LiveData在数据更改时不更新

Question

我使用LiveData从服务器获取数据并观察它。我的onChanged()方法只是第一次被调用，当服务器中的数据更新时不会被调用。

用户片段：

UserViewModel userViewModel = ViewModelProviders.of(this).get(UserViewModel.class);
userViewModel.getUser().observe(this, new Observer<User>() {
@Override
public void onChanged(User user) {
//Set UI
}
});

UserViewModel:

public class UserViewModel extends AndroidViewModel {
private LiveData<User> user;
public UserViewModel(Application application) {
super(application);
user = UserRepository.getInstance().fetchUser();
}
public LiveData<User> getUser() {
return user;
}    
}

用户存储库：

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;
}
public LiveData<User> fetchUser() {
final MutableLiveData<User> data = new MutableLiveData<>();
Call<User> call = apiService.getUser();
call.enqueue(new Callback<User>() {
@Override
public void onResponse(@NonNull Call<User> call, @NonNull Response<User> response) {
if (response.body() != null) {
data.postValue(response.body());
}
}
@Override
public void onFailure(@NonNull Call<User> call, @NonNull Throwable t) {
data.postValue(null);
t.printStackTrace();
}
});
return data;
}
}

Answer 1

问题是每次调用fetchUser时都会创建一个新的LiveData<>。

这意味着你的第一个永远不会收到更新。

请看一下这些。。。

存储库

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<User> 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<User> data = new MutableLiveData<>();
Call<User> call = apiService.getUser();
call.enqueue(new Callback<User>() {
@Override
public void onResponse(@NonNull Call<User> call, @NonNull Response<User> response) {
if (response.body() != null) {
data.postValue(response.body());
}
}
@Override
public void onFailure(@NonNull Call<User> call, @NonNull Throwable t) {
data.postValue(null);
t.printStackTrace();
}
});
return data;
}
// My alterations below:
private MutableLiveData<User> userLiveData = new MutableLiveData<>();
public LiveData<User> getUser() {
return userLiveData;
}
public LiveData<User> fetchUser2() {
Call<User> call = apiService.getUser();
call.enqueue(new Callback<User>() {
@Override
public void onResponse(@NonNull Call<User> call, @NonNull Response<User> 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<User> call, @NonNull Throwable t) {
userLiveData.postValue(null);
t.printStackTrace();
}
});
return userLiveData;
}
}

ViewModel

我也会稍微改变一下。我会直接观察LiveData，而不是观察fetch。

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

稍后，您可以随时从服务器请求更新的数据。

UserRepository.getInstance().fetchUser2();

您也可以在UserRepository的第一个构造上调用fetchUser2()。那么只有更新才会直接调用fetchUser2()。

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

碎片

此外，在您的片段中，不要在this上进行观察。而是使用getViewLifecycleOwner()

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