Android MVVM retrofit2 模型未更新

Question

尝试使用改造调用我的 API 并更新模型。它正确初始化并且能够提取数据，但是当我通过按下更新按钮手动触发它时，onChange QuestionResponse 模型为空。这似乎是一种竞争条件，因为在调试时它会在调用 onResponse 之前触发 onChange。我尝试从 setValue 更改为 postvalue 等。

活动

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

    mViewModel = ViewModelProviders.of(this).get(BasicViewModel.class);
    mViewModel.getQuestions().observe(this, new Observer<QuestionResponse>() {
        @Override
        public void onChanged(QuestionResponse response) {
            QuestionResponse temp = mViewModel.getQuestions().getValue();
            Toast.makeText(MainActivity.this, "Questions Updated", Toast.LENGTH_SHORT).show();
        }
    });

    updateButton = findViewById(R.id.updateButton);
    updateButton .setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            update();
        }
    });
}

private void update(){
    mViewModel.updateQuestions();
}

查看模型

private Repository repository;
private Application application;
private MutableLiveData<QuestionResponse> questionMutableLiveData;

public BasicViewModel(Application application){
    super(application);
    this.application = application;
    repository = new Repository();
    questionMutableLiveData = repository.getQuotes();
}

public void updateQuestions(){
    questionMutableLiveData.setValue(repository.getQuotes().getValue());
}

public LiveData<QuestionResponse> getQuestions(){
    if(questionMutableLiveData == null){
        System.out.println("Something Is Wrong");
    }
    return questionMutableLiveData;
}

改装服务

public class Repository {
    MutableLiveData<QuestionResponse> questionData;
    ApiService apiService;

    public QuoteMRepository(){
        apiService = RetrofitSingleton.getRetrofitInstance().create(ApiService.class);
    }

    public MutableLiveData<QuestionResponse> getQuotes(){
        Call<QuestionResponse> call = apiService.getQuestions();

        //Old version of doing it 
        //MutableLiveData<QuestionResponse> questionData = new MutableLiveData<>();

        //New version, seems to be working but triggering twice 
        if(questionData == null) {
            questionData = new MutableLiveData<>();
        }    

        call.enqueue(new Callback<QuestionResponse>() {
            @Override
            public void onResponse(Call<QuestionResponse> call, Response<QuestionResponse> response) {
                if(response.isSuccessful()){
                    QuestionResponse reply = response.body();
                    questionData.setValue(reply);
                }
            }

            @Override
            public void onFailure(Call<QuestionResponse> call, Throwable t) {
                System.out.println("Failure");
            }
        });

        return questionData;
    }
}

Answer 1

您将返回一个空的 LiveData 对象，然后使用存储库响应对其进行更新，因此您的观察者会触发两次，一次用于 null，一次用于响应。

public MutableLiveData<QuestionResponse> getQuotes(){
    Call<QuestionResponse> call = apiService.getQuestions();
    MutableLiveData<QuestionResponse> questionData = new MutableLiveData<>();

    call.enqueue(new Callback<QuestionResponse>() {
        @Override
        public void onResponse(Call<QuestionResponse> call, Response<QuestionResponse> response) {
            if(response.isSuccessful()){
                QuestionResponse reply = response.body();
                questionData.setValue(reply);
            }
        }

        @Override
        public void onFailure(Call<QuestionResponse> call, Throwable t) {
            System.out.println("Failure");
        }
    });

    // THIS RETURNS AN EMPTY OBJECT AND LATER UPDATES
    return questionData;
}

例如，您可以在观察者上按null 过滤，或者为您的LiveData 使用SwitchMap 并返回一个指示加载的包装器。

Answer 2

相信我解决了这个问题。第一个问题是我总是在我的存储库中创建并返回一个新的可变列表。这个新列表没有被观察到，但仍然被分配了一个值。 onChange observable 在主要活动中仍然被触发的原因是因为在 ViewModel questionMutableLiveData.setValue(repository.getQuotes().getValue()) 实际上正在更改数据，但新创建的可变列表的值为空。

如果有任何反馈，如果这是正确的或组织存储库/视图模型的更好方法，我很乐意听到！

活动

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

    mViewModel = ViewModelProviders.of(this).get(BasicViewModel.class);
    mViewModel.getQuestions().observe(this, new Observer<QuestionResponse>() {
        @Override
        public void onChanged(QuestionResponse response) {
            QuestionResponse temp = mViewModel.getQuestions().getValue();
            Toast.makeText(MainActivity.this, "Questions Updated", Toast.LENGTH_SHORT).show();
        }
    });

    updateButton = findViewById(R.id.updateButton);
    updateButton .setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            update();
        }
    });
}

private void update(){
    mViewModel.updateQuestions();
}

查看模型

private Repository repository;
private Application application;
private MutableLiveData<QuestionResponse> questionMutableLiveData;

public BasicViewModel(Application application){
    super(application);
    this.application = application;
    repository = new Repository();
    questionMutableLiveData = repository.getQuotes();
}

public void updateQuestions(){
    repository.getQuotes().getValue();
}

public LiveData<QuestionResponse> getQuestions(){
    if(questionMutableLiveData == null){
        System.out.println("Something Is Wrong");
    }
    return questionMutableLiveData;
}

改装服务

public class Repository {
    MutableLiveData<QuestionResponse> questionData;
    ApiService apiService;

    public QuoteMRepository(){
        apiService = RetrofitSingleton.getRetrofitInstance().create(ApiService.class);
    }

    public MutableLiveData<QuestionResponse> getQuotes(){
        Call<QuestionResponse> call = apiService.getQuestions();

        if(questionData == null) {
            questionData = new MutableLiveData<>();
        }    

        call.enqueue(new Callback<QuestionResponse>() {
            @Override
            public void onResponse(Call<QuestionResponse> call, Response<QuestionResponse> response) {
                if(response.isSuccessful()){
                    QuestionResponse reply = response.body();
                    questionData.setValue(reply);
                }
            }

            @Override
            public void onFailure(Call<QuestionResponse> call, Throwable t) {
                System.out.println("Failure");
            }
        });

        return questionData;
    }
}