List setter is not being called when appending a new value - kotlin

Question

I had a workaround solution for this problem which involves reassigning the variable to itself; however, this doesn't seem efficient.
fun main() {
val x = Example1()
x.listOfInt = mutableListOf(3,4)
x.listOfInt.add(455)
}
class Example1 {
var listOfInt: MutableList<Int> = mutableListOf()
set(value) {
println("setting it here to value => $value")
field = value
}
}
// It only prints:
// setting it here to value => [3, 4]
As you can see, the setter doesn't get triggered when the value is appended which effectively changing the value of the collection. Is there a better way to do this?
Here's a link to the code on Kotlin's playground
The reason this is odd to me because of the below similar code in Swift which does the expected when the Array is appended:
import Foundation
class Example1 {
var fieldProperty: Int = 0
var listOfInt: [Int] = [] {
didSet {
print("setting it here to value => \(listOfInt) vs oldValue: \(oldValue)")
}
}
}
let x = Example1()
x.listOfInt = [3,4]
x.listOfInt.append(455)
// this prints:
// setting it here to value => [3, 4] vs oldValue: []
// setting it here to value => [3, 4, 455] vs oldValue: [3, 4]
I know I might be comparing apples to oranges, but I'm just curious if there's a better solution.

Answer 1

The use of set(value) { } is the equivalent of implementing a setter on that variable.
The code you provided could be interpreted as:
class Example1 {
var listOfInt: MutableList<Int> = mutableListOf()
fun set(value: MutableList<Int>) {
println("setting it here to value => $value")
this.listOfInt = value
}
}
When you call x.listOfInt.add(455) you are calling the add method on the list defined in the x instance of Example1.
Try calling x.listOfInt = mutableListOf(1,2,3) and you'll see your setter will correctly get called.
To implement what you want to do, there is several ways:
Implement a new version of MutableList<> which overrides the method add (this might be overkill).
Create a add method in your Example1 class which will look like this:
fun add(value: Int) {
println("adding value to list => $value")
listOfInt.add(value)
}
You main will now look like this:
fun main() {
val x = Example1()
x.listOfInt = mutableListOf(3,4)
x.add(455)
}