android - 恢复 fragment 时如何维护在ViewPager中 fragment 的onCreateView中创建的 View 实例？

Question

故事是 ViewPager 中有五个 fragment ，每个 fragment 在出现在 ViewPager 上时都必须运行一个 AsyncTask。我正在尝试与正确答案完全相同的 here .我将 AsyncTask 放在 onResumeFragment 中，它工作正常。但是当我在 AsyncTask 成功运行后更新 View 时，问题就开始了。因为当我更新数据集时，我在 onCreateView 中创建的 View (RecyclerView) 为 null。我还在 onActivityCreated 中尝试了 setRetainInstance(true)。它没有用。

这是我的代码，

主 Activity .java

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

    //Tab & Pager

    viewPager = (ViewPager) findViewById(R.id.main_tab_content);
    adapter = new ViewPagerAdapter(getSupportFragmentManager());
    viewPager.setAdapter(adapter);
    addPagerListener();
}

private void addPagerListener(){
    viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int newPosition) {
            FragmentLifecycle fragmentToShow = (FragmentLifecycle)adapter.getItem(newPosition);
            fragmentToShow.onResumeFragment();

            FragmentLifecycle fragmentToHide = (FragmentLifecycle)adapter.getItem(currentPosition);
            fragmentToHide.onPauseFragment();

            currentPosition = newPosition;
        }

        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        }

        @Override
        public void onPageScrollStateChanged(int state) {

        }
    });
}

class ViewPagerAdapter extends FragmentPagerAdapter {
    public ViewPagerAdapter(FragmentManager manager) {
        super(manager);
    }

    @Override
    public Fragment getItem(int position) {
        switch (position){
            case 0: return new HomeFragment();
            case 1: return new ArtistFragment();
            case 2: return new AlbumsFragment();
            case 3: return new CollectionsFragment();
            case 4: return new SavedFragments();
            default: return null;
        }
    }

    @Override
    public int getCount() {
        return 5;
    }

}

这是我的第二个 fragment 代码，到目前为止其余 fragment 都是空白的。

public class ArtistFragment extends Fragment implements FragmentLifecycle{

@BindView(R.id.recyclerview_frag_artist)
RecyclerView mRecyclerView;

RecyclerView.Adapter mAdapter;
RecyclerView.LayoutManager mLayoutManager;

@BindView(R.id.radioBtnAll_frag_artist)
RadioButton radioAll;

@BindView(R.id.radioBtnMale_frag_artist)
RadioButton radioMale;

@BindView(R.id.radioBtnFemale_frag_artist)
RadioButton radioFemale;

ArrayList<String> vocalistNameList;
Context context;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_artists, container, false);
    ButterKnife.bind(this, rootView);
    mRecyclerView.setHasFixedSize(true);
    mLayoutManager = new LinearLayoutManager(rootView.getContext());
    mRecyclerView.setLayoutManager(mLayoutManager);
    vocalistNameList = getDummyDS();
    mAdapter = new MyListAdapter(vocalistNameList);
    mRecyclerView.setAdapter(mAdapter);
    radioAll.setChecked(true);
    return rootView;
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    setRetainInstance(true);
}

private ArrayList<String> getDummyDS(){
    ArrayList<String> dataset = new ArrayList<>();
    dataset.add("test");
    dataset.add("test");
    dataset.add("test");
    dataset.add("test");
    dataset.add("test");
    dataset.add("test");
    dataset.add("test");
    return dataset;
}

private void getVocalistList(Gender gender){
    RequestParams params = new RequestParams();
    params.put("action","query");
    params.put("list","allcategories");
    params.put("acprefix","Vocalist-");
    params.put("format","json");
    params.put("aclimit","5000");

    MyRestClient.get("", params, new JsonHttpResponseHandler(){
        ProgressDialog progress;
        @Override
        public void onStart() {
            progress = ProgressDialog.show(AppConstants.activity, "Please wait",
                        "Loading data..", true);
            progress.setCancelable(true);
        }

        @Override
        public void onSuccess(int statusCode, Header[] headers, JSONObject response) {
            progress.dismiss();
            vocalistNameList = new ArrayList<String>();
            try {
                JSONArray vocalListJsonArray = response.getJSONObject("query").getJSONArray("allcategories");
                for(int i=0; i<vocalListJsonArray.length(); i++){
                    String vocalListName = vocalListJsonArray.getJSONObject(i).getString("*").replace("Vocalist-","");
                    vocalistNameList.add(vocalListName);
                }
                mAdapter = new MyListAdapter(vocalistNameList);
                mRecyclerView.setAdapter(mAdapter); // <= RecyclerView is null and app is crashing
                mAdapter.notifyDataSetChanged();
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onFailure(int statusCode, Header[] headers, String responseString, Throwable throwable) {
            progress.dismiss();
            Log.i("failure", "failed "+statusCode);
        }
    });
}

@Override
public void onPauseFragment() {
    Toast.makeText(AppConstants.applicationContext, "onPauseFragment():", Toast.LENGTH_SHORT).show();
}

@Override
public void onResumeFragment() {
    getVocalistList(Gender.All);
    Toast.makeText(AppConstants.applicationContext, "onResumeFragment(): Artist", Toast.LENGTH_SHORT).show();
}

自一周前以来，我一直在尝试解决该问题。任何的想法？ (P.S 我正在使用 Android Asynchronous Http Client 而不是 AsyncTask)

Answer 1

你必须做这样的事情:

pager.setOffscreenPageLimit([quantity of tabs that you would like to keep in memory]);

看看这个答案。 ViewPager inside Fragment loses content when returning to it