Question

我正在使用简单的“房间”表进行设置

@Entity
data class Settings(var user: String = "") {
    @PrimaryKey
    var id: Long = 1

    var activeItem: Int = 0
    var developerMode: Boolean = false
    var logoUrl: String = ""
    var advertisment: String = ""
}

我观察到LiveData表的变化。问题是，如果所有数据都在id = 1的一个条目中，那么当表中的任何值更新时，总是会触发所有观察者。有什么方法可以强制LiveData仅在参数更改时（而不是整个条目）进行观察？我不想通过其他表来解决它，也不想将其存储在具有不同ID的条目中

以下是来自Dao的查询：

@Query("SELECT activeItem FROM Settings WHERE id = 1")
abstract fun getActiveItem(): LiveData<Int>

@Query("SELECT user FROM Settings WHERE id = 1")
abstract fun getUser(): LiveData<String>

@Query("SELECT developerMode FROM Settings WHERE id = 1")
abstract fun getDeveloperMode(): LiveData<Boolean>

@Query("SELECT logoUrl FROM Settings WHERE id = 1")
abstract fun getLogoUrl(): LiveData<String>

@Query("SELECT advertisment FROM Settings WHERE id = 1")
abstract fun getAdvertisment(): LiveData<String>

Answer 1

您应使用SharedPreferences保留用户首选项。您可以通过首选项更改监听器收听首选项更新

preferences.registerOnSharedPreferenceChangeListener { sharedPreferences, key -> 
    // handle preference update
}

或者您可以实现自己的首选项注册表。

将您的偏好设置模型定义为sealed class

sealed class MyPreference

data class ActiveItemPreference(val number: Int) : MyPreference()
data class UserPreference(val name: String, val age: Int) : MyPreference()

将PreferenceRegistry作为类型安全的异构容器（有效Java（第3版）。第33条）

object PreferencesRegistry {

    private val preferenceLiveDataMap: Map<KClass<*>, MutableLiveData<MyPreference>> =
            HashMap<KClass<*>, MutableLiveData<MyPreference>>().apply {
                addPreferenceLiveData(ActiveItemPreference::class)
                addPreferenceLiveData(UserPreference::class)
                /*
                 Or you if you use kotlin-reflect you can register your preferences like this
                Preference::class.sealedSubclasses.forEach { preferenceClass ->
                    addPreferenceLiveData(preferenceClass)
                }
                */
            }

    operator fun <T : MyPreference> get(preferenceType: KClass<T>): LiveData<T> {
        @Suppress("UNCHECKED_CAST") val liveData = preferenceLiveDataMap[preferenceType] as? LiveData<T>
        return liveData ?: throw AssertionError("Unexpected preference type $preferenceType")
    }

    /**
     * Store receiver [T] of [toPreferences] extension to receiver [SharedPreferences] of [action]
     */
    fun <T : MyPreference> T.toPreferences(context: Context, action: SharedPreferences.(T) -> Unit) {
        // get shared preferences to store T
        context.getSharedPreferences("preferences", Context.MODE_PRIVATE).action(this)
        // send new value of preference T to observers
        preferenceLiveDataMap[this::class]?.postValue(this) ?: AssertionError(
                "Unable to update preference live data of type ${this::class}")
    }

    /**
     * Helper extension for type safety
     */
    private fun <T : MyPreference> MutableMap<KClass<*>, MutableLiveData<MyPreference>>.addPreferenceLiveData(type: KClass<T>) {
        put(type, MutableLiveData())
    }
}

看看以DSL方式实现的toPreferences扩展，您可以在《行动中的科特琳》（项目11.2.1）或Type-Safe Builders文章中阅读有关此方法的更多信息。

现在您可以保存属性

UserPreference("Tom", 12).toPreferences(context) { (name, age) ->
    // extension for SharedPreferences from Android SDK
    edit {
        putString("USER_NAME", name)
        putInt("USER_AGE", age)
    }
}

观察变化

PreferencesRegistry[UserPreference::class].observe(this::getLifecycle) { userPref ->
    if (userPref == null)
        Log.w("PREFERENCES", "Unexpected nullable user preference")
    else
        Log.d("PREFERENCES", "User preference updated. New name is ${userPref.name} and new age is ${userPref.age}")
}

加载实施偏好设置并在应用程序启动时调用

存储在“房间”表中的设置只有一个条目，可触发任何参数更新的所有观察者

1 个答案: