(Paint.ANTI_ALIAS_FLAG);
        mDividerPaint.setStyle(Paint.Style.FILL);
        mDividerPaint.setColor(Color.WHITE);
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(c, parent, state);
        int childCount=parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            View view=parent.getChildAt(i);
            int left=parent.getPaddingLeft();
            int right=parent.getWidth()-parent.getPaddingRight();
            int top=view.getTop()-1;
            int bottom=view.getTop();
            //Item分割线
            c.drawRect(left,top,right,bottom,mDividerPaint);
        }
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

        int childCount=parent.getChildCount();
        int itemCount=state.getItemCount();
        int left=parent.getPaddingLeft();
        int right=parent.getWidth()-parent.getPaddingRight();
        String preGroupTitle;
        String groupTitle="";
        for (int i = 0; i < childCount; i++) {
            View child=parent.getChildAt(i);
            int pos=parent.getChildLayoutPosition(child);
            preGroupTitle=groupTitle;
            groupTitle=mISticky.getGroupTitle(pos);
            //如果当前分组名和之前分组名一样，忽略此次循环
            if (groupTitle.equals(preGroupTitle)) {
                continue;
            }

            //文字的基线，保证显示完全
            int textBaseLine=Math.max(mRectHeight,child.getTop());

            //分组标题
            String title=mISticky.getGroupTitle(pos);

            int viewBottom=child.getBottom();
            //加入限定 防止数组越界
            if (pos + 1 < itemCount) {
                String nextGroupTitle=mISticky.getGroupTitle(pos+1);
                //当分组不一样  并且改组要向上移动时候
                if (!nextGroupTitle.equals(groupTitle) && viewBottom < textBaseLine) {
                    //将上一个往上移动
                    textBaseLine = viewBottom;
                }
            }
            //绘制边框
            c.drawRect(left, textBaseLine - mRectHeight, right, textBaseLine, mRectPaint);

            //绘制文字并且实现文字居中
            int value= (int) Math.abs(mTxtPaint.getFontMetrics().descent
                    +mTxtPaint.getFontMetrics().ascent);
            c.drawText(title, left,
                    textBaseLine-(mRectHeight+value)/2,
                    mTxtPaint);
        }

    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        int pos=parent.getChildLayoutPosition(view);
        if (mISticky.isFirstPosition(pos)) {
            outRect.top=mRectHeight;
            outRect.bottom=1;
        }else {
            outRect.bottom=1;
        }
    }
}

5.最后只要设置一下即可

final List modelList=new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            modelList.add(new Model("标题1","Content:1_"+i,R.mipmap.ic_launcher_round));
        }
        for (int i = 0; i < 10; i++) {
            modelList.add(new Model("标题2","Content:2_"+i,R.mipmap.ic_launcher_round));
        }
        for (int i = 0; i < 10; i++) {
            modelList.add(new Model("标题3","Content:3_"+i,R.mipmap.ic_launcher_round));
        }
        mRecyclerView.addItemDecoration(new StickyItemDecoration(this, new ISticky() {
            @Override
            public boolean isFirstPosition(int pos) {
                return pos==0||
                        !modelList.get(pos).getTitle().equals(modelList.get(pos-1).getTitle());
            }

            @Override
            public String getGroupTitle(int pos) {
                return modelList.get(pos).getTitle();
            }
        }));

备注：第一次写博客 如有不好之处 还望大家见谅