泛型preference委托

class Preference<T>(val context: Context, val name: String, val default: T)
    : ReadWriteProperty<Any?, T> {
	
	val prefs by lazy {
	    context.getSharedPreferences("default", Context.MODE_PRIVATE)
    }
	
	override fun getValue(thisRef: Any?, property: KProperty<*>): T {
		return findPreference(name, default)
	}

	override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        putPreference(name, value)
    }
	...
}

private fun <T> findPreference(name: String, default: T): T = with(prefs) {
        val res: Any = when (default) {
        is Long -> getLong(name, default)
        is String -> getString(name, default)
        is Int -> getInt(name, default)
        is Boolean -> getBoolean(name, default)
        is Float -> getFloat(name, default)
        else -> throw IllegalArgumentException(
            "This type can be saved into Preferences")
    }

	res as T
}

private fun <U> putPreference(name: String, value: U) = with(prefs.edit()) {
    when (value) {
        is Long -> putLong(name, value)
        is String -> putString(name, value)
        is Int -> putInt(name, value)
        is Boolean -> putBoolean(name, value)
        is Float -> putFloat(name, value)
        else -> throw IllegalArgumentException("This type can be saved into Pref\
erences")
    }.apply()
}

object DelegatesExt {
	...
	fun preference<T : Any>(context: Context, name: String, default: T)
	    = Preference(context, name, default)
}