android - 无法恢复 RecyclerView 的位置/状态

Question

每当我从 DetailsActivity 移动到 MainActivity 时，RecyclerView 都会重置并且列表始终从顶部位置显示。

完整源码可见here .该代码在很大程度上仍在进行中。

MainActivityFragment is as follows.

public class MainActivityFragment extends Fragment {

    private ArrayList<Movie> mMovieArrayList = new ArrayList<Movie>();
    private static final String PAGE = "1";
    private RecyclerView mRecyclerView;

    private SharedPreferences mSettings;
    private SharedPreferences.Editor mEditor;

    private static final String LOG = MainActivityFragment.class.getSimpleName();

    public MainActivityFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_movie_list, container, false);

        // Set column size to 2 for default and portrait
        // and 3 for landscape orientations
        int column = Integer.parseInt(getString(R.string.grid_portrait));
        if (getResources().getConfiguration().orientation == 1) {
            column = Integer.parseInt(getString(R.string.grid_portrait));
        } else if (getResources().getConfiguration().orientation == 2) {
            column = Integer.parseInt(getString(R.string.grid_landscape));
        }

        if (getActivity().findViewById(R.id.movie_detail_container) != null) {
            column = Integer.parseInt("2");
        }

        mSettings = PreferenceManager.getDefaultSharedPreferences(getActivity());
        mEditor = mSettings.edit();
        mEditor.apply();

        mRecyclerView = (RecyclerView) rootView.findViewById(R.id.recyclerview);
        mRecyclerView.setHasFixedSize(true);
        mRecyclerView.setLayoutManager(new GridLayoutManager(getActivity(), column));

        mRecyclerView.setAdapter(new MovieAdapter(getActivity(), mMovieArrayList));
        return rootView;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);
    }

    @Override
    public void onStart() {
        super.onStart();
        updateMovieList();
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        inflater.inflate(R.menu.menu_main_fragment, menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.popularity:
                mEditor.putString(getResources().getString(R.string.perf_sort),
                        getResources().getString(R.string.url_popularity));
                mEditor.apply();
                updateMovieList();
                item.setChecked(true);
                Log.d(LOG, "onOptionsItemSelected: popularity");
                return true;
            case R.id.rating:
                mEditor.putString(getResources().getString(R.string.perf_sort),
                        getResources().getString(R.string.url_top_rated));
                mEditor.apply();
                updateMovieList();
                item.setChecked(true);
                Log.d(LOG, "onOptionsItemSelected: rating");
                return true;
            case R.id.favorite:
                mEditor.putString(getResources().getString(R.string.perf_sort),
                        getResources().getString(R.string.url_favorite));
                mEditor.apply();
                updateMovieList();
                item.setChecked(true);
                Log.d(LOG, "onOptionsItemSelected: favorite");
                return true;
        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onPrepareOptionsMenu(Menu menu) {
        super.onPrepareOptionsMenu(menu);

        String sortBy = mSettings.getString(getResources().getString(R.string.perf_sort),
                getResources().getString(R.string.url_popularity));

        if (sortBy.equals(getResources().getString(R.string.url_popularity))) {
            menu.findItem(R.id.popularity).setChecked(true);
        } else if (sortBy.equals(getResources().getString(R.string.url_top_rated))) {
            menu.findItem(R.id.rating).setChecked(true);
        } else if (sortBy.equals(getResources().getString(R.string.url_favorite))) {
            menu.findItem(R.id.favorite).setChecked(true);
        }
    }

    private void updateMovieList() {
        mMovieArrayList = new ArrayList<>();
        String sortBy = mSettings.getString(getResources().getString(R.string.perf_sort),
                getResources().getString(R.string.url_popularity));

        if (sortBy.equals(getResources().getString(R.string.url_popularity)) ||
                sortBy.equals(getResources().getString(R.string.url_top_rated))) {

            try {
                mMovieArrayList =
                        new FetchMoviesTask().execute(sortBy, PAGE).get();
            } catch (ExecutionException | InterruptedException ei) {
                ei.printStackTrace();
            }
        } else if (sortBy.equals(getResources().getString(R.string.url_favorite))) {
            ContentResolver resolver = getActivity().getContentResolver();
            Cursor cursor =
                    resolver.query(MovieContract.MovieEntry.CONTENT_URI,
                            null,
                            null,
                            null,
                            null);

            if (cursor != null) {
                if (cursor.moveToFirst()) {
                    do {
                        String title = cursor.getString(cursor.getColumnIndex(MovieContract.MovieEntry.TITLE));
                        String movie_id = cursor.getString(cursor.getColumnIndex(MovieContract.MovieEntry.MOVIE_ID));
                        String poster = cursor.getString(cursor.getColumnIndex(MovieContract.MovieEntry.POSTER));
                        String backdrop = cursor.getString(cursor.getColumnIndex(MovieContract.MovieEntry.BACKDROP));
                        String overview = cursor.getString(cursor.getColumnIndex(MovieContract.MovieEntry.OVERVIEW));
                        String vote_average = cursor.getString(cursor.getColumnIndex(MovieContract.MovieEntry.VOTE_AVERAGE));
                        String release_date = cursor.getString(cursor.getColumnIndex(MovieContract.MovieEntry.DATE));

                        Movie movie = new Movie(title, release_date, poster,
                                vote_average, overview, backdrop, movie_id);
                        mMovieArrayList.add(movie);
                    } while (cursor.moveToNext());
                }
            }

            if (cursor != null)
                cursor.close();
        }

        mRecyclerView.setAdapter(new MovieAdapter(getActivity(), mMovieArrayList));
        mRecyclerView.getAdapter().notifyDataSetChanged();
    }
}

MainActivity如下。

public class MainActivity extends AppCompatActivity {

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

        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

//        if (id == android.R.id.home) {
//            onBackPressed();
//            return true;
//        }

        return super.onOptionsItemSelected(item);
    }
}

我已尝试设置保存 RecyclerView 的状态，如 post 所示.

我也试过在 onSaveInstanceState 中保存 mArrayList 并在 onActivityCreated 或 onViewCreated 中恢复它，就像这样post .

这两个都不适合我。一旦我从 DetailsActivity 返回到 MainActivity，位置就消失了。

谢谢。

Answer 1

1. 您正在 fragment 生命周期的 onStart 回调中调用一种方法来更新电影列表。这意味着当您的主要 Activity 重新显示时，您正在重置您的列表。你不应该。
2. 您在更新列表方法的末尾设置了一个新的适配器实例。这会重绘整个列表。你应该重新考虑你的生命周期。仅在 onCreate 或 onCreateView 中调用更新电影列表，并始终检查适配器对象是否为空，然后才在回收器 View 上使用 setAdapter 方法。如果适配器不为空，只需调用 notifyDataSetChanged 或更新项目所需的任何方法。