public static abstract class ItemDecoration {
 ...
 public void onDraw(Canvas c, RecyclerView parent, State state) {
  onDraw(c, parent);
 }
 public void onDrawOver(Canvas c, RecyclerView parent, State state) {
  onDrawOver(c, parent);
 }
 public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
  getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
    parent);
 }
}

private int mHeight = 5; //分割线高度

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
 super.getItemOffsets(outRect, view, parent, state);
 int position = parent.getChildAdapterPosition(view);
 if (position != 0) {
  //第一个item预留空间
  outRect.top = mHeight;
 }
}

@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
 super.onDraw(c, parent, state);
 final int left = parent.getLeft();
 final int right = parent.getRight();
 final int childCount = parent.getChildCount();
 for (int i = 0; i < childCount; i++) {
  final View childView = parent.getChildAt(i);
  final int bottom = childView.getTop();
  final int top = bottom - mHeight;
  c.drawRect(left, top, right, bottom, mPaint);
 }
}

public interface GroupListener {
 String getGroupName(int position);
}

//判断是不是组中的第一个位置
//根据前一个组名，判断当前是否为新的组
private boolean isFirstInGroup(int pos) {
 if (pos == 0) {
 return true;
 } else {
 String prevGroupId = getGroupName(pos - 1);
 String groupId = getGroupName(pos);
 return !TextUtils.equals(prevGroupId, groupId);
 }
}

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
 super.getItemOffsets(outRect, view, parent, state);
 int pos = parent.getChildAdapterPosition(view);
 String groupId = getGroupName(pos);
 if (groupId == null) return;
 //只有是同一组的第一个才显示悬浮栏
 if (pos == 0 || isFirstInGroup(pos)) {
 outRect.top = mGroupHeight;
 }
}

@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDrawOver(c, parent, state);
final int itemCount = state.getItemCount();
final int childCount = parent.getChildCount();
final int left = parent.getLeft() + parent.getPaddingLeft();
final int right = parent.getRight() - parent.getPaddingRight();
String preGroupName;  //标记上一个item对应的Group
String currentGroupName = null;  //当前item对应的Group
for (int i = 0; i < childCount; i++) {
 View view = parent.getChildAt(i);
 int position = parent.getChildAdapterPosition(view);
 preGroupName = currentGroupName;
 currentGroupName = getGroupName(position);
 if (currentGroupName == null || TextUtils.equals(currentGroupName, preGroupName))
  continue;
 int viewBottom = view.getBottom();
 float top = Math.max(mGroupHeight, view.getTop());//top 决定当前顶部第一个悬浮Group的位置
 if (position + 1 < itemCount) {
  //获取下个GroupName
  String nextGroupName = getGroupName(position + 1);
  //下一组的第一个View接近头部
  if (!currentGroupName.equals(nextGroupName) && viewBottom < top) {
   top = viewBottom;
  }
 }
 //根据top绘制group
 c.drawRect(left, top - mGroupHeight, right, top, mGroutPaint);
 Paint.FontMetrics fm = mTextPaint.getFontMetrics();
 //文字竖直居中显示
 float baseLine = top - (mGroupHeight - (fm.bottom - fm.top)) / 2 - fm.bottom;
 c.drawText(currentGroupName, left + mLeftMargin, baseLine, mTextPaint);
 }
}

float top = Math.max(mGroupHeight, view.getTop());

if (!currentGroupName.equals(nextGroupName) && viewBottom < top) {
  top = viewBottom;
 }

public interface PowerGroupListener {
 String getGroupName(int position);
 View getGroupView(int position);
}

@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
 ...
 for (int i = 0; i < childCount; i++) {
  ...
  //根据position获取View
  View groupView = mGroupListener.getGroupView(position);
  if (groupView == null) return;
  ViewGroup.LayoutParams layoutParams = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, mGroupHeight);
groupView.setLayoutParams(layoutParams);
  groupView.setDrawingCacheEnabled(true);
  groupView.measure(
    View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED),
    View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED));
  //指定高度、宽度的groupView
  groupView.layout(0, 0, right, mGroupHeight);
  groupView.buildDrawingCache();
  Bitmap bitmap = groupView.getDrawingCache();
  c.drawBitmap(bitmap, left, top - mGroupHeight, null);
 }
}

repositories {
 jcenter()// If not already there
}
dependencies {
 compile 'com.gavin.com.library:stickyDecoration:1.0.2'
}