我的代码遇到了一个我怀疑可能是自己造成的问题。所以我应该先回答这个问题。我可以/如何观察从基于内部联接查询和List参数的Dao返回的LiveData对象?
不幸的是我在Stackoverflow上还没有“10声望”,所以显然我无法嵌入图像。但这是我的ERD快照,因为它可以帮助您了解我的实体如何捆绑在一起:https://i.ibb.co/9YW0Vbx/Screenshot-at-2019-04-06-13-04-43.png
PrayerListFragment
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
mTagViewModel = ViewModelProviders.of(this).get(TagViewModel.class);
mPrayerTagViewModel =
ViewModelProviders.of(this).get(PrayerTagViewModel.class);
...
//Update the tag list with the selected tags
mTagViewModel.getSelectedTags().observe(this, new Observer<List<Tag>>() {
@Override
public void onChanged(@Nullable List<Tag> tags) {
if(tags.size() > 0) {
mPrayerTagViewModel.setTagList(tags);
}
}
});
//Observe whatever prayers the view model has to show us
mPrayerTagViewModel.getPrayers().observe(this, new Observer<List<Prayer>>() {
@Override
public void onChanged(@Nullable List<Prayer> prayers) {
mPrayersAdapter.setPrayers(prayers);
}
});
...
}
PrayerTagViewModel
...
private List<String> mTagNames = new ArrayList<>();
...
public LiveData<List<Prayer>> getPrayers() { return getPrayersForTags(mTagNames); }
...
public void setTagList(List<Tag> tags) {
mTagNames = new ArrayList<>();
for (Tag tag: tags) {
mTagNames.add(tag.getName());
}
}
ITagDAO 这将返回我没有遇到任何问题的LiveData对象:
@Query("SELECT * FROM tag_table ORDER BY name")
LiveData<List<Tag>> getAll();
@Query("SELECT * FROM tag_table WHERE selected ORDER BY name")
LiveData<List<Tag>> getSelected();
IPrayerTagDAO 但是我遇到了观察这个的问题,所以我想首先确保它是有效的语法:
@Query("SELECT * FROM prayer_table " +
"INNER JOIN prayertag_table " +
"ON summary=fk_summary " +
"WHERE fk_name IN (:names)")
LiveData<List<Prayer>> getPrayersForTags(final List<String> names);
如果它是有效的语法,我可能在我的片段中丢失了我的observable,因为在PrayerTagViewModel中调用getPrayers()会返回一个新的ViewModel,即一个不同于我在片段中开始观察的ViewModel?
坚持得到了回报!我有一种偷偷摸摸的怀疑神秘的Transformations.switchMap可以解决我的问题,但只有经过更多的阅读后我才意识到这一点。
PrayerTagViewModel(修改)
...
private PrayerTagRepository mPrayerTagRepository;
private MutableLiveData<List<String>> mTags = new MutableLiveData<>();
private LiveData<List<Prayer>> mPrayers =
Transformations.switchMap(mTags, mTags -> getPrayersForTags(mTags));
public PrayerTagViewModel(@NonNull Application application) {
super(application);
mPrayerTagRepository = PrayerTagRepository.getRepository(application);
}
...
public LiveData<List<Prayer>> getPrayersForTags(final List<String> names) {
return mPrayerTagRepository.getPrayersForTags(names);
}
public LiveData<List<Prayer>> getPrayers() { return mPrayers; }
...
public void setTagList(List<Tag> tags) {
List<String> tagNames = new ArrayList<>();
for (Tag tag: tags)
tagNames.add(tag.getName());
mTags.setValue(tagNames);
}
PrayersListFragment(已修改)
...
//Update the tag list with the selected tags
mTagViewModel.getSelectedTags().observe(this, new Observer<List<Tag>>() {
@Override
public void onChanged(@Nullable List<Tag> tags) {
Log.i(this.getClass().getName(),"onChanged :: Tag size = " + tags.size());
if(tags.size() > 0)
mPrayerTagViewModel.setTagList(tags);
}
});
//Observe whatever prayers the view model has to show us
mPrayerTagViewModel.getPrayers().observe(this,
prayers -> mPrayersAdapter.setPrayers(prayers));
解决方案:switchMap让我的Fragment从我的ViewModel中观察到一个动态变化的LiveData(实际上是一个MutableLiveData)。