如何 link Retrofit 和 Repository/ViewModel MVVM 模式？

Question

我在使用 MVVM 架构进行 Retrofit link 时遇到了麻烦。事实上，在阅读了他们只谈论用于 SQLite 本地数据库的 Room 的文档之后，我搜索了相同的但来自 Rest Server 的数据。所以，我尝试做类似的事情，但没有成功： https://proandroiddev.com/mvvm-architecture-viewmodel-and-livedata-part-1-604f50cda1

我有一个 Activity 观察 ViewModel :

Activity代码：

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

            mFlightPlanAdapter.setFlightPlans(flightPlans);
        });

视图模型：

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 响应使用单例模式的存储库：

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);
    }
}

它使用这个接口：

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;
}

这是一个 return 空列表。我想我明白为什么：调用 enqueue() 方法发出了另一个线程中的请求，所以这里我们 return 数据而不等待结果。

所以我的问题是如何 link 改造和我的 ViewModel？

Answer 1

嘿凯文只是对你的 code.Instead MutableLiveData return LiveData 从你的存储库做了一个小改动：

public LiveData<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.postValue(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.postValue(flightPlans);
            Log.d(TAG, t.getMessage());
        }
    });

    return data;
}

Answer 2

如果我这样做：

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

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


    List<FlightPlan> flightPlans = new ArrayList<>();
    flightPlans.add(new FlightPlan(5, "Test5", 3.551, 50.52, 3.55122, 50.52625));
    data.setValue(flightPlans);
    Log.d(TAG, data.getValue().toString());


    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;
}

我可以在 Logcat 中看到默认值，以及来自我的其余服务器的两个值，但字符串为空值且 int / double 为 0，并且它不会添加两个其余条目到回收站视图。

这是我的 activity 代码：

public class FlightPlanActivity extends AppCompatActivity implements View.OnClickListener, SwipeRefreshLayout.OnRefreshListener, LifecycleOwner {
    private static final String TAG = "FlightPlanActivity";

    private FlightPlanAdapter mFlightPlanAdapter;
    private FlightPlanViewModel mFlightPlanViewModel;

    /**
     * UI
     */
    private FloatingActionButton mAddPlanButton;
    private RecyclerView mRecyclerView;
    private SwipeRefreshLayout mSwipeRefreshLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.setContentView(R.layout.activity_flight_plan);

        Log.d(TAG, "onCreate called");

        initUI();
    }

    private void initUI() {
        this.setTitle("Flight Plans");

        mRecyclerView = findViewById(R.id.flight_plan_list);
        mAddPlanButton = findViewById(R.id.add_flight_plan);
        mSwipeRefreshLayout = findViewById(R.id.refresh_flight_plan_list);

        mAddPlanButton.setOnClickListener(this);
        mSwipeRefreshLayout.setOnRefreshListener(this);

        mFlightPlanAdapter = new FlightPlanAdapter();

        mRecyclerView.setHasFixedSize(true);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
        mRecyclerView.setAdapter(mFlightPlanAdapter);

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

            mFlightPlanAdapter.setFlightPlans(flightPlans);
        });
    }

    @Override
    public void onClick(View v) {
        if (v.getId() == mAddPlanButton.getId()) {
            // TODO: A implémenter
        }
    }

    @Override
    public void onRefresh() {
        mFlightPlanAdapter.setFlightPlans(mFlightPlanViewModel.getFlightPlans().getValue());

        mSwipeRefreshLayout.setRefreshing(false);
    }
}

Answer 3

那么，如果我这样做，当有回应时会发生什么？

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

    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) {
                data.setValue(response.body());
            }
        }

        @Override
        public void onFailure(Call<List<FlightPlan>> call, Throwable t) {
            // Do something
        }
    });

    return data;
}

你运行那个，首先是一个空对象 returned 但如果有响应，它会 return 什么吗？

我试过了：

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

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

    try {
        Response<List<FlightPlan>> response = mRestApi.getFlightPlanList().execute();
        if (response.isSuccessful()) {
            flightPlans = response.body();
        } else {
            Log.d(TAG, "Can't get data !");
            throw new Exception("Can't get data !");
        }
    } catch (Exception e) {
        e.printStackTrace();

        flightPlans = new ArrayList<>();
        flightPlans.add(new FlightPlan(5, "Test5", 3.551, 50.52, 3.55122, 50.52625));
    }

    data.setValue(flightPlans);
    Log.d(TAG, data.getValue().toString());

    return data;
}

现在它在执行行崩溃了，因为我在主线程中启动它.. 所以我必须做一个异步任务但是在哪里？

其他选项：像最初一样创建一个异步任务，将 setter 放入模型视图对象，当有响应时，我调用 setter.. 这样做好吗？

谢谢你给我的答案！很有帮助。

编辑：为什么 Google 文档中有此代码：

public class UserRepository {
    private Webservice webservice;
    // ...
    public LiveData<User> getUser(int userId) {
        // This isn't an optimal implementation. We'll fix it later.
        final MutableLiveData<User> data = new MutableLiveData<>();
        webservice.getUser(userId).enqueue(new Callback<User>() {
            @Override
            public void onResponse(Call<User> call, Response<User> response) {
                data.setValue(response.body());
            }

            // Error case is left out for brevity.
        });
        return data;
    }
}

意思是可以做这样的事？

解决方案！

编辑：

这对我很有帮助！！！

我找到了解决办法！我很笨：我在 gradle 文件中插入了旧的依赖项！

如何 link Retrofit 和 Repository/ViewModel MVVM 模式？

How to link Retrofit and Repository/ViewModel for a MVVM pattern?

java

android

mvvm

retrofit2

android-architecture-components