SharedPreferencesX.kt

class MyViewModel(private val sharedPreferences: SharedPreferences) : ViewModel {
  var isAdult by sharedPreferences.isAdult

  init {
    println(isAdult) // null
    isAdult = true
    println(isAdult) // true
    isAdult.value = false
    println(isAdult) // false

    sharedPreferences.isAdultX = true
    println(isAdult) // true
  }

}

class SharedPreferencesX {
  companion object {
    var SharedPreferences.isAdultX by ofBoolean { PREF_IS_ADULT } // How can we do something like this?
  }
}

val SharedPreferences.isAdult get() = ofBoolean { PREF_IS_ADULT }

const val PREF_IS_ADULT = "isAdult"

fun SharedPreferences.ofBoolean(
    onKey: () -> String,
): MutableSharedPreferencesState<Boolean?> = BooleanPreference(
    onKey = onKey,
    sharedPreferences = this,
    scope = scope,
)

class BooleanPreference(
    private val sharedPreferences: SharedPreferences,
    private val onKey: () -> String,
): MutableSharedPreferencesState<Boolean?> {
    override var value: Boolean?
        get() = sharedPreferences.getBooleanOrNull(onKey())
        set(value) {
           sharedPreferences.edit { putOrNull(onKey(), value) }
        }

    override fun component1(): Boolean? = value

    override fun component2(): (Boolean?) -> Unit = { value = it }
}


fun SharedPreferences.getBooleanOrNull(key: String) =
    if (contains(key)) getBoolean(key, false) else null

fun SharedPreferences.Editor.putOrNull(key: String, value: Boolean?): SharedPreferences.Editor =
    if (value != null) putBoolean(key, value)
    else remove(key)

    
    interface SharedPreferencesState<T> {
    val value: T
}

interface MutableSharedPreferencesState<T> : SharedPreferencesState<T> {
    override var value: T
    operator fun component1(): T
    operator fun component2(): (T) -> Unit
}

operator fun <T> SharedPreferencesState<T>.getValue(thisObj: Any?, property: KProperty<*>): T = value

operator fun <T> MutableSharedPreferencesState<T>.setValue(thisObj: Any?, property: KProperty<*>, value: T) {
    this.value = value
}