我收到以下错误

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

来 self 的主要 fragment 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 是我如何向 fragment 提供数据的不可或缺的一部分。由于我现在在我的应用程序中维护 6 个 fragment ，我需要为所有六个 fragment 使用一个 ViewModel。每个 fragment 都有一个包含相同类型数据(对象和 View )的回收 View ，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，此参数将过滤器应用于数据。当然，我在 ViewModel 中维护所有 6 个 fragment 。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)

最佳答案

您有异步代码，在实现这些方法时没有考虑到这些代码，最终得到一个空的 LiveData，您在该代码上调用 observe() 并抛出异常。您目前使用的流程:

• 使用未知过滤器调用 getUpcomingReleases()
• 此过滤器不在 map 中，因此您调用 loadReleases()
• loadReleases() 只是为后台操作 (firebase) 设置监听器并立即返回
• 您到达 return comcomingReleasesListMap.get(filter); 这行将返回 null，因为 firebase 监听器很可能没有完成并且您没有在该过滤器的映射中放置任何内容值(value)
• 使用空 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);                
        }
    });
}

关于android - ViewModel 观察者方法返回 null，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/53184059/