我正在从以下位置迁移应用程序LoaderManager with Callbacks到一个实现使用ViewModel and LiveData。我想继续使用现有的SQLiteDatabase.
主要实现工作正常。这Activity实例化ViewModel并创建一个Observer这更新了View如果它观察到变化MutableLiveData生活在ViewModel. The ViewModel从获取数据（光标）SQLiteDatabase通过使用查询ContentProvider.
但我还有其他可以更改数据库的活动，同时MainActivity被停止但未被破坏。还有一个后台服务可以在运行时对数据库进行更改MainActivity位于前台。
其他活动和后台服务可以更改数据库中的值，因此可能会影响MutableLiveData in the ViewModel.
我的问题是：如何观察变化SQLiteDatabase为了更新LiveData?
这是一个简化版本MainActivity:
public class MainActivity extends AppCompatActivity {
private DrawerAdapter mDrawerAdapter;
HomeActivityViewModel homeActivityViewModel;
private Observer<Cursor> leftDrawerLiveDataObserver = new Observer<Cursor>() {
@Override
public void onChanged(@Nullable Cursor cursor) {
if (cursor != null && cursor.moveToFirst()) { // Do we have a non-empty cursor?
mDrawerAdapter.setCursor(cursor);
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
homeActivityViewModel = ViewModelProviders.of(this).get(HomeActivityViewModel.class);
homeActivityViewModel.getLiveData().observe(this, leftDrawerLiveDataObserver);
homeActivityViewModel.updateLiveData(); //,LEFT_DRAWER_LIVEDATA_ID);
}
@Override
protected void onResume(){ // update the LiveData on Resume
super.onResume();
homeActivityViewModel.updateLiveData();
}
}
这是我的ViewModel:
public class HomeActivityViewModel extends AndroidViewModel {
public HomeActivityViewModel(Application application) {
super(application);
}
@NonNull
private final MutableLiveData<Integer> updateCookie = new MutableLiveData<>();
@NonNull
private final LiveData<Cursor> cursorLeftDrawer =
Transformations.switchMap(updateCookie,
new Function<Integer, LiveData<Cursor>>() {
private QueryHandler mQueryHandler;
@Override
public LiveData<Cursor> apply(Integer input) {
mQueryHandler = new QueryHandler(getApplication().getContentResolver());
MutableLiveData<Cursor> cursorMutableLiveData = new MutableLiveData<>();
mQueryHandler.startQuery(ID, cursorMutableLiveData, URI,
new String[]{FeedData.ID, FeedData.URL},
null,null,null
);
return cursorMutableLiveData;
}
}
);
// By changing the value of the updateCookie, LiveData gets refreshed through the Observer.
void updateLiveData() {
Integer x = updateCookie.getValue();
int y = (x != null) ? Math.abs(x -1) : 1 ;
updateCookie.setValue(y);
}
@NonNull
LiveData<Cursor> getLiveData() {
return cursorLeftDrawer;
}
/**
* Inner class to perform a query on a background thread.
* When the query is completed, the result is handled in onQueryComplete
*/
private static class QueryHandler extends AsyncQueryHandler {
QueryHandler(ContentResolver cr) {
super(cr);
}
@Override
protected void onQueryComplete(int token, Object cookie, Cursor cursor) {
MutableLiveData<Cursor> cursorMutableLiveData = (MutableLiveData<Cursor>) cookie;
cursorMutableLiveData.setValue(cursor);
}
}
}
