问题
I'm playing with LiveData and want to understand what it can do. I want to fill my RecyclerView with data from different sources by switch(using filters if you like).
Filtering values inside an adapter is not an option. So, I decided to use MediatorLiveData inside my view model.
Dao:
@Query("SELECT * FROM tasks WHERE completed = 0")
LiveData<List<Task>> getActiveTasksLiveData();
@Query("SELECT * FROM tasks")
LiveData<List<Task>> getAllTasksLiveData();
@Query("SELECT * FROM tasks WHERE completed = 1")
LiveData<List<Task>> getClosedTasksLiveData();
Repo:
public LiveData<List<Task>> getActiveTasks() {
return mTaskDao.getActiveTasksLiveData();
}
public LiveData<List<Task>> getAllTasks() {
return mTaskDao.getAllTasksLiveData();
}
public LiveData<List<Task>> getClosedTasks() {
return mTaskDao.getClosedTasksLiveData();
}
ViewModel
public class MainViewModel extends AndroidViewModel {
private final String TAG = "MainViewModel";
private final AppDataRepository mData;
private MediatorLiveData<List<Task>> mMediatorTasks;
public MainViewModel(@NonNull Application application) {
super(application);
mData = AppDataInjector.getDataRepository(application.getApplicationContext());
mMediatorTasks = new MediatorLiveData<>();
mMediatorTasks.setValue(null);
}
public LiveData<List<Task>> getTasks(){
return mMediatorTasks;
}
public void changeTasksOption(int index){
mMediatorTasks.removeSource(mData.getAllTasks());
mMediatorTasks.removeSource(mData.getActiveTasks());
mMediatorTasks.removeSource(mData.getClosedTasks());
if (index == R.id.navigation_all){
Log.i(TAG, "Add source: all");
mMediatorTasks.addSource(mData.getAllTasks(), new Observer<List<Task>>() {
@Override
public void onChanged(List<Task> tasks) {
Log.i(TAG, "Add source: all - setValue");
mMediatorTasks.setValue(tasks);
}
});
} else if (index == R.id.navigation_closed){
Log.i(TAG, "Add source closed");
mMediatorTasks.addSource(mData.getClosedTasks(), new Observer<List<Task>>() {
@Override
public void onChanged(List<Task> tasks) {
Log.i(TAG, "Add source: closed - setValue");
mMediatorTasks.setValue(tasks);
}
});
} else {
Log.i(TAG, "Add source active");
mMediatorTasks.addSource(mData.getActiveTasks(), new Observer<List<Task>>() {
@Override
public void onChanged(List<Task> tasks) {
Log.i(TAG, "Add source: active - setValue");
mMediatorTasks.setValue(tasks);
}
});
}
}
}
Fragment
public View onCreateView(@NonNull LayoutInflater inflater,
@Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_main, container, false);
mNavigationView = view.findViewById(R.id.navigation);
mFab = view.findViewById(R.id.fabMain);
mRecyclerView = view.findViewById(R.id.mainRecyclerView);
tasksAdapterLive = new TasksAdapterLive(mAdapterCallback);
RecyclerView.LayoutManager manager = new GridLayoutManager(getContext(), 1);
mRecyclerView.setLayoutManager(manager);
mRecyclerView.setAdapter(tasksAdapterLive);
// set up bottom navigation listener
mNavigationView.setOnNavigationItemSelectedListener(item -> {
mViewModel.changeTasksOption(item.getItemId());
return true;
});
return view;
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
mViewModel = ViewModelProviders.of(this).get(MainViewModel.class);
mViewModel.getTasks().observe(this, tasks -> {
if (tasks != null) {
tasksAdapterLive.setTasks(tasks);
tasksAdapterLive.notifyDataSetChanged();
}
});
mViewModel.changeTasksOption(mNavigationView.getSelectedItemId());
}
As you can see, I've decided to use MediatorLiveData inside my view model. My main goal - change data inside adapter when changeTasksOption() called from fragment.
I use removeSource(), because how I understand it removes LiveData source from observing.
But, in my case it does not.
When I launch app, logs are:
MainViewModel: Add source active
MainViewModel: Add source: active - setValue
When I try switch to another source - logs are
MainViewModel: Add source: all
MainViewModel: Add source: all - setValue
MainViewModel: Add source: active - setValue
MainViewModel: Add source: all - setValue
MainViewModel: Add source: active - setValue
*** repeats about 100 times
RecyclerView is blinking
So, I kindly ask. What am I doing wrong? Did I misunderstood the documentation? What really removeSourse() does? Because in my case it does not remove sources.
In case my method implementing this is wrong, how do you suggest I do?
Thank you!
EDTITED:
After experimenting for couple of hours I've found solution. Yeep, this is bad(or maybe not?). But clearly this is not universal, because we do not use Romm + LiveData
Create normal Room functions that return List
@Query("SELECT * FROM tasks WHERE completed = 0")
List<Task> getActiveTasks();
@Query("SELECT * FROM tasks")
List<Task> getAllTasks();
@Query("SELECT * FROM tasks WHERE completed = 1")
List<Task> getClosedTasks();
Created MutableLiveData in repo
private MutableLiveData<List<Task>> mTasksTestActive, mTasksTestAll, mTasksTestClosed;
Add theese functions to repo
public LiveData<List<Task>> getActiveTasksTest() {
Executors.newSingleThreadExecutor().execute(() -> {
List<Task> taskList = mTaskDao.getActiveTasks();
mTasksTestActive.postValue(taskList);
});
return mTasksTestActive;
}
public LiveData<List<Task>> getAllTasksTest() {
Executors.newSingleThreadExecutor().execute(() -> {
List<Task> taskList = mTaskDao.getAllTasks();
mTasksTestAll.postValue(taskList);
});
return mTasksTestAll;
}
public LiveData<List<Task>> getClosedTasksTest() {
Executors.newSingleThreadExecutor().execute(() -> {
List<Task> taskList = mTaskDao.getClosedTasks();
mTasksTestClosed.postValue(taskList);
});
return mTasksTestClosed;
}
ViewModel changes:
public void changeTasksOption(int index) {
mMediatorTasks.removeSource(mData.getAllTasksTest());
mMediatorTasks.removeSource(mData.getActiveTasksTest());
mMediatorTasks.removeSource(mData.getClosedTasksTest());
if (index == R.id.navigation_all) {
Log.i(TAG, "Add source: all");
mMediatorTasks.addSource(mData.getAllTasksTest(), tasks -> {
Log.i(TAG, "Add source: all - postValue");
mMediatorTasks.postValue(tasks);
});
} else if (index == R.id.navigation_closed) {
Log.i(TAG, "Add source closed");
mMediatorTasks.addSource(mData.getClosedTasksTest(), tasks -> {
Log.i(TAG, "Add source: closed - postValue");
mMediatorTasks.postValue(tasks);
});
} else {
Log.i(TAG, "Add source active");
mMediatorTasks.addSource(mData.getActiveTasksTest(), tasks -> {
Log.i(TAG, "Add source: active - postValue");
mMediatorTasks.postValue(tasks);
});
}
}
And now, by switching UI, I have my result. No more loops and everything seems go ok.
But still! This is a bad solution. Maybe something is wrong with Room?
回答1:
public void changeTasksOption(int index){ mMediatorTasks.removeSource(mData.getAllTasks()); mMediatorTasks.removeSource(mData.getActiveTasks()); mMediatorTasks.removeSource(mData.getClosedTasks());
No this is not how it should be!
The selected option should be in a LiveData. Then you can use Transformations.switchMap { against that LiveData to select the correct LiveData<List<Task>>.
private MutableLiveData<Integer> mSelectedIndex = new MutableLiveData<>();
private final LiveData<List<Task>> mMediatorTasks = Transformations.switchMap(mSelectedIndex, (index) -> {
if (index == R.id.navigation_all) {
return mData.getAllTasksTest();
} else if (index == R.id.navigation_closed) {
return mData.getClosedTasksTest();
} else {
return mData.getActiveTasksTest();
}
});
public void changeTasksOption(int index) {
mSelectedIndex.setValue(index);
}
public LiveData<List<Task>> getTasks(){
return mMediatorTasks;
}
Also, you should bring your mData.get*() methods to return LiveData<List<Task>> from the DAO again, that was a better solution.
回答2:
You are returning values from your repo synchronously in your previous repo code -
public LiveData<List<Task>> getActiveTasks() {
return mTaskDao.getActiveTasksLiveData();
}
public LiveData<List<Task>> getAllTasks() {
return mTaskDao.getAllTasksLiveData();
}
public LiveData<List<Task>> getClosedTasks() {
return mTaskDao.getClosedTasksLiveData();
}
So when you call removeSource(mData.getAllTasksTest()), it synchronously fetches data from the repo and that is why you are receiving data from all the repos.
In your edited code, you are using a worker thread to fetch data, which means that your source livedata gets removed from the mediator live data before the repo returns any value.
来源：https://stackoverflow.com/questions/54403479/filling-an-adapter-from-different-livedata-sources