flatMap() as transforming operator

FlatMap.swift

struct Person {
    var name: String
    let age: CurrentValueSubject<Int, Never>

    init(name: String, age: Int) {
        self.name = name
        self.age = CurrentValueSubject(age)
    }
}

var person = Person(name: "Mark", age: 18)
let age = CurrentValueSubject<Person, Never>(person)

age
    .flatMap { $0.age }
    .sink { print($0) }

person.age.value = 45
person.name = "Kobra"

// Output:
//  18
//  45