使用一个条目存储在Room表中的设置会在任何参数更新时触发所有观察者

Posted 2021-04-15

我正在使用简单的Room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 = ""
}

我用LiveData观察表中的变化。问题是如果所有数据都在id为1的一个条目中，那么当表中的任何值更新时，所有观察者都会被触发。有没有办法如何强制LiveData仅在参数更改时观察，而不是整个条目？我不想通过附加表解决它或将其存储在具有不同ID的条目中

以下是Dao的询问：

@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>

答案

您应该使用SharedPreferences来保持用户首选项。您可以通过首选项更改侦听器侦听首选项更新

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

或者您可以实现自己的首选项注册表。

---

1. 将您的偏好模型定义为sealed class sealed class MyPreference data class ActiveItemPreference(val number: Int) : MyPreference() data class UserPreference(val name: String, val age: Int) : MyPreference()
2. 将PreferenceRegistry实现为类型安全的异构容器（Effective Java（3rd）。Item 33） 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，你可以在Kotlin in Action书（第11.2.1项）或Type-Safe Builders文章中阅读更多关于这种方法的内容。
3. 现在您可以保存您的属性 UserPreference("Tom", 12).toPreferences(context) { (name, age) -> // extension for SharedPreferences from android SDK edit { putString("USER_NAME", name) putInt("USER_AGE", age) } }
4. 并观察变化 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}") }
5. 实现首选项加载并在应用程序启动时调用它