Kotlin: Compare property values of different target objects with(out) reflection

Question

I want to compare values between multiple instances of a data class so that I know which value changed:

data class A(val name : String)
val firstA = A("hello")
val secondA = A("you")

if (secondA.name.changed(firstA)) {
   // Do something
}

Can I somehow access the property function of .name and execute it on another target value (in this example on firstA) without explicitly defining it? Can delegates help here or how do I solve this with and without reflection?

Answer 1

Without Reflection

I found a way without using reflection:

interface DiffValue<T> {
    val old : T?
}

fun <T,R> T.changed(memberAccess : T.() -> R) : Boolean {
    if (this is DiffValue<*>) {
        val old = this.old as? T
        if (old != null) {
            val currentValue = with(this, memberAccess)
            val previousValue = with(old, memberAccess)
            return currentValue != previousValue
        }
    }
    return true
}

And this is how you use it:

data class A(val name: String, override val old : A? = null): DiffValue<A>
val firstA = A("hello")
val secondA = A("you", firstA)

if (secondA.changed {name}) {
   // Do something
}

This is based on the concept of lambda literals with receivers that allows Kotlin to build powerful builders and DSLs.

Note: The DiffValue interface is optional and is just used to make it a bit easier to keep values together and avoids an additional parameter in changed.