android - 用列表保存 RecyclerView 状态

Question

我正在尝试保存 RecyclerView 的状态，以便即使在旋转时也能显示数据。我实际上是通过保存 LayoutManager 的状态来解决这个问题的。我在旋转时遇到的错误是:

Process: com.example.android.guardiannewsapp, PID: 25829
                  java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Parcelable android.support.v7.widget.RecyclerView$LayoutManager.onSaveInstanceState()' on a null object reference
                      at com.example.android.guardiannewsapp.MainActivity.onSaveInstanceState(MainActivity.java:80)

我想我了解问题所在，但不确定如何解决。错误指向这个方法:

@Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        mListState = mLayoutManager.onSaveInstanceState();
        outState.putParcelable(STATE_LIST, mListState);
    }

还有这一行

mListState = mLayoutManager.onSaveInstanceState();

这是我的 MainActivity 代码:

包 com.example.android.guardiannewsapp;

import android.app.LoaderManager;
import android.app.LoaderManager.LoaderCallbacks;
import android.content.Context;
import android.content.Loader;
import android.graphics.Canvas;
import android.graphics.drawable.Drawable;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.os.Parcelable;
import android.os.PersistableBundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.DefaultItemAnimator;
import android.support.v7.widget.DividerItemDecoration;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.SearchView;
import android.widget.TextView;

import java.util.List;

public class MainActivity extends AppCompatActivity implements LoaderCallbacks<List<News>> {

    //Constant value for the news loader ID.  We can choose any integer. We do this if we
    //are using multiple loaders. We aren't in this app but good practice.
    private static final int EARTHQUAKE_LAODER_ID = 1;

    RecyclerView recyclerView;
    private TextView emptyView;
    private ProgressBar progressBar;
    private ConnectivityManager cm;
    private TextView internetConnectionEmptyView;
    private List<News> mNews;
    private LoaderManager loaderManager;
    private static final String STATE_LIST = "State Adapter Data";
    RecyclerView.LayoutManager mLayoutManager;
    private Parcelable mListState;

    //Adapter for the list of news articles
    private NewsAdapter mAdapter;

    //User search term
    private String userSearch = "";

    //URL to query the Guardian dataset for search query
    private String GUARDIAN_REQUEST_URL = "";


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Find a reference to the RecyclerView in the layout
        recyclerView = findViewById(R.id.recyclerView);

        //Find a reference to the empty view
        emptyView = findViewById(R.id.empty_view);

        //Find a reference to the progress bar and keep it hidden
        progressBar = findViewById(R.id.progress_bar);
        progressBar.setVisibility(View.GONE);

        //Find a reference to the no internet connection message.
        internetConnectionEmptyView = findViewById(R.id.no_internet_connection);

        recyclerView.addItemDecoration(new DividerItemDecoration(getApplicationContext()));

    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        mListState = mLayoutManager.onSaveInstanceState();
        outState.putParcelable(STATE_LIST, mListState);
    }

    @Override
    public void onRestoreInstanceState(Bundle outState) {
        super.onRestoreInstanceState(outState);

        if (outState != null) {
            mListState = outState.getParcelable(STATE_LIST);
        }

    }

    @Override
    protected void onResume() {
        super.onResume();
        if (mListState != null) {
            mLayoutManager.onRestoreInstanceState(mListState);
        }
    }



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.options_menu, menu);
        // Associate searchable configuration with the SearchView
        final SearchView searchView =
                (SearchView) menu.findItem(R.id.search).getActionView();
        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
            @Override
            public boolean onQueryTextSubmit(String query) {
                userSearch = query;

                cm = (ConnectivityManager)getSystemService(Context.CONNECTIVITY_SERVICE);
                NetworkInfo activeNetwork = cm.getActiveNetworkInfo();

                //Initialize the loader.  Pass in the int ID constant defined above and pass in null
                //for the bundle.  Pass in this activity for the LoaderCallbacks parameter (which is valid
                //because this activity implements the LoaderCallbacks interace).
                if (activeNetwork != null && activeNetwork.isConnected()) {
                    // Get a reference to the LoaderManager, in order to interact with loaders.
                    loaderManager = getLoaderManager();

                    //Initialize the loader.  Pass in the int ID constant defined above and pass in null
                    //for the bundle.  Pass in the activity for the LoaderCallbacks parameter (which is valid
                    //because this activity implements the LoaderCallbacks interface.)
                    loaderManager.initLoader(EARTHQUAKE_LAODER_ID, null, MainActivity.this);

                    loaderManager.restartLoader(EARTHQUAKE_LAODER_ID, null, MainActivity.this);
                } else {
                    //If there is no network connection, hide the loading indicator and show the
                    //no internet connection message.
                    internetConnectionEmptyView.setVisibility(View.VISIBLE);
                    progressBar.setVisibility(View.INVISIBLE);
                }

                searchView.clearFocus();
                return true;
            }

            @Override
            public boolean onQueryTextChange(String newText) {
                return false;
            }
        });{
        }

        return true;
    }

    //When the LoaderManager determines that the loader with our specified ID isn't running to
    //create a new one.
    @Override
    public Loader<List<News>> onCreateLoader(int i, Bundle bundle) {
        GUARDIAN_REQUEST_URL = "http://content.guardianapis.com/search?show-fields=thumbnail&q="+userSearch+
                "&api-key=23a6ee65-f55d-452f-a073-0bc71e36bb8b";

        return new NewsLoader(this, GUARDIAN_REQUEST_URL);
    }

    @Override
    public void onLoadFinished(Loader<List<News>> loader, List<News> news) {

        mNews = news;
        //If the list is empty, the app will show the emptyView message
        if (news.isEmpty()){
            emptyView.setVisibility(View.VISIBLE);
            recyclerView.setVisibility(View.INVISIBLE);
            progressBar.setVisibility(View.INVISIBLE);
        } else {

            //Once the view populates, hide the progress bar.
            progressBar.setVisibility(View.INVISIBLE);

            //Create a new adapter that takes an empty list of news articles as input
            mAdapter = new NewsAdapter(this, news);

            //Create a new adapter that takes an empty list of news articles as input
            mAdapter = new NewsAdapter(this, news);

            //Set the adapter on the RecyclerView so the list can be populated in the user interface.
            mLayoutManager = new LinearLayoutManager(getApplicationContext());
            recyclerView.setLayoutManager(mLayoutManager);
            recyclerView.setItemAnimator(new DefaultItemAnimator());
            recyclerView.setAdapter(mAdapter);
        }

    }

    @Override
    public void onLoaderReset(Loader<List<News>> loader) {

    }

    public class DividerItemDecoration extends RecyclerView.ItemDecoration {
        private Drawable mDivider;

        public DividerItemDecoration(Context context){
            mDivider = context.getResources().getDrawable(R.drawable.horizontal_line);
        }

        @Override
        public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
            int dividerLeft = parent.getPaddingLeft();
            int dividerRight = parent.getWidth() - parent.getPaddingRight();

            int childCount = parent.getChildCount();
            for (int i = 0; i < childCount - 1; i++) {
                View child = parent.getChildAt(i);

                RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();

                int dividerTop = child.getBottom() + params.bottomMargin;
                int dividerBottom = dividerTop + mDivider.getIntrinsicHeight();

                mDivider.setBounds(dividerLeft, dividerTop, dividerRight, dividerBottom);
                mDivider.draw(c);
            }
        }
    }

}

Answer 1

看起来mLayoutManager还没有设置。由于是在onLoadFinished()中设置的，要么onLoadFinished()没有运行，要么news.isEmpty()为true，所以mLayoutManager 仍然不会被设置。

应该没问题，在onCreate()中设置mLayoutManager即可。那应该可以解决问题。您也可以在 onSaveInstanceState() 中使用它之前检查 mLayoutManager 是否为 null。