Recyclerview滚动侦听器无法正常工作

Question

我在片段中使用了recyclerview。我想在向下滚动时实现移动到顶部功能。所以我在recyclerview中添加了一个滚动监听器，但是在片段onViewCreated方法调用时触发了一次。滚动此方法时不会触发，因此dy值始终为零。我不知道为什么这个问题在片段的情况下，请回答我，谢谢。

// HomeFragment.java

    public class HomeFragment extends Fragment {
  ....
   public HomeFragment() {

}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment

    view = inflater.inflate(R.layout.fragment_home, container, false);



    recyclerView = (RecyclerView)view.findViewById(R.id.indiafragment_rview);
    fab = (FloatingActionButton) view.findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
            smoothScroller.setTargetPosition(0);
            mLayoutManager.startSmoothScroll(smoothScroller);

        }
    });
    recyclerView.setNestedScrollingEnabled(false);

    sharedPreferences = getActivity().getSharedPreferences("News", Context.MODE_PRIVATE);



    mLayoutManager = new LinearLayoutManager(getContext());
    recyclerView.setLayoutManager(mLayoutManager);



    getdata();


    recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);
            Toast.makeText(getActivity(), "Scroll changed", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);
            if (dy > 0 && fab.getVisibility() == View.VISIBLE) {
                Toast.makeText(getActivity(), "Scroll down", Toast.LENGTH_SHORT).show();
                fab.hide();
            } else if (dy < 0 && fab.getVisibility() != View.VISIBLE) {
                Toast.makeText(getActivity(), "Scroll up", Toast.LENGTH_SHORT).show();
                fab.show();
            }

            if (dy > 0 ) {
                Toast.makeText(getActivity(), "Scroll down", Toast.LENGTH_SHORT).show();
                fab.hide();
            } else if (dy < 0 ) {
                Toast.makeText(getActivity(), "Scroll up", Toast.LENGTH_SHORT).show();
                fab.show();
            }else
    {
      Toast.makeText(getActivity(), "dy = "+dy, Toast.LENGTH_SHORT).show();
    }


        }

    });



    smoothScroller = new LinearSmoothScroller(getContext()) {
        @Override protected int getVerticalSnapPreference() {
            return LinearSmoothScroller.SNAP_TO_START;
        }
    };




    return view;
}}

// fragment_home.xml

     <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:app="http://schemas.android.com/apk/res-auto"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context=".HomeFragment">

<!-- TODO: Update blank fragment layout -->
<android.support.v4.widget.NestedScrollView
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">




        <ImageView
            android:layout_width="match_parent"
            android:layout_height="60dp"
            android:scaleType="fitXY"
            android:id="@+id/bannerimg"
            android:src="@drawable/banner_sample"/>
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_below="@id/bannerimg">

            <android.support.v7.widget.CardView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:cardElevation="8dp"
                >
                <android.support.v7.widget.RecyclerView
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:id="@+id/fragment_rview"
                    >
                </android.support.v7.widget.RecyclerView>
            </android.support.v7.widget.CardView>

        </LinearLayout>


    </RelativeLayout>
</android.support.v4.widget.NestedScrollView>

<android.support.design.widget.FloatingActionButton
    android:id="@+id/fab"
    app:backgroundTint="#FFFFFF"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom|end"
    android:layout_margin="@dimen/fab_margin"
    app:srcCompat="@drawable/movetop" />