数据更改时LiveData不更新

问题是fetchUser LiveData<>每次调用都会创建一个新的。

这意味着您的第一个将永远不会收到更新。

请看看这些...

public class UserRepository {    
    private ApiService apiService;
    private static UserRepository userRepository;

    private UserRepository() {
        apiService = RestClient.getClient().create(ApiService.class);
    }

    public synchronized static UserRepository getInstance() {
        if (userRepository == null) userRepository = new UserRepository();
        return userRepository;
    }

    // Your example code
    public LiveData fetchUser() {
        // Your problem lies here. Every time you fetch user data, you create a new LiveData.
        // Instead, fetch user should update the data on a pre-existing LiveData.
        final MutableLiveData data = new MutableLiveData<>();
        Call call = apiService.getUser();
        call.enqueue(new Callback() {
            @Override
            public void onResponse(@NonNull Call call, @NonNull Response response) {
                if (response.body() != null) {
                    data.postValue(response.body());
                }
            }

            @Override
            public void onFailure(@NonNull Call call, @NonNull Throwable t) {
                data.postValue(null);
                t.printStackTrace();
            }
        });
        return data;
    }

    // My alterations below:
    private MutableLiveData userLiveData = new MutableLiveData<>();

    public LiveData getUser() {
        return userLiveData;
    }

    public LiveData fetchUser2() {
        Call call = apiService.getUser();
        call.enqueue(new Callback() {
            @Override
            public void onResponse(@NonNull Call call, @NonNull Response response) {
                if (response.body() != null) {
                    userLiveData.postValue(response.body());
                }
                // TODO: Consider a fallback response to the LiveData here, in the case that bad data is returned. Perhaps null?
            }

            @Override
            public void onFailure(@NonNull Call call, @NonNull Throwable t) {
                userLiveData.postValue(null);
                t.printStackTrace();
            }
        });
        return userLiveData;
    }
}

我也会对此稍作更改。与其观察获取，不如直接观察LiveData。

user = UserRepository.getInstance().getUser();

以后，您可以随时从服务器请求更新的数据。

UserRepository.getInstance().fetchUser2();

您也可以调用fetchUser2()的第一个构造UserRepository。然后，只有更新会fetchUser2()直接调用。

private UserRepository() {
    apiService = RestClient.getClient().create(ApiService.class);
    fetchUser2();
}

另外，在您的片段中，请勿观察this。改为使用getViewLifecycleOwner()

userViewModel.getUser().observe(getViewLifecycleOwner(), new Observer() {
    @Override
    public void onChanged(User user) {
        //Set UI
    }
});