Android StickHeaderRecyclerView - 让recyclerview头部固定

介绍
在项目中有时会需要recyclerview滑动式时某个view滑出后会固定在头部显示，比较常用的比如手机联系人界面、地区选择界面等。 StickHeaderRecyclerView就是实现这个功能的。效果图：

这样的控件网上一抓一大把了，本控件的优点就是使用简单- lib简单 - 使用的语法也简单（之前下了2个类似开源项目，都是上万行代码。读起来麻烦、改起来麻烦就自己写了这个控件）

使用
只需要让你的adapter实现StickHeaderDecoration.StickHeaderInterface接口，方法boolean isStick(int position)中返回的值就标识当前位置的view是否需要固定。
同时需要让Adapter中的item不复用（如果怕影响性能也可以单独让需要固定的view不复用）在adapter构造方法中setHasStableIds(true); 同时复写adapter的public long getItemId(int position) {return position;}
上代码

public class NormalAdapter  extends RecyclerView.Adapter<NormalAdapter.InnerHolder> implements StickHeaderDecoration.StickHeaderInterface{

    NormalAdapter(Activity activity, List<String> dates){

        this.activity = activity;

        this.dates = dates;

    }

    @Override

    public boolean isStick(int position) {

        return position %  == ;

    }

    Activity activity;

    private List<String> dates;

    @Override

    public InnerHolder onCreateViewHolder(ViewGroup parent, int viewType) {

        View inflate = LayoutInflater.from(activity).inflate(R.layout.item, parent,

                false);

        return new InnerHolder(inflate);

    }

    @Override

    public void onBindViewHolder(InnerHolder holder, int position) {

        if(isStick(position)){

            holder.itemView.setBackgroundResource(R.color.colorAccent);

            holder.tvText.setText(position /  +"");

        }else{

            holder.itemView.setBackgroundResource(R.color.white);

            holder.tvText.setText(dates.get(position));

        }

    }

    @Override

    public int getItemCount() {

        return dates.size();

    }

    class InnerHolder extends RecyclerView.ViewHolder{

        TextView tvText;

        public InnerHolder(View itemView) {

            super(itemView);

            tvText = (TextView) itemView.findViewById(R.id.tvText);

        }

    }

}

activity代码

public class MainActivity extends Activity {

    private RecyclerView recycle;

    private List<String> dates = new ArrayList<>();

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        recycle = (RecyclerView) findViewById(R.id.recycle);

        for(int i=;i<;i++){

            dates.add("date : "+i);

        }

        recycle.setLayoutManager(new LinearLayoutManager(this));

        recycle.setAdapter(new NormalAdapter(this, dates));

        recycle.addItemDecoration(new StickHeaderDecoration(recycle));

    }

}

完成了

原理

先上核心类代码

public class StickHeaderDecoration extends RecyclerView.ItemDecoration {

    public interface StickHeaderInterface {

        /**

         * is this item need stick

         * @param position now item position in the recyclerView

         * @return true : need stick else not

         */

        boolean isStick(int position);

    }

    private RecyclerView recyclerView;

    private RecyclerView.LayoutManager manager;

    private RecyclerView.Adapter adapter;

    private StickHeaderInterface stickHeaderInterface;

    /**

     * 进行一些容错检查

     */

    public StickHeaderDecoration(RecyclerView recyclerView) {

        this.recyclerView = recyclerView;

        this.manager = recyclerView.getLayoutManager();

        this.adapter = recyclerView.getAdapter();

        if (adapter == null) {

            throw new RuntimeException("please set Decoration after set adapter");

        }

        if (adapter instanceof StickHeaderInterface) {

            stickHeaderInterface = (StickHeaderInterface) adapter;

            return;

        }

        throw new RuntimeException("please make your adapter implements StickHeaderInterface");

    }

    /**

     * 绘制头部的stick view

     */

    @Override

    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {

        super.onDrawOver(c, parent, state);

        View childAt = parent.getChildAt();

        if (childAt == null)

            return;

        RecyclerView.ViewHolder childViewHolder = parent.getChildViewHolder(childAt);

        int position = childViewHolder.getPosition();

        for (int i = position; i >= ; i--) {

            if (stickHeaderInterface.isStick(i)) {

                int top = ;

                if (position +  < adapter.getItemCount()) {

                    if (stickHeaderInterface.isStick(position + )) {

                        View childNext = parent.getChildAt();

                        top = manager.getDecoratedTop(childNext) <  ?  : manager

                                .getDecoratedTop(childNext);

                    }

                }

                RecyclerView.ViewHolder inflate = recyclerView.getAdapter().createViewHolder(parent,

                        recyclerView.getAdapter().getItemViewType(i));

                recyclerView.getAdapter().bindViewHolder(inflate, i);

                int measureHeight = getMeasureHeight(inflate.itemView);

                c.save();

                if (top < inflate.itemView.getMeasuredHeight() && top > ) {

                    c.translate(, top - measureHeight);

                }

                inflate.itemView.draw(c);

                c.restore();

                return;

            }

        }

    }

    /**

     * 测量控件的高度

     *

     * @param header

     */

    private int getMeasureHeight(View header) {

        int widthSpec = View.MeasureSpec.makeMeasureSpec(recyclerView.getWidth(), View

                .MeasureSpec.EXACTLY);

        int heightSpec = View.MeasureSpec.makeMeasureSpec(, View.MeasureSpec.UNSPECIFIED);

        header.measure(widthSpec, heightSpec);

        header.layout(, , header.getMeasuredWidth(), header.getMeasuredHeight());

        return header.getMeasuredHeight();

    }

}

首先ItemDecoration是一个接口，通过RecyclerView的 recycle.addItemDecoration方法设置进去
其中只有6个方法其中3个过时了。我们这儿只需要对onDrawOver进行操作。
onDrawOver是当前RecyclerView绘制完毕后调用，可以其中进行绘制。我们的头部固定其实就是在这个方法中绘制进去的。

算法
1.这儿我们需要判断当前显示item的前面是否有需要固定的item（这儿取名为beforitem）如果有则绘制在顶部
2.我们还需要当第二个固定的item把前面的item慢慢顶上去的效果，这儿通过判断当前显示的第一个item的下一个item是否需要固定，如果需要则通过manager.getDecoratedTop(childNext)获取这个item距离顶部的距离然后通过计算把beforitem先上移动一定的距离。
基本原理就这样,相信代码更加有说服力，github 地址
https://github.com/LiuLinXin/StickHeaderRecyclerView-philer

待优化
头部view现在是通过ondraw绘制进去的，不能相应点击事件等。暂时没相处好的解决办法，希望有想法的朋友提示下。

巴特西

Android StickHeaderRecyclerView - 让recyclerview头部固定

原理

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