从其他活动返回到同一片段时，如何保存和恢复片段中 RecyclerView 的滚动位置？

Question

对版主的要求：这不是重复的问题，请阅读以下信息。

在问这个问题之前，我尝试了几乎所有可用的 SO 解决方案，但没有一个对我有用，有些导致崩溃，有些根本不起作用（我是初学者，有可能我在我的代码中做错了什么，我没有责怪任何人在 SO 上提供的答案在我的情况下不起作用）。以下是我的代码，请看一下：

fragment_tab1.xml（包含recyclerview）：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:background="#333333"
    android:orientation="vertical"
    xmlns:android="http://schemas.android.com/apk/res/android">

    <RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginTop="96dp">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="42dp"
            android:background="@color/colorPrimaryDark"

            android:id="@+id/sort1"
            android:orientation="horizontal">
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/sort"
            android:layout_marginLeft="10dp"
            android:fontFamily="@font/quicksand"
            android:textStyle="bold"
            android:text="Sort by:"
            android:textColor="#ffffff"/>

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginLeft="10dp"
                style="@style/Widget.AppCompat.Button.Colored"
                android:text="Oldest first"/>
            <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
                style="@style/Widget.AppCompat.Button.Colored"
            android:text="Newest first"/>
        </LinearLayout>

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_below="@id/sort1"
        android:clipToPadding="false"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10dp"
        android:padding="5dp" />

    </RelativeLayout>

</LinearLayout>

Fragmenttab1.java

public class tab1  extends Fragment {
    RecyclerView mRecyclerView;
    FirebaseDatabase mFirebaseDatabase;
    DatabaseReference mRef;
    LinearLayoutManager manager;
    private static final String TAG = "tab1";
    ProgressDialog progressDialog;
    View rootView;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        //Returning the layout file after inflating
        //Change R.layout.tab1 in you classes
         rootView = inflater.inflate(R.layout.fragment_tab1, container, false);


return rootView;

    }


    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        mRecyclerView = (RecyclerView) rootView.findViewById(R.id.recycler_view);



        manager = new LinearLayoutManager(getActivity());
        mRecyclerView.setLayoutManager(manager);
        mRecyclerView.setItemViewCacheSize(20);
        mRecyclerView.setDrawingCacheEnabled(true);
        mFirebaseDatabase = FirebaseDatabase.getInstance();
        mRef = mFirebaseDatabase.getReference("memes");

        mFirebaseDatabase.getInstance().getReference().keepSynced(true);

        progressDialog = new ProgressDialog(getActivity());
        progressDialog.setMessage("Please Wait");
        progressDialog.show();

    }

    public void onStart() {

        super.onStart();


        FirebaseRecyclerAdapter<Model, ViewHolder> firebaseRecyclerAdapter =
                new FirebaseRecyclerAdapter<Model, ViewHolder>(
                        Model.class,
                        R.layout.row2,
                        ViewHolder.class,
                        mRef

                ) {



                    @Override
                    protected void populateViewHolder(ViewHolder viewHolder, Model model, int position) {
                        viewHolder.setDetails(getActivity(), model.getTitle(), model.getDesc(), model.getImage());
                        progressDialog.cancel();
                    }



                    @Override
                    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {

                        ViewHolder viewHolder = super.onCreateViewHolder(parent, viewType);

                        viewHolder.setOnClickListener(new ViewHolder.ClickListener(){




                            @Override
                            public void onItemClick(View view, int position){



                                //get data from firebase here
                                String mTitle = getItem(position).getTitle();
                                String mDesc= getItem(position).getDesc();
                                String mImage = getItem(position).getImage();

                                //pass this data to new activity
                                Intent intent = new Intent(view.getContext(), ArticleDetail.class);
                                intent.putExtra("title", mTitle);// put title
                                intent.putExtra("desc", mDesc);// put description
                                intent.putExtra("image", mImage); //put image urls

                                startActivity(intent);
                            }




                            @Override
                            public void onItemLongClick(View view, int position){

                                //TODO your own implementation on long item click


                            }

                        });

                        return viewHolder;
                    }
                };


        mRecyclerView.setAdapter(firebaseRecyclerAdapter);

    }

    @Override
    public void  onPause() {
        super.onPause();
        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(getActivity());

        View firstChild = mRecyclerView.getChildAt(0);
        int firstVisiblePosition = mRecyclerView.getChildAdapterPosition(firstChild);
        int offset = firstChild.getTop();

        Log.d(TAG, "Postition: " + firstVisiblePosition);
        Log.d(TAG, "Offset: " + offset);

        preferences.edit()
                .putInt("position", firstVisiblePosition)
                .putInt("offset", offset)
                .apply();
    }


    @Override
    public void onResume(){
        super.onResume();

    }
    @Override
    public void onSaveInstanceState(Bundle outState) {


        super.onSaveInstanceState(outState);
    }

    @Override
    public void onViewStateRestored(@Nullable Bundle savedInstanceState) {


        super.onViewStateRestored(savedInstanceState);
    }

}

Answer 1

花了这么多天，尝试了很多解决方案，终于找到了解决我问题的有效解决方案，感谢https://stackoverflow.com/a/42563804/2128364

我已将工作解决方案移至其他方法以使其在片段中工作。

也许这可以节省某人的一天，这是它的工作原理：

1. onConfigurationChanged（按照原方案的建议），onSaveInstanceState 或 onViewRestoredState 方法不起作用，因此您需要 onPause 和 onResume 方法分别保存和恢复状态。
2. 您必须在 onPause 方法中保存状态：

mBundleRecyclerViewState = new Bundle();    
mListState = mRecyclerView.getLayoutManager().onSaveInstanceState();
mBundleRecyclerViewState.putParcelable(KEY_RECYCLER_STATE, mListState);

1. 然后在onResume方法中恢复：

if (mBundleRecyclerViewState != null) {
    new Handler().postDelayed(new Runnable() {
        @Override
        public void run() {
            mListState = mBundleRecyclerViewState.getParcelable(KEY_RECYCLER_STATE);
            mRecyclerView.getLayoutManager().onRestoreInstanceState(mListState);
        }
    }, 50);
}    
mRecyclerView.setLayoutManager(staggeredGridLayoutManager);

Answer 2

如果您还没有，请在您的 build.gradle 中添加 recyclerView 依赖项

implementation "androidx.recyclerview:recyclerview:1.2.0"

不仅仅是在初始化列表适配器之后设置stateRestorationPolicy。

 listAdapter = ListAdapter()

 // Set scroll restore policy
 listdapter.stateRestorationPolicy =
      RecyclerView.Adapter.StateRestorationPolicy.PREVENT_WHEN_EMPTY

有关stateRestorationPolicy 和不同类型政策visit this article 的更多信息

Answer 3

你应该使用这个方法来返回第一个可见的项目：

int findFirstVisibleItemPosition();

那么你必须在 onPause() 中保存这个索引，当用户返回时在 onResume() 中检索并设置它：

void scrollToPosition (int position)

Answer 4

问题是作用域，当您移动到另一个活动或片段时，此片段中的变量会被破坏

Answer 5

如果有人介意它的接受答案使用静态对象 (static Bundle mBundleRecyclerViewState) 来保持状态，您可以改用ViewModel。

public class MainViewModel extends ViewModel {
    private Parcelable mRecyclerViewState;
    
    /**
     * Back up RecyclerView's state to ViewModel in Fragment's onDestroyView
     * or in Activity's onStop (or in somewhere else appropriate)
     *
     * @param layoutManager RecyclerView.LayoutManager object
     */
    public void saveRecyclerViewState(RecyclerView.LayoutManager layoutManager) {
        mRecyclerViewState = layoutManager.onSaveInstanceState();
    }

    /**
     * Restore backed up RecyclerView's state from ViewModel in onCreate
     *
     * @param layoutManager RecyclerView.LayoutManager object
     */
    public void restoreRecyclerViewState(RecyclerView.LayoutManager layoutManager) {
        layoutManager.onRestoreInstanceState(mRecyclerViewState);
    }
}

用法示例（在 Fragment 中）：

private MainViewModel mViewModel;
private GridLayoutManager mLayoutManager;

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    mViewModel = new ViewModelProvider(requireActivity()).get(MainViewModel.class);

    mLayoutManager = new GridLayoutManager(requireContext(), ITEMS_PER_ROW);
    mViewModel.restoreRecyclerViewState(mLayoutManager);
}

@Override
public void onDestroyView() {
    mViewModel.saveRecyclerViewState(mLayoutManager);

    super.onDestroyView();
}