RecyclerView分割线附上顶吸

参考:RecyclerView 自定义ItemDecoration从入门到实现吸顶效果
Solartisan/WaveSideBar

简单分割线

实现分割线要继承RecyclerView.ItemDecoration，重写三个方法

public class MyItemDecoration extends RecyclerView.ItemDecoration {
    public MyItemDecoration(Context context) {
    }
    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(c, parent, state);int count=parent.getChildCount();
    }
    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

    }
    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
    }
}

• onDraw方法和自定义View中的onDraw方法一样，是用来在屏幕上画东西的
• onDrawOver 英文Over的意思在…的上面 ,可以理解成是图层关系,item的内容和分割线是第一层(要在第一层画东西要调用onDraw),而onDrawOver是第二层,位于onDraw的上面
• getItemOffsets 看名字可以知道是设置item的偏移值,其实效果和padding一样
• 注意：三个方法中都有一个RecyclerView类型的parent，这个不是整个RecyclerView，而是当前屏幕显示的那几个RecyclerView，所以当滚动屏幕的时候会重新调用此类来绘制分割线。同理调用parent.getChildCount()获取到的时当前屏幕上显示的item个数

一条简单的红色分割线

public class MyItemDecoration extends RecyclerView.ItemDecoration {
    private int width;
    private int height;
    private int item_height;
    private int item_padding;
    private Paint paint;
    private Context context;
    public MyItemDecoration(Context context) {
        this.context = context;
        width=context.getResources().getDisplayMetrics().widthPixels;
        height=context.getResources().getDisplayMetrics().heightPixels;
        paint=new Paint(Paint.ANTI_ALIAS_FLAG|Paint.DITHER_FLAG);
        paint.setColor(Color.RED);
        item_height=DensityUtil.dip2px(context, 1);
        item_padding=DensityUtil.dip2px(context, 10);
    }
    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(c, parent, state);int count=parent.getChildCount();
        for (int i = 0; i < count; i++) {
            View view=parent.getChildAt(i);
            int top=view.getBottom();
            int bottom=top+item_height;
            c.drawRect(0,top,width,bottom,paint);
        }
    }
    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

    }
    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
    }
}

• 以上是实现item底部的分割线
• 主要是调用onDraw方法画分割线，c.drawRect是画矩形，参数分别是左上右下和画笔对象
• 设置分割线左右边距就改成c.drawRect(0+padding,top,width-padding,bottom,paint);

• 因为onDraw和item内容处于一个图层，所以把分割线画的比较粗的话会和item内容重合，此时第三个方法就派上用场了，设置一个偏移值

  @Override
  public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
      super.getItemOffsets(outRect, view, parent, state);
      outRect.bottom=item_height;
  }

• 该偏移值的意思是让列表中每一个item的底部向下空出一个item_height的宽度用来绘制分割线

实现顶吸

顶部绘制一个固定的区域

public class MyItemDecoration extends RecyclerView.ItemDecoration {
    private int width;
    private int item_height;
    private Paint paint;
    private Paint paint2;
    public MyItemDecoration(Context context) {
        width=context.getResources().getDisplayMetrics().widthPixels;
        paint=new Paint(Paint.ANTI_ALIAS_FLAG|Paint.DITHER_FLAG);
        paint.setColor(Color.GRAY);
        item_height=DensityUtil.dip2px(context, 20);
        paint2=new Paint(Paint.ANTI_ALIAS_FLAG|Paint.DITHER_FLAG);
        paint2.setColor(Color.parseColor("#52ff0000"));
    }
    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(c, parent, state);int count=parent.getChildCount();
        for (int i = 0; i < count; i++) {
            View view=parent.getChildAt(i);
            int bottom=view.getTop();
            int top=bottom-item_height;
            c.drawRect(0,top,width,bottom,paint);
        }
    }
    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);
        c.drawRect(0,0,width,item_height,paint2);
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        outRect.top=item_height;
    }
}

• 先把分隔条写到每一个item的上面，修改了onDraw和getItemOffsets

• 实现每一个间隔把顶部固定区域顶上去

  @Override
  public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
      super.onDrawOver(c, parent, state);
      //获取屏幕上的第一个item
      View child0 = parent.getChildAt(0);
      //如果第一个item的Bottom<=分割线的高度
      if (child0.getBottom() <= item_height) {
          //随着RecyclerView滑动 分割线的top=固定为0不动,bottom则赋值为child0的bottom值.
          c.drawRect(0, 0, width,child0.getBottom() , paint2);
      } else {
          //固定不动
          c.drawRect(0, 0, width, item_height, paint2);
  
      }
  }

• 实现方法是获取屏幕第一个recyclerview的item，看他底部的距离，如果小于一个分割线的高度，那就把分割线高度弄成第一个item到底部的距离

一个简单完整的吸顶，用Canvas绘制

参考:RecyclerView 自定义ItemDecoration从入门到实现吸顶效果
效果:

public class MyItemDecoration extends RecyclerView.ItemDecoration {
    private Paint.FontMetrics fontMetrics;
    private int wight;
    private int itemDecorationHeight;
    private Paint paint;
    private ObtainTextCallback callback;
    private float itemDecorationPadding;
    private TextPaint textPaint;
    private Rect text_rect=new Rect();
    public MyItemDecoration(Context context, ObtainTextCallback callback) {
        //获取item宽度
        wight=context.getResources().getDisplayMetrics().widthPixels;

        //绘制间隔背景为红色
        paint=new Paint(Paint.ANTI_ALIAS_FLAG|Paint.DITHER_FLAG);
        paint.setColor(Color.RED);

        //间隔高度
        itemDecorationHeight=DensityUtil.dip2px(context, 30);
        //间隔左右padding
        itemDecorationPadding=DensityUtil.dip2px(context, 10);
        this.callback = callback;

        //绘制间隔上的文字
        textPaint = new TextPaint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
        textPaint.setColor(Color.WHITE);
        textPaint.setTextAlign(Paint.Align.LEFT);
        textPaint.setTextSize(DensityUtil.dip2px(context, 25));

        //绘制文字相关的参数
        fontMetrics = new Paint.FontMetrics();
        textPaint.getFontMetrics(fontMetrics);
    }
    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(c, parent, state);
        int count=parent.getChildCount();
        for (int i = 0; i < count; i++) {
            View view=parent.getChildAt(i);
            int top=view.getTop()-itemDecorationHeight;
            int bottom=top+itemDecorationHeight;

            //获取该item在总列表里是第几个
            int position = parent.getChildAdapterPosition(view);
            //通过callback获取到这个item的第一个字
            String content = callback.getText(position);

            textPaint.getTextBounds(content,0, content.length(),text_rect);

            //如果需要画分割线
            if(isFirstInGroup(position)) {
                c.drawRect(0,top,wight,bottom,paint);
                c.drawText(content, itemDecorationPadding, bottom-fontMetrics.descent, textPaint);
            }
        }
    }
    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

        //获取到屏幕上第一个item
        View child0=parent.getChildAt(0);
        //获取到屏幕上第一个item是总列表中第几个
        int position = parent.getChildAdapterPosition(child0);
        //获取到这个item的第一个字
        String content = callback.getText(position);
        //如果已经重合了并且第一个item前需要画分割线，那么就按照第一个item的底部高度来画分割线
        if(child0.getBottom()<=itemDecorationHeight&&isFirstInGroup(position+1)){
            c.drawRect(0, 0, wight, child0.getBottom(), paint);
            c.drawText(content, itemDecorationPadding, child0.getBottom()-fontMetrics.descent, textPaint);
        }
        //否则就按照一个完整的分割线来画，这里通过textPaint来写不同的字
        else {
            c.drawRect(0, 0, wight, itemDecorationHeight, paint);
            c.drawText(content, itemDecorationPadding, itemDecorationHeight-fontMetrics.descent, textPaint);
        }
    }
    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        int position= parent.getChildAdapterPosition(view);
        //如果不是在同一组就腾出分割线需要的高度
        if(isFirstInGroup(position)){
            outRect.top=itemDecorationHeight;
        }
    }

    //回调接口,通过该回调获取item的内容的第一个文字
    public interface ObtainTextCallback {
        String getText(int position);
    }

    //判断当前item和下一个item的第一个文字是否相同,如果相同说明是同一组,不需要画分割线
    private boolean isFirstInGroup(int pos) {
        //如果是adapter的第一个position直接return,因为第一个item必须有分割线
        if (pos == 0) {
            return true;
        } else {
            //否者判断前一个item的字符串 与 当前item字符串 是否相同
            String prevGroupId = callback.getText(pos - 1);
            String groupId = callback.getText(pos);
            //如果前一个item和这个item的第一个字相同那就不用画分割线
            if (prevGroupId.equals(groupId)) {
                return false;
            } else {
                return true;
            }
        }
    }
}

Activity

recyclerView = (RecyclerView)findViewById(R.id.recycler_view);
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
final List<String> list = new ArrayList<String>();
initList(list);
Adapter adapter = new Adapter(list);
recyclerView.setAdapter(adapter);
//通过每一个item内容的第一个字进行分类
recyclerView.addItemDecoration(new MyItemDecoration(this, new MyItemDecoration.ObtainTextCallback() {
    @Override
    public String getText(int position) {
        return list.get(position).substring(0,1);
    }
}));

附上一个更完整的吸顶，用布局绘制

参考：Solartisan/WaveSideBarhttps://github.com/Solartisan/TurboRecyclerViewHelper)

1. 首先是bean
   Fruit.java

   public class Fruit {
       private String name;
       private int type;
   
       public Fruit(String name, int type) {
           this.name = name;
           this.type = type;
       }
   
       public String getName() {
           return name;
       }
   
       public void setName(String name) {
           this.name = name;
       }
   
       public int getType() {
           return type;
       }
   
       public void setType(int type) {
           this.type = type;
       }
   }

这个bean包含了item和分割线的内容，用type区分

2. 适配器
   FruitAdapter.java

   public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.BaseHolder> {
       private List<Fruit> mFruitList;
       private static final String TAG = "FruitAdapter";
   
       //重写区分不同的元素，不同的type
       @Override
       public int getItemViewType(int position) {
           return mFruitList.get(position).getType();
       }
   
       //不同的type创建不同的布局 
       @Override
       public BaseHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int type) {
           View view;
           final BaseHolder holder;
           if (type== 1) {
               view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.fruit_item, viewGroup, false);
               holder = new ItemHolder(view);
           } else {
               view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.head_item, viewGroup, false);
               holder = new HeadHolder(view);
           }
           return holder;
       }
   
       //填充不同的数据
       @Override
       public void onBindViewHolder(@NonNull BaseHolder viewHolder, int i) {
           Fruit fruit = mFruitList.get(i);
           viewHolder.setData(fruit);
       }
   
       @Override
       public int getItemCount() {
           return mFruitList.size();
       }
   
       //不同布局的Viewholder，先创建一个baseHolder，然后item和顶吸的分别继承
       public abstract class BaseHolder extends RecyclerView.ViewHolder {
           public abstract void setData(Fruit fruit);
   
           public BaseHolder(@NonNull View itemView) {
               super(itemView);
           }
       }
   
       
       public class ItemHolder extends BaseHolder {
           private TextView textView;
   
           @Override
           public void setData(Fruit fruit) {
               textView.setText(fruit.getName());
           }
   
           public ItemHolder(@NonNull View itemView) {
               super(itemView);
               textView = (TextView) itemView.findViewById(R.id.fruit_name);
           }
       }
   
       public class HeadHolder extends BaseHolder {
           private TextView textView;
   
           @Override
           public void setData(Fruit fruit) {
               textView.setText(fruit.getName().substring(0,1));
           }
   
           public HeadHolder(@NonNull View itemView) {
               super(itemView);
               textView = (TextView) itemView.findViewById(R.id.head_name);
           }
       }
   
       public FruitAdapter(List<Fruit> fruitList) {
           mFruitList = fruitList;
       }
   }

• 这个adapter中提供了两个viewholder，一个是普通item，另一个是分割线
• 重写getItemViewType方法定义区分两种holder的方法，上面的区分方法是通过bean的type区分
• onCreateViewHolder通过上面的方法判定不同的viewtype构造不同的holder
• onBindViewHolder填充数据

3. 分割线
   PinnedHeaderDecoration.java

   public class PinnedHeaderDecoration extends RecyclerView.ItemDecoration {
       private int mHeaderPosition;
       private int mPinnedHeaderTop;
       private static final String TAG = "PinnedHeaderDecoration";
       private boolean mIsAdapterDataChanged;
       private Rect mClipBounds;
       //是在顶部固定的那个框框
       private View mPinnedHeaderView;
       //父布局的整个adapter
       private RecyclerView.Adapter mAdapter;
       //一个安卓特有的稀疏数组，取代hashmap，key-value保存值，只需要指定value的类型即可
       private final SparseArray<PinnedHeaderCreator> mTypePinnedHeaderFactories = new SparseArray<>();
       //recyclerview适配器的监听
       private final RecyclerView.AdapterDataObserver mAdapterDataObserver = new RecyclerView.AdapterDataObserver() {
           @Override
           public void onChanged() {
               mIsAdapterDataChanged = true;
           }
       };
       //初始化
       public PinnedHeaderDecoration() {
           this.mHeaderPosition = -1;
       }
       @Override
       public void onDraw(Canvas c, final RecyclerView parent, RecyclerView.State state) {
           createPinnedHeader(parent);
   
           //mPinnedHeaderView是顶部固定的框框
           if (mPinnedHeaderView != null) {
               int headerEndAt = mPinnedHeaderView.getTop() + mPinnedHeaderView.getHeight();
               //获取列表可见部分的第一个view
               View v = parent.findChildViewUnder(c.getWidth() / 2, headerEndAt + 1);
   
               //判断这个v是不是分隔，如果是分割那么顶部的框框的顶要向上移动，否则就在0处，即最顶端
               if (isPinnedView(parent, v)) {
                   mPinnedHeaderTop = v.getTop() - mPinnedHeaderView.getHeight();
               } else {
                   mPinnedHeaderTop = 0;
               }
   
               //开始画分隔线，分隔线的头部就是顶部框框的底部
               mClipBounds = c.getClipBounds();
               mClipBounds.top = mPinnedHeaderTop + mPinnedHeaderView.getHeight();
               c.clipRect(mClipBounds);
           }
       }
       @Override
       public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
           if (mPinnedHeaderView != null) {
               //先保存c的状态
               c.save();
   
               mClipBounds.top = 0;
               c.clipRect(mClipBounds, Region.Op.UNION);
               //平移到框框顶部
               c.translate(0, mPinnedHeaderTop);
               //绘制出顶部的框框
               mPinnedHeaderView.draw(c);
   
               //把c还原到save的状态
               c.restore();
           }
       }
       private void createPinnedHeader(RecyclerView parent) {
           updatePinnedHeader(parent);
   
           RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
           if (layoutManager == null || layoutManager.getChildCount() <= 0) {
               return;
           }
   
           //获取第一个元素在整个recyclerview中是第几个
           int firstVisiblePosition = ((RecyclerView.LayoutParams) layoutManager.getChildAt(0).getLayoutParams()).getViewAdapterPosition();
           int headerPosition = findPinnedHeaderPosition(parent, firstVisiblePosition);
   
           if (headerPosition >= 0 && mHeaderPosition != headerPosition) {
               mHeaderPosition = headerPosition;
               int viewType = mAdapter.getItemViewType(headerPosition);
   
               RecyclerView.ViewHolder pinnedViewHolder = mAdapter.createViewHolder(parent, viewType);
               mAdapter.bindViewHolder(pinnedViewHolder, headerPosition);
               mPinnedHeaderView = pinnedViewHolder.itemView;
   
               // read layout parameters
               ViewGroup.LayoutParams layoutParams = mPinnedHeaderView.getLayoutParams();
               if (layoutParams == null) {
                   layoutParams = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT);
                   mPinnedHeaderView.setLayoutParams(layoutParams);
               }
   
               int heightMode = View.MeasureSpec.getMode(layoutParams.height);
               int heightSize = View.MeasureSpec.getSize(layoutParams.height);
   
               if (heightMode == View.MeasureSpec.UNSPECIFIED) {
                   heightMode = View.MeasureSpec.EXACTLY;
               }
   
               int maxHeight = parent.getHeight() - parent.getPaddingTop() - parent.getPaddingBottom();
               if (heightSize > maxHeight) {
                   heightSize = maxHeight;
               }
   
               // measure & layout
               int ws = View.MeasureSpec.makeMeasureSpec(parent.getWidth() - parent.getPaddingLeft() - parent.getPaddingRight(), View.MeasureSpec.EXACTLY);
               int hs = View.MeasureSpec.makeMeasureSpec(heightSize, heightMode);
               mPinnedHeaderView.measure(ws, hs);
               mPinnedHeaderView.layout(0, 0, mPinnedHeaderView.getMeasuredWidth(), mPinnedHeaderView.getMeasuredHeight());
           }
       }
       private int findPinnedHeaderPosition(RecyclerView parent, int fromPosition) {
           if (fromPosition > mAdapter.getItemCount() || fromPosition < 0) {
               return -1;
           }
   
           for (int position = fromPosition; position >= 0; position--) {
               final int viewType = mAdapter.getItemViewType(position);
               if (isPinnedViewType(parent, position, viewType)) {
                   return position;
               }
           }
   
           return -1;
       }
       private boolean isPinnedViewType(RecyclerView parent, int adapterPosition, int viewType) {
           PinnedHeaderCreator pinnedHeaderCreator =  mTypePinnedHeaderFactories.get(viewType);
   
           return pinnedHeaderCreator != null && pinnedHeaderCreator.create(parent, adapterPosition);
       }
   
       private boolean isPinnedView(RecyclerView parent, View v) {
           int position = parent.getChildAdapterPosition(v);
           //如果获取的是非法值
           if (position == RecyclerView.NO_POSITION) {
               return false;
           }
   
           return isPinnedViewType(parent, position, mAdapter.getItemViewType(position));
       }
       private void updatePinnedHeader(RecyclerView parent) {
           RecyclerView.Adapter adapter = parent.getAdapter();
           //获取的是recyclerview所有的adapter，如果adapter发生改变或者监听到内容发生改变执行
           //重新绘制
           if (mAdapter != adapter || mIsAdapterDataChanged) {
               //重置
               resetPinnedHeader();
               if (mAdapter != null) {
                   mAdapter.unregisterAdapterDataObserver(mAdapterDataObserver);
               }
   
               mAdapter = adapter;
               if (mAdapter != null) {
                   //设置监听
                   mAdapter.registerAdapterDataObserver(mAdapterDataObserver);
               }
           }
       }
       //重置header
       private void resetPinnedHeader() {
           mHeaderPosition = -1;
           mPinnedHeaderView = null;
       }
       public void registerTypePinnedHeader(int itemType, PinnedHeaderCreator pinnedHeaderCreator) {
           mTypePinnedHeaderFactories.put(itemType, pinnedHeaderCreator);
       }
       public interface PinnedHeaderCreator {
           boolean create(RecyclerView parent, int adapterPosition);
       }
   }

4. 使用
   MainActivity.java

   final PinnedHeaderDecoration decoration = new PinnedHeaderDecoration();
   decoration.registerTypePinnedHeader(1, new PinnedHeaderDecoration.PinnedHeaderCreator() {
       @Override
       public boolean create(RecyclerView parent, int adapterPosition) {
           return true;
       }
   });
   mRecyclerView.addItemDecoration(decoration);

• registerTypePinnedHeader传的参数1是当adapter那个getviewtype中返回的是1的时候调用create方法，如果返回的是true，那就让他顶吸