medium_11.kt

class StringDelegate : ReadOnlyPropery<ContextProvider, String> {
 
    private val stringRes: Int
    private var string: String? = null
  
    constructor(@StringRes stringRes: Int) {
        this.stringRes = stringRes
    }
  
    override fun getValue(ref: ContextProvider, property: KProperty<*>): String {
        if (string == null) {
            string = ref.provideContext().getString(stringRes) 
        }
        return string ?: throw IllegalStateException()
    }
}