我们如何在ViewModel中将LiveData从Room“分配”到MutableLiveData

Question

最近，我坚持使用以下代码。

public class NoteViewModel extends ViewModel {

    private final MutableLiveData<List<Note>> notesLiveData = new MutableLiveData<>();

    public NoteViewModel() {
        LiveData<List<Note>> notesLiveDataFromRepository = NoteRepository.INSTANCE.getNotes();

        // How can I "assign" LiveData from Room, to MutableLiveData?
    }

}

我想知道，我如何从LiveData“Room”分配到MutableLiveData？

使用Transformation.map和Transformation.switchMap不起作用，因为它们都返回LiveData，而不是MutableLiveData。

可能的解决方法

可能的解决方案之一是，而不是

@Dao
public abstract class NoteDao {
    @Transaction
    @Query("SELECT * FROM plain_note")
    public abstract LiveData<List<Note>> getNotes();

我会用

@Dao
public abstract class NoteDao {
    @Transaction
    @Query("SELECT * FROM plain_note")
    public abstract List<Note> getNotes();

然后，在我的ViewModel中，我会写

public class NoteViewModel extends ViewModel {
    private final MutableLiveData<List<Note>> notesLiveData = new MutableLiveData<>();

    public NoteViewModel() {
        new Thread(() -> {
            List<Note> notesLiveDataFromRepository = NoteRepository.INSTANCE.getNotes();
            notesLiveData.postValue(notesLiveDataFromRepository);
        }).start();
    }

}

我真的不喜欢这种方法，因为我被迫明确处理线程化问题。

有没有更好的方法，以避免明确处理线程？

Answer 1

诀窍是不在视图模型中进行任何实际的提取。

从网络或数据库获取数据应该在存储库中完成。在这方面，ViewModel应该是不可知的。

在ViewModel中，使用LiveData类，而不是MutableLiveData。除非你真的找到了一个用例。

// In your constructor, no extra thread
notesLiveData = notesLiveDataFromRepository.getAllNotes();

然后在您的存储库中，您可以使用getAllNotes（）方法中的逻辑来确定这些注释的来源。在存储库中，您拥有MutableLiveData。然后，您可以从获取数据的线程postValue。但这对于房间来说并不是必需的，这是为您处理的。

因此，在您的存储库中，您将返回另一个直接从DAO方法支持的LiveData。

在这种情况下，您需要坚持使用public abstract LiveData<List<Note>> getNotes();。

活动

public class MyActivity extends AppCompatActivity {

    private MyViewModel viewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Set up your view model
        viewModel = ViewModelProviders.of(this).get(MyViewModel.class);

        // Observe the view model
        viewModel.getMyLiveData().observe(this, s -> {
            // You work with the data provided through the view model here.
            // You should only really be delivering UI updates at this point. Updating
            // a RecyclerView for example.
            Log.v("LIVEDATA", "The livedata changed: "+s);
        });

        // This will start the off-the-UI-thread work that we want to perform.
        MyRepository.getInstance().doSomeStuff();
    }
}

视图模型

public class MyViewModel extends AndroidViewModel {

    @NonNull
    private MyRepository repo = MyRepository.getInstance();

    @NonNull
    private LiveData<String> myLiveData;

    public MyViewModel(@NonNull Application application) {
        super(application);
        // The local live data needs to reference the repository live data
        myLiveData = repo.getMyLiveData();
    }

    @NonNull
    public LiveData<String> getMyLiveData() {
        return myLiveData;
    }
}

存储库

public class MyRepository {

    private static MyRepository instance;

    // Note the use of MutableLiveData, this allows changes to be made
    @NonNull
    private MutableLiveData<String> myLiveData = new MutableLiveData<>();

    public static MyRepository getInstance() {
        if(instance == null) {
            synchronized (MyRepository.class) {
                if(instance == null) {
                    instance = new MyRepository();
                }
            }
        }
        return instance;
    }

    // The getter upcasts to LiveData, this ensures that only the repository can cause a change
    @NonNull
    public LiveData<String> getMyLiveData() {
        return myLiveData;
    }

    // This method runs some work for 3 seconds. It then posts a status update to the live data.
    // This would effectively be the "doInBackground" method from AsyncTask.
    public void doSomeStuff() {
        new Thread(() -> {
            try {
                Thread.sleep(3000);
            } catch (InterruptedException ignored) {
            }

            myLiveData.postValue("Updated time: "+System.currentTimeMillis());
        }).start();
    }

}

Answer 2

非常简单。

class MainViewModel: ViewModel() {

    @Inject lateinit var currencyRepository: CurrencyRepository

    val notifyCurrencyList = MediatorLiveData<List<Currency?>>()

    init {
        CurrencyApplication.component.inject(this)
    }

    fun loadCurrencyList() {
        notifyCurrencyList.addSource(currencyRepository.loadLatestRates()) {
            notifyCurrencyList.value = it
        }
    }


}