OptimisticCommand.swift

struct AddPlayer: Command {
    var session = URLSession.shared
    var player: Player
    
    func execute(state: State, reactor: Reactor<State>) {
        let originalPlayers = state.players
        
        let task = session.dataTask(with: player.putRequest()) { data, response, error in
            if error {
                // 3. whoops. switch back and display error
                reactor.fire(event: Update(value: originalPlayers))
                reactor.fire(event: DisplayError(message: "Error adding \(player.name)"))
            }
            else {
                // handle json and ...
                // 2. update with the real values from the server, hopefully the same
                reactor.fire(event: Update(value: players))
            }
        }
        
        // 1. optimistically assume success and immediately update UI
        var optimisticPlayers = originalPlayers
        optimisticPlayers.append(player)
        reactor.fire(event: Update(value: optimisticPlayers))
        
        task.resume()
    }
}

// Caller uses:
reactor.fire(command: AddPlayer(player: player))