Specify field to copy in data classes

Is there a way to specify the field to be copied in copy by, say, passing the variable name as a parameter in a function? Here is an example to illustrate what I mean. Supposed we have below:

val myData = User()

data class User(
    val firstName: String = "",
    val lastName: String = "",
    val email: String = ""
)

What I would like to do is create a function that does the following:

fun update(fieldToBeUpdated: String, value: String) {
    myData.update {
        it.copy(fieldToBeUpdated = value)
    }
}

Of course, this doesn't work, because the compiler says

cannot find a parameter with this name: fieldToBeUpdated

Does anybody know how to achieve this, or is this not possible in Kotlin?

CodePudding user response：

It is possible but it is not pretty. First thing is to make sure to add kotlin-reflect dependency to your pom.xml:

<dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-reflect</artifactId>
    <version>your-kotlin-version</version>
</dependency>

And then you will need some reflection features to make this possible. Something along the following lines should do the trick:

import kotlin.reflect.full.instanceParameter
import kotlin.reflect.full.memberFunctions

fun main(args: Array<String>) {
    val myData = User()

    println(myData)
    println(myData.update("firstName", "Test"))
}

fun User.update(fieldToBeUpdated: String, value: String): User {
    val copy = this::class.memberFunctions.first { it.name == "copy" }
    val instanceParameter = copy.instanceParameter!!
    val parameterToBeUpdated = copy.parameters.first { it.name == fieldToBeUpdated }
    return copy.callBy(mapOf(instanceParameter to this, parameterToBeUpdated to value)) as User
}

data class User(val firstName: String = "",
                val lastName: String = "",
                val email: String = "")