ViewModel 观察者方法返回 null

Question

我收到以下错误

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

从我的主片段中的以下代码部分

    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);
        }
    }); 

错误是专门在这一行抛出的（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 onError(Exception e) {
                // Log.e(TAG, e.getMessage());
                MutableLiveData<List<_Release>> releases = new MutableLiveData<>();
                releases.setValue(null);
                upcomingReleasesListMap.put(filter, releases);
            }
        });
    }
}

Hashmap 是我如何将数据提供给我的片段的一个组成部分。由于我现在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）的回收器视图，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platforms列表等操作时）

Answer 1

您在实现这些方法时没有考虑异步代码，最终得到一个 null LiveData，您在该代码上调用 observe() 并抛出异常。您当前使用的流程：

• 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() 只是为后台操作（firebase）设置一个监听器并立即返回
• 您到达 returncomingcomingReleasesListMap.get(filter); 行，这将返回 null，因为 firebase 侦听器很可能没有完成，并且您没有在地图中为该过滤器添加任何内容价值
• 使用 null LiveData 并失败

您需要类似下面的代码才能使其工作：

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 onError(Exception e) {
            // Log.e(TAG, e.getMessage());
            MutableLiveData<List<_Release>> releases = upcomingReleasesListMap.get(filter);
            releases.setValue(null);                
        }
    });
}