Kotlin extension function and interface

Question

I have defined an interface as,

interface Manga {
    fun title(): String
    fun rating(): String
    fun coverUrl(): String
    val id: String
}

I want to change set id without affecting the mutability of the interface. So I have created an extension function that sets the id field.

fun Manga.setId(id_: String): Manga {
    return object : Manga {
        override fun title() = this@setId.title()

        override fun rating() = this@setId.rating()

        override fun coverUrl() = this@setId.coverUrl()

        override val id: String
            get() = id_
    }
}

If I want to add field to manga interface then I have to modify the extension function.

Is there a way to override only id while creating the new object without modify the extension function? Or any other way to achieve the same effect.

Answer 1

Use delegation

fun Manga.setId(id_: String): Manga {
    return object : Manga by this {
        override val id: String
            get() = id_
    }
}