I am trying to understand how does this works.

list.sortedWith(compareBy ({
    when(it.state) {
       A -> -1
       B -> 0
       C -> 1
   }
}))

This returns a list were all the elements with state A come first, followed by B and then followed by C. I am just not able to wrap head around this though. I understand how comparable works by overriding compareTo function and comparing two objects that the function gets. But not this. Can someone please explain this to me :)

sortedWith(comparator)

uses

comparator.compare(a, b)

instead of

a.compareTo(b)

as

sorted()

would use

compareBy(f).compare(a, b)

returns

f(a).compareTo(f(b))

you don't need

sortedWith(compareBy())

, there's a standard function

list.sortedBy {
    when(it.state) {
        A -> -1
        B -> 0
        C -> 1
    }
}

I am passing multiple selectors to

compareBy

, how can I use

a

and

b

when doing it in this to implement some complex logic?

you can just implement

Comparator { a, b -> ... }

but what are you trying to do, it should not be common to need that

Note that if your states are an

enum class

, it's already done for you:

list.sortedBy { it.state }

will sort them in the same order as they are declared in code.