[英]Reading from Preferences DataStore returns null
即使我使用 elvis 運算符在首選項管理器上設置了默認值,我的 DataStore 也一直返回 null。 此外,我在鍵值對上設置首選項的編輯功能沒有被調用,所以我什至不確定我的數據存儲是否正確設置。 不過,我很確定該類已正確注入,因為我可以在使用斷點時將其視為變量。
基本上val countryCode = viewModel.countrySettings.value
在 ViewModel 上總是返回 null
PreferencesManager 類
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")
}
}
視圖模型
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)
}
}
}
分段
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
}
}
是的,ianhanniballake 是正確的,這是一個新手錯誤 - 我完全忘記了我必須觀察 livedata 值然后才設置 UI 參數。 我試圖根據幾個開關的值設置首選項(反之亦然)。 這是設置它的正確功能:
fun setupSwitches() {
viewModel.countrySettings.observe(viewLifecycleOwner, { preference ->
binding.radioGroup.check(adaptPreferenceFromDataStore(preference))
})
binding.radioGroup.setOnCheckedChangeListener { _, checkedId ->
viewModel.setCountryPreference(adaptPreferenceToDataStore(checkedId))
}
}
然后在onViewCreated
上調用setupSwitches()
。
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