ReactiveCocoa contract was that the executing observable would emit false as a first value when subscribed to and the command was not executing. This would allow to directly use it in the enabledIf of another opposite action. Instead we have to manually send false which can have side effects if the action is actually executing.

let stopAction = Action<(), ()> { _ in .just(()) }
let startAction = Action<(), ()>(enabledIf: stopAction.startWith(false).not()) { _ in
   .just(())
}

Instead of

let stopAction = Action<(), ()> { _ in .just(()) }
let startAction = Action<(), ()>(enabledIf: stopAction.not()) { _ in
   .just(())
}