public int getScollYDistance() {
  int position = layoutManager.findFirstVisibleItemPosition();
  View firstVisiableChildView = layoutManager.findViewByPosition(position);
  int firstVisiableChildViewTop = firstVisiableChildView.getTop();
  int itemHeight = firstVisiableChildView.getHeight();
  //可见的item的index*item高度-最顶端位置
  return (position) * itemHeight - firstVisiableChildViewTop;
 }

 public void selectItem() {

  //弹出top返回顶部按钮
  if (getScollYDistance()>=400){
   backTop.setVisibility(View.VISIBLE);
  }else {
   backTop.setVisibility(View.GONE);
  }

 }

if (getScollYDistance() <= 0) {   
   float scale = (float) getScollYDistance() / 400;
   float alpha = (255 * scale);
   // 只是layout背景透明(仿知乎滑动效果)
   tvTitle.setBackgroundColor(Color.argb((int) alpha, 254, 184, 6));
  } else {
//   tvTitle.setBackgroundColor(Color.argb((int) 0, 254, 184, 6));
   tvTitle.setVisibility(View.GONE);
  }

   @Override
   public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
    super.onScrolled(recyclerView, dx, dy);

    selectItem();
}

  backTop.setOnClickListener(new View.OnClickListener() {
   @Override
   public void onClick(View v) {
    recyclerView.scrollToPosition(0);
   }
  });

 <RelativeLayout
  android:id="@+id/view_relat"
  android:layout_width="match_parent"
  android:layout_height="match_parent">
<android.support.v4.widget.SwipeRefreshLayout
 android:id="@+id/swipeRefreshLayout"
 android:layout_width="match_parent"
 android:layout_height="match_parent">
<android.support.v7.widget.RecyclerView
 android:id="@+id/recyclerView"
 android:dividerHeight="2dp"
 android:layout_width="match_parent"
 android:layout_height="match_parent"/>
</android.support.v4.widget.SwipeRefreshLayout>
 <RelativeLayout
  android:layout_width="80dp"
  android:layout_height="80dp"
  android:layout_alignParentBottom="true"
  android:layout_alignParentEnd="true">
 <ImageView
  android:id="@+id/back_top"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:src="@drawable/back_top_icon"
  android:visibility="gone"/>
 </RelativeLayout>
 </RelativeLayout>