FlatMapExample.swift

let disposeBag = DisposeBag()

struct Player {
    var level: Variable<Int>
}

// players
let killua = Player(level: Variable(50))
let gon = Player(level: Variable(50))

let player = PublishSubject<Player>()

player.asObservable()
    .flatMap { $0.level.asObservable() }
    .subscribe(onNext: { print($0) })
    .disposed(by: disposeBag)

player.onNext(killua) // 50
killua.level.value = 51 // 51
player.onNext(gon) //50
gon.level.value = 51 // 51
killua.level.value = 52 // 52