从 Preferences DataStore 读取返回 null

如何解决从 Preferences DataStore 读取返回 null

即使我使用 elvis 运算符在首选项管理器上设置了默认值，我的 DataStore 也一直返回 null。此外，我在键值对上设置首选项的编辑功能没有被调用，所以我什至不确定我的数据存储是否正确设置。不过，我很确定该类已正确注入，因为我可以在使用断点时将其视为变量。

基本上在 viewmodel 上的 val countryCode = viewmodel.countrySettings.value 总是返回 null

PreferencesManager 类

 
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,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")
    }
 
}

视图模型

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)
        }
    }
}

片段

 
    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
    }
}

解决方法

是的，ianhanniballake 是正确的，这是一个新手错误 - 我完全忘记了我必须观察 livedata 值然后才设置 UI 参数。我试图根据几个开关的值设置首选项（反之亦然）。这是设置它的正确功能：

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

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

然后在 setupSwitches() 上调用了 onViewCreated。