如何使用 LiveData 观察共享偏好的变化？

Question

最近，我的一个项目需要观察整个应用程序中 SharedPreferences 的变化（我有活动、前台服务和 BroadcastReciever，它们会对 SharedPreferences 的变化做出不同的反应）。我还想仅在相应的 AppComponents 处于活动状态时才监听这些更改。因此，LiveData 将适合我的要求，但最好的方法是什么？

（出于文档目的提出问题，希望看到比下面的答案更好的实现）

Answer 1

以下代码是我在项目中一直使用的 SharedPreference 的 LiveData 实现。效果很好。

package com.chargingwatts.chargingalarm.util.preference;

import android.arch.lifecycle.LiveData
import android.content.SharedPreferences

abstract class SharedPreferenceLiveData<T>(val sharedPrefs: SharedPreferences,
                                           val key: String,
                                           val defValue: T) : LiveData<T>() {

    private val preferenceChangeListener = SharedPreferences.OnSharedPreferenceChangeListener { sharedPreferences, key ->
        if (key == this.key) {
            value = getValueFromPreferences(key, defValue)
        }
    }

    abstract fun getValueFromPreferences(key: String, defValue: T): T

    override fun onActive() {
        super.onActive()
        value = getValueFromPreferences(key, defValue)
        sharedPrefs.registerOnSharedPreferenceChangeListener(preferenceChangeListener)
    }

    override fun onInactive() {
        sharedPrefs.unregisterOnSharedPreferenceChangeListener(preferenceChangeListener)
        super.onInactive()
    }
}

class SharedPreferenceIntLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Int) :
        SharedPreferenceLiveData<Int>(sharedPrefs, key, defValue) {
    override fun getValueFromPreferences(key: String, defValue: Int): Int = sharedPrefs.getInt(key, defValue)
}

class SharedPreferenceStringLiveData(sharedPrefs: SharedPreferences, key: String, defValue: String) :
        SharedPreferenceLiveData<String>(sharedPrefs, key, defValue) {
    override fun getValueFromPreferences(key: String, defValue: String): String = sharedPrefs.getString(key, defValue)
}

class SharedPreferenceBooleanLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Boolean) :
        SharedPreferenceLiveData<Boolean>(sharedPrefs, key, defValue) {
    override fun getValueFromPreferences(key: String, defValue: Boolean): Boolean = sharedPrefs.getBoolean(key, defValue)
}

class SharedPreferenceFloatLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Float) :
        SharedPreferenceLiveData<Float>(sharedPrefs, key, defValue) {
    override fun getValueFromPreferences(key: String, defValue: Float): Float = sharedPrefs.getFloat(key, defValue)
}

class SharedPreferenceLongLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Long) :
        SharedPreferenceLiveData<Long>(sharedPrefs, key, defValue) {
    override fun getValueFromPreferences(key: String, defValue: Long): Long = sharedPrefs.getLong(key, defValue)
}

class SharedPreferenceStringSetLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Set<String>) :
        SharedPreferenceLiveData<Set<String>>(sharedPrefs, key, defValue) {
    override fun getValueFromPreferences(key: String, defValue: Set<String>): Set<String> = sharedPrefs.getStringSet(key, defValue)
}

fun SharedPreferences.intLiveData(key: String, defValue: Int): SharedPreferenceLiveData<Int> {
    return SharedPreferenceIntLiveData(this, key, defValue)
}

fun SharedPreferences.stringLiveData(key: String, defValue: String): SharedPreferenceLiveData<String> {
    return SharedPreferenceStringLiveData(this, key, defValue)
}

fun SharedPreferences.booleanLiveData(key: String, defValue: Boolean): SharedPreferenceLiveData<Boolean> {
    return SharedPreferenceBooleanLiveData(this, key, defValue)
}

fun SharedPreferences.floatLiveData(key: String, defValue: Float): SharedPreferenceLiveData<Float> {
    return SharedPreferenceFloatLiveData(this, key, defValue)
}

fun SharedPreferences.longLiveData(key: String, defValue: Long): SharedPreferenceLiveData<Long> {
    return SharedPreferenceLongLiveData(this, key, defValue)
}

fun SharedPreferences.stringSetLiveData(key: String, defValue: Set<String>): SharedPreferenceLiveData<Set<String>> {
    return SharedPreferenceStringSetLiveData(this, key, defValue)
}