如何为MVVM模式链接Retrofit和Repository / ViewModel？

Question

我无法将Retrofit与MVVM架构联系起来。实际上，在阅读了他们只讨论用于SQLite本地数据库的Room的文档之后，我搜索了相同的但是来自Rest服务器的数据。所以，我尝试做类似这样的事情并没有奏效：https://proandroiddev.com/mvvm-architecture-viewmodel-and-livedata-part-1-604f50cda1

我有一个观察ViewModel的Activity：

活动代码：

mFlightPlanViewModel = ViewModelProviders.of(this).get(FlightPlanViewModel.class);
        mFlightPlanViewModel.getFlightPlans().observe(this, (flightPlans) -> {
            Log.d(TAG, "ON_CHANGED");

            mFlightPlanAdapter.setFlightPlans(flightPlans);
        });

ViewModel：

public class FlightPlanViewModel extends androidViewModel {
    private static final String TAG = "FlightPlanViewModel";

    private LiveData<List<FlightPlan>> mFlightPlans;
    private FlightPlanRepository mFlightPlanRepository;

    public FlightPlanViewModel(@NonNull Application application) {
        super(application);
        Log.d(TAG, "CONSTRUCTOR");

        mFlightPlanRepository = FlightPlanRepository.getInstance();
        mFlightPlans = mFlightPlanRepository.getFlightPlans();
    }

    public LiveData<List<FlightPlan>> getFlightPlans() {
        Log.d(TAG, "GET_FLIGHT_PLANS");

        return mFlightPlans;
    }
}

ViewModel回答使用单例模式的Repository：

public class FlightPlanRepository {
    private static final String TAG = "FlightPlanRepository";

    private static FlightPlanRepository instance;
    private RestApi mRestApi;

    private FlightPlanRepository() {
        Log.d(TAG, "CONSTRUCTOR");

        mRestApi = RestDao.getRestDao();
    }

    public static FlightPlanRepository getInstance() {
        Log.d(TAG, "GET_INSTANCE");

        if (instance == null) {
            instance = new FlightPlanRepository();
        }
        return instance;
    }

    public MutableLiveData<List<FlightPlan>> getFlightPlans() {
        Log.d(TAG, "GET_FLIGHT_PLANS");

        final MutableLiveData<List<FlightPlan>> data = new MutableLiveData<>();

        mRestApi.getFlightPlanList().enqueue(new Callback<List<FlightPlan>>() {
            @Override
            public void onResponse(Call<List<FlightPlan>> call, Response<List<FlightPlan>> response) {
                if (response.code() == 200) {
                    List<FlightPlan> temp = response.body();
                    for (FlightPlan flightPlan : temp) {
                        Log.d(TAG + "res", flightPlan.toString());
                    }
                    data.setValue(response.body());
                    Log.d(TAG + "res", response.toString());
                }
            }

            @Override
            public void onFailure(Call<List<FlightPlan>> call, Throwable t) {
                List<FlightPlan> flightPlans = new ArrayList<>();
                flightPlans.add(new FlightPlan(0, "Test", 3.551, 50.52, 3.55122, 50.52625));
                data.setValue(flightPlans);
                Log.d(TAG, t.getMessage());
            }
        });

        return data;
    }
}

存储库使用Retrofit实例：

public class RestDao {
    private static final String BASE_URL = "http://192.168.1.78:8080";
    private static Retrofit instance;

    private static Retrofit getInstance() {
        if (instance == null) {
            instance = new Retrofit.Builder()
                    .baseUrl(BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .build();
        }
        return instance;
    }

    public static RestApi getRestDao() {
        return getInstance().create(RestApi.class);
    }
}

它使用这个界面：

public interface RestApi {
    @GET("/plan/list")
    Call<List<FlightPlan>> getFlightPlanList();
}

代码中不起作用的部分是：

public MutableLiveData<List<FlightPlan>> getFlightPlans() {
    Log.d(TAG, "GET_FLIGHT_PLANS");

    final MutableLiveData<List<FlightPlan>> data = new MutableLiveData<>();

    mRestApi.getFlightPlanList().enqueue(new Callback<List<FlightPlan>>() {
        @Override
        public void onResponse(Call<List<FlightPlan>> call, Response<List<FlightPlan>> response) {
            if (response.code() == 200) {
                List<FlightPlan> temp = response.body();
                for (FlightPlan flightPlan : temp) {
                    Log.d(TAG + "res", flightPlan.toString());
                }
                data.setValue(response.body());
                Log.d(TAG + "res", response.toString());
            }
        }

        @Override
        public void onFailure(Call<List<FlightPlan>> call, Throwable t) {
            List<FlightPlan> flightPlans = new ArrayList<>();
            flightPlans.add(new FlightPlan(0, "Test", 3.551, 50.52, 3.55122, 50.52625));
            data.setValue(flightPlans);
            Log.d(TAG, t.getMessage());
        }
    });

    return data;
}

这返回一个空列表。我认为我理解为什么：调用enqueue（）方法发出了一个在另一个Thread中的请求，所以在这里我们返回数据而不等待结果。

所以我的问题是如何链接Retrofit和我的ViewModel？

Answer 1

另一答案

Answer 2

另一答案