如何在Room Dao中更改LiveData的ViewModel源代码。在WordDao中,我有两个问题:
@Dao
public interface WordDao {
@Query("SELECT * FROM " + Word.TABLE_NAME + " ORDER BY text ASC")
LiveData<List<Word>> getWordsByAsc();
@Query("SELECT * FROM " + Word.TABLE_NAME + " ORDER BY text DESC")
LiveData<List<Word>> getWordsByDesc();
}
我还有Repository类:
public class WordRepository {
public LiveData<List<Word>> getWordsByAsc() {
return wordDao.getWordsByAsc();
}
public LiveData<List<Word>> getWordsByDesc() {
return wordDao.getWordsByDesc();
}
}
和我的ViewModel类:
public class WordViewModel extends AndroidViewModel {
private boolean isSortAsc = true;
private LiveData<Word> words;
private WordRepository wordRepository;
public WordViewModel(@NonNull Application application) {
super(application);
wordRepository = new WordRepository(application);
words = wordRepository.getWordsByAsc();
}
public LiveData<List<Word>> getWords() {
return words;
}
public void sortButtonClicked() {
isSortAsc = !isSortAsc;
//TODO how change here source of data depending on the isSortAsc
//It works only after change configuration of screen, not insta
words = isSortAsc ? wordRepository.getWordsByAsc() :
wordRepository.getWordsByDesc()
}
}
在我的活动中,我添加了观察者:
wordViewModel.getWords().observe(this, new Observer<List<Word>>() {
@Override
public void onChanged(@Nullable List<Word> words) {
adapter.setWords(words);
}
});
在setWords(words)方法中,我也调用“notifyDataSetChanged()”。
如何根据“isSortAsc”参数更改LiveData的viewModel类源(words = isSortAsc?wordRepository.getWordsByAsc():wordRepository.getWordsByDesc()
它仅在更改屏幕配置后才能工作,而不是在LiveData字的insta更改源之后
一种方法可能是使用MediatorLiveData
....例如:
val words = MediatorLiveData<Word>().apply {
this.addSource(sortDirection) {
if (sortDirection.value) {
this.value = wordRepository.getWordsByAsc()
} else {
this.value = wordRepository.getWordsByDesc()
}
}
}
我在下面做了类似的事情(设置了direction
)https://github.com/joreilly/galway-bus-android/blob/master/app/src/main/java/com/surrus/galwaybus/ui/viewmodel/BusStopsViewModel.kt
我认为这是一个简单的解决方案,总是查询表ASC即反转列表,以防用户选择降序。 Collection.Sort()由Android提供,因此它应该表现更好。
public void sortButtonClicked() {
isSortAsc = !isSortAsc;
//TODO how change here source of data depending on the isSortAsc
//It works only after change configuration of screen, not insta
words = wordRepository.getWordsByAsc();
if(!isSortAsc) {
Collections.reverse(words);
}
}
我希望这能帮到您。
我相信需要修改以下功能
wordViewModel.getWords().observe(this, new Observer<List<Word>>() {
@Override
public void onChanged(@Nullable List<Word> words) {
adapter.setAdapter(words);
notifyDataSetChanged();
}
});
在触发onChanged之前,单词变量将不再具有任何值。所以你必须确保只有在onChanged()之后才能调用getWords()