android – ViewModel observer方法返回null

我收到以下错误

Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observe on a null object reference

从我的主要片段中的以下部分代码

    mReleasesViewModel = ViewModelProviders.of(this).get(ReleasesViewModel.class);
    mReleasesViewModel.getUpcomingReleases(filter).observe(this, new Observer<List<_Release>>() {
        @Override
        public void onChanged(@Nullable List<_Release> releases) {
            // whenever the list is changed
            if (releases != null) {
                mUpcomingGamesAdapter.setData(releases);
                mUpcomingGamesAdapter.notifyDataSetChanged();
            }
            mDatabaseLoading.setVisibility(View.GONE);
        }
    }); 

错误是在这一行上特别抛出的(当我附加观察者时)

mReleasesViewModel.getUpcomingReleases(filter) ...

我的ViewModel类：

public class ReleasesViewModel extends ViewModel {
    // fragment name and list
    private HashMap<String, MutableLiveData<List<_Release>>> upcomingReleasesListMap = new HashMap<>();

    private ReleasesRepository releasesRepository;
    private ArrayList<Integer> platforms;
    private String region;

    public ReleasesViewModel() {
        // Shared to all fragments : User settings region & platforms
        region = SharedPrefManager.read(SharedPrefManager.KEY_PREF_REGION, "North America");
        Set<String> defaultPlatformsSet = new HashSet<>();
        platforms = SharedPrefManager.read(SharedPrefManager.PLATFORM_IDS, defaultPlatformsSet);
    }

    public MutableLiveData<List<_Release>> getUpcomingReleases(String filter) {
        // ReleasesRepository takes a different monthly filter
        releasesRepository = new ReleasesRepository(region, filter, platforms);

        if (upcomingReleasesListMap.containsKey(filter)) {
            // Double check if it isn't null, just in case
            if (upcomingReleasesListMap.get(filter) == null) {
                // if null; try again to send a new request
                loadReleases(filter);
            } // else just don't do anything, the list is already in the Map
        } else {
            // Load it in if this filter was never added to the map [New filter and new list]
            loadReleases(filter);
        }
        return upcomingReleasesListMap.get(filter);
    }


    private void loadReleases(final String filter) {
        releasesRepository.addListener(new FirebaseDatabaseRepository.FirebaseDatabaseRepositoryCallback<_Release>() {
            @Override
            public void onSuccess(List<_Release> result) {
                // sort by release date
                if (platforms.size() > 1) {
                    // Will only sort for multiple platforms filter
                    Collections.sort(result);
                }
                MutableLiveData<List<_Release>> releases = new MutableLiveData<>();
                releases.setValue(result);
                upcomingReleasesListMap.put(filter, releases);
            }

            @Override
            public void one rror(Exception e) {
                // Log.e(TAG, e.getMessage());
                MutableLiveData<List<_Release>> releases = new MutableLiveData<>();
                releases.setValue(null);
                upcomingReleasesListMap.put(filter, releases);
            }
        });
    }
}

Hashmap是我如何向片段提供数据的不可或缺的一部分.由于我现在如何在我的应用程序中维护6个片段,我需要为所有六个片段使用一个ViewModel.每个片段都有一个具有相同类型数据(对象和视图)的recyclelerview,但它们对该数据应用不同的过滤器.请查看ViewModel中的String filter参数,该参数将过滤器应用于数据.当然,我在ViewModel中维护所有6个片段.因为它们都需要同时更新(当用户完成更改平台列表等操作时)

解决方法:

您有异步代码,在实现这些方法时没有考虑这些异步代码,并且您最终得到一个null的LiveData,您可以在其上调用observe()抛出异常.您目前使用的流程：

>使用未知过滤器调用getUpcomingReleases()
>此过滤器不在地图中,因此您调用loadReleases()
> loadReleases()只为后台操作(firebase)设置一个监听器并立即返回
>你得到的行返回comingReleasesListMap.get(过滤器);这将返回null,因为firebase侦听器很可能没有完成,你不会在地图中为该过滤器值添加任何内容
>使用null LiveData并失败

你需要像下面的代码一样使它工作：

public MutableLiveData<List<_Release>> getUpcomingReleases(String filter) {
   ...
   if (upcomingReleasesListMap.get(filter) == null) {
       // we don't have a mapping for this filter so create one in the map
       MutableLiveData<List<_Release>> releases = new MutableLiveData<>();
       upcomingReleasesListMap.put(filter, releases);  
       // also call this method to update the LiveData
       loadReleases(filter);
    }
    // for now just return the empty LiveData so our ui can use it
    // when the firebase listener returns we will update it
    return upcomingReleasesListMap.get(filter);
}

private void loadReleases(final String filter) {
    releasesRepository.addListener(new FirebaseDatabaseRepository.FirebaseDatabaseRepositoryCallback<_Release>() {
        @Override
        public void onSuccess(List<_Release> result) {
            // sort by release date
            if (platforms.size() > 1) {
                // Will only sort for multiple platforms filter
                Collections.sort(result);
            }
            // just use the previous created LiveData, this time with the data we got 
            MutableLiveData<List<_Release>> releases = upcomingReleasesListMap.get(filter);
            releases.setValue(result);
        }

        @Override
        public void one rror(Exception e) {
            // Log.e(TAG, e.getMessage());
            MutableLiveData<List<_Release>> releases = upcomingReleasesListMap.get(filter);
            releases.setValue(null);                
        }
    });
}

标签：android,viewmodel,android-livedata
来源： https://codeday.me/bug/20190705/1387618.html