Make Android's SharedPreferences as simple as a field in kotlin.

MyPreferences.kt

package com.mypackage.myapp.prefs

import android.content.Context
import com.mypackage.myapp.appCtx

object MyPreferences : Preferences(name = "MyPrefsFileName") {

    var userWantsFancyUi by BoolPref(Keys.FANCY_UI, false)
    var isFcmTokenSent by BoolPref(Keys.FCM_TOKEN_SENT_TO_SERVER, false)
    var eggTapsCount by LongPref(Keys.EGG_TAPS_COUNT, 0L)

    object Keys {
        const val FANCY_UI = "fancyUI"
        const val FCM_TOKEN_SENT_TO_SERVER = "fcmTokenSent"
        const val EGG_TAPS_COUNT = "eggTapsCount"
    }
}

ObservablePreferences.kt

package com.mypackage.myapp.prefs

import android.content.Context
import android.content.SharedPreferences
import com.google.android.agera.BaseObservable
import com.google.android.agera.Observable
import com.google.android.agera.Updatable
import com.mypackage.myapp.appCtx

abstract class ObservablePreferences(ctx: Context = appCtx, name: String) : Preferences(ctx, name), Observable {

    private val changeListener by lazy { ChangesObservable() }
    /**
     * Updated before the update is dispatched. The value should be only used to check which
     * preference changed.
     */
    var lastChangedKey = ""
        private set

    override fun addUpdatable(updatable: Updatable) = changeListener.addUpdatable(updatable)
    override fun removeUpdatable(updatable: Updatable) = changeListener.removeUpdatable(updatable)

    inner private class ChangesObservable : BaseObservable(), SharedPreferences.OnSharedPreferenceChangeListener {
        override fun observableActivated() = prefs.registerOnSharedPreferenceChangeListener(this)

        override fun onSharedPreferenceChanged(sharedPreferences: SharedPreferences, key: String) {
            check(sharedPreferences in this@ObservablePreferences)
            lastChangedKey = key
            dispatchUpdate()
        }

        override fun observableDeactivated() = prefs.unregisterOnSharedPreferenceChangeListener(this)
    }
}

Preferences.kt

package com.mypackage.myapp.prefs

import android.content.Context
import com.mypackage.myapp.appCtx

abstract class Preferences(ctx: Context = appCtx, name: String) {
    private val prefs = ctx.getSharedPreferences(name, Context.MODE_PRIVATE)
    
    operator fun contains(o: Any) = prefs === o

    internal inner class BoolPref(key: String, defaultValue: Boolean)
        : BoolPrefField(prefs, key, defaultValue)

    internal inner class IntPref(key: String, defaultValue: Int)
        : IntPrefField(prefs, key, defaultValue)

    internal inner class FloatPref(key: String, defaultValue: Float)
        : FloatPrefField(prefs, key, defaultValue)

    internal inner class LongPref(key: String, defaultValue: Long)
        : LongPrefField(prefs, key, defaultValue)

    internal inner class StringPref(key: String, defaultValue: String)
        : StringPrefField(prefs, key, defaultValue)
}

PrefsDelegates.kt

package com.mypackage.myapp.prefs

@file:Suppress("NOTHING_TO_INLINE")

import android.content.SharedPreferences
import kotlin.reflect.KProperty

internal abstract class BoolPrefField(private val prefs: SharedPreferences,
                                       private val key: String,
                                       defaultValue: Boolean) {

    private var cachedValue = prefs.getBoolean(key, defaultValue)

    inline operator fun getValue(thisRef: Any?, prop: KProperty<*>) = cachedValue
    inline operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: Boolean) {
        cachedValue = value
        prefs.edit().putBoolean(key, value).apply()
    }
}

internal abstract class IntPrefField(private val prefs: SharedPreferences,
                                      private val key: String,
                                      defaultValue: Int) {

    private var cachedValue = prefs.getInt(key, defaultValue)

    inline operator fun getValue(thisRef: Any?, prop: KProperty<*>) = cachedValue
    inline operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: Int) {
        cachedValue = value
        prefs.edit().putInt(key, value).apply()
    }
}

internal abstract class LongPrefField(private val prefs: SharedPreferences,
                                     private val key: String,
                                     defaultValue: Long) {

    private var cachedValue = prefs.getLong(key, defaultValue)

    inline operator fun getValue(thisRef: Any?, prop: KProperty<*>) = cachedValue
    inline operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: Long) {
        cachedValue = value
        prefs.edit().putLong(key, value).apply()
    }
}

internal abstract class FloatPrefField(private val prefs: SharedPreferences,
                                     private val key: String,
                                     defaultValue: Float) {

    private var cachedValue = prefs.getFloat(key, defaultValue)

    inline operator fun getValue(thisRef: Any?, prop: KProperty<*>) = cachedValue
    inline operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: Float) {
        cachedValue = value
        prefs.edit().putFloat(key, value).apply()
    }
}

internal abstract class StringPrefField(private val prefs: SharedPreferences,
                                     private val key: String,
                                     defaultValue: String) {

    private var cachedValue = prefs.getString(key, defaultValue)!!

    inline operator fun getValue(thisRef: Any?, prop: KProperty<*>) = cachedValue
    inline operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: String) {
        cachedValue = value
        prefs.edit().putString(key, value).apply()
    }
}