从 Preferences DataStore 读取返回 null

Question

My DataStore keeps returning null even though I've set a default value on the preferences manager using the elvis operator. Also, my edit function to set a preference on a key-value pair isn't being called so I'm not even sure my datastore is properly setup in general. I'm pretty sure the class is properly injected though, because I can see it as a variable while using breakpoints.

Basically val countryCode = viewModel.countrySettings.value on the ViewModel always returns null

PreferencesManager class

 
const val TAG = "PreferencesManager"
 
const val DEFAULT_COUNTRY_PREFERENCE = "us"
const val DEFAULT_CATEGORY_PREFERENCE = "general"
 
private val Context.dataStore by preferencesDataStore(name = PREFERENCES_NAME)
 
@Singleton
class PreferencesManager @Inject constructor(@ApplicationContext appContext: Context) {
 
    private val preferencesDataStore = appContext.dataStore
 
    //Pairs are separated but I'll create an appropriate data class later.
    val countrySettings = preferencesDataStore.data
        .catch { exception ->
            if (exception is IOException) {
                Log.e(TAG, "Error while trying to read user preferences", exception)
                emit(emptyPreferences())
            } else {
                throw exception
            }
        }
        .map { preference ->
            val country = preference[PreferenceKeys.COUNTRY] ?: DEFAULT_COUNTRY_PREFERENCE
            country
        }
 
    val categorySettings = preferencesDataStore.data
        .catch { exception ->
            if (exception is IOException) {
                Log.e(TAG, "Error while trying to read user preferences", exception)
                emit(emptyPreferences())
            } else {
                throw exception
            }
        }
        .map { preferences ->
            val category = preferences[PreferenceKeys.CATEGORY] ?: DEFAULT_CATEGORY_PREFERENCE
            category
        }
 
    suspend fun setCountryPreference(country: String) {
        preferencesDataStore.edit { preference ->
            preference[PreferenceKeys.COUNTRY] = country
        }
    }
 
    suspend fun setCategoryPreference(category: String) {
        preferencesDataStore.edit { preference ->
            preference[PreferenceKeys.CATEGORY] = category
        }
    }
 
 
    private object PreferenceKeys {
        val COUNTRY = stringPreferencesKey("country")
        val CATEGORY = stringPreferencesKey("category")
    }
 
}

ViewModel

class MainViewModel @Inject constructor(
    private val repository: Repository,
    private val preferencesManager: PreferencesManager
): ViewModel() {
 
 
    val countrySettings = preferencesManager.countrySettings.asLiveData()
    val categorySettings = preferencesManager.categorySettings.asLiveData()
 
/* .... */
 
    fun setCountryPreference(country: String) {
        viewModelScope.launch {
            preferencesManager.setCountryPreference(country)
        }
    }
 
    fun setCategoryPreference(category: String) {
        viewModelScope.launch {
            preferencesManager.setCategoryPreference(category)
        }
    }
}

Fragment

 
    val viewModel: MainViewModel by activityViewModels()
 
    private var _binding: FragmentSettingsCountryScreenBinding? = null
    private val binding get() = _binding!!
 
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
 
        _binding = FragmentSettingsCountryScreenBinding.bind(view)
 
        //Using breakpoints I've noticed this function isn't even called on the preferences manager to set the value, which is weird
        viewModel.setCountryPreference("us")
 
        val countryCode = viewModel.countrySettings.value
 
        binding.radiogroup.check(adaptPreferenceFromDataStore(countryCode!!))
 
 
        binding.radiogroup.setOnCheckedChangeListener { _, checkedId ->
            viewModel.setCountryPreference(adaptPreferenceToDataStore(checkedId))
        }
 
    }
 
    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null
    }
}

Answer 1

Yes, ianhanniballake is correct, it was a newbie mistake - I completely forgot I had to observe the livedata value and just then set the UI parameters. I was trying to set the preferences based on the value of a few switches (and vice-versa). Here's the proper function for setting it up:

fun setupSwitches() {
        viewModel.countrySettings.observe(viewLifecycleOwner, { preference ->
            binding.radioGroup.check(adaptPreferenceFromDataStore(preference))
        })

        binding.radioGroup.setOnCheckedChangeListener { _, checkedId ->
            viewModel.setCountryPreference(adaptPreferenceToDataStore(checkedId))
        }
    }

Then called setupSwitches() on onViewCreated .