medium_15.kt

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