SharedPreferences as LiveData

MainViewModel.kt

package io.github.yaraki.playground

import android.app.Application
import android.arch.lifecycle.AndroidViewModel
import android.content.Context

class MainViewModel(application: Application) : AndroidViewModel(application) {

    companion object {
        const val PREF = "main"
        const val PREF_MESSAGE = "message"
    }

    private val prefs = application.getSharedPreferences(PREF, Context.MODE_PRIVATE)

    val message = prefs.liveString(PREF_MESSAGE, "")

    fun saveMessage(message: String) {
        prefs.edit().putString(PREF_MESSAGE, message).apply()
    }

}

SharedPreferenceLiveData.kt

/*
 * Copyright (C) 2018 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package io.github.yaraki.playground

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


fun SharedPreferences.liveString(key: String, defaultValue: String): LiveData<String> {
    return SharedPreferenceLiveData(this, key, defaultValue, SharedPreferences::getString)
}

fun SharedPreferences.liveInt(key: String, defaultValue: Int): LiveData<Int> {
    return SharedPreferenceLiveData(this, key, defaultValue, SharedPreferences::getInt)
}

fun SharedPreferences.liveBoolean(key: String, defaultValue: Boolean): LiveData<Boolean> {
    return SharedPreferenceLiveData(this, key, defaultValue, SharedPreferences::getBoolean)
}

fun SharedPreferences.liveFloat(key: String, defaultValue: Float): LiveData<Float> {
    return SharedPreferenceLiveData(this, key, defaultValue, SharedPreferences::getFloat)
}

fun SharedPreferences.liveLong(key: String, defaultValue: Long): LiveData<Long> {
    return SharedPreferenceLiveData(this, key, defaultValue, SharedPreferences::getLong)
}

fun SharedPreferences.liveStringSet(key: String, defaultValue: Set<String>): LiveData<Set<String>> {
    return SharedPreferenceLiveData(this, key, defaultValue, SharedPreferences::getStringSet)
}

internal class SharedPreferenceLiveData<T>(
        private val prefs: SharedPreferences,
        private val key: String,
        private val defaultValue: T,
        private val getter: (SharedPreferences).(String, T) -> T
) : LiveData<T>() {

    private val listener = SharedPreferences.OnSharedPreferenceChangeListener { _, key ->
        if (key == this.key) {
            value = prefs.getter(key, defaultValue)
        }
    }

    override fun onActive() {
        super.onActive()
        value = prefs.getter(key, defaultValue)
        prefs.registerOnSharedPreferenceChangeListener(listener)
    }

    override fun onInactive() {
        super.onInactive()
        prefs.unregisterOnSharedPreferenceChangeListener(listener)
    }

}