我遇到以下错误
Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observe on a null object reference
摘自我的主要片段的以下这段代码
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);
}
});
该错误专门在此行上抛出(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是我将数据提供给片段的必不可少的部分。由于我现在要在应用程序中维护6个片段,因此我需要对所有6个片段使用一个ViewModel。每个片段都有一个带有相同类型数据(对象和视图)的recyclerview,但是它们对数据应用了不同的过滤器。请查看ViewModel中的String filter
参数,此参数将过滤器应用于数据。当然,我将在ViewModel中保留所有6个片段。因为它们都需要同时进行更新(当用户完成诸如更改platforms
列表之类的操作时)
答案 0 :(得分:1)
您拥有异步代码,在实现这些方法时没有考虑这些异步代码,最后得到的是一个空LiveData,在其上调用watch()引发异常。您当前使用的流程:
您需要类似下面的代码才能使其正常工作:
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);
}
});
}