ActionsWithPriority.swift

import Foundation
import ReactiveCocoa

final class PriorityAction<T> {
    
    let identifier : String
    let action : Action<Void, [T], Error>
    
    init(identifier: String, action: Action<Void, [T], Error>) {
        
        self.identifier = identifier
        self.action = action
    }
}

final class PriorityEngine<T> {
    
    private var queue: [PriorityAction<T>] = []
    private var inProgress : Bool = false
    
    func addActionWithPriority(action: PriorityAction<T>) {
        
        queue.enumerate()
            .filter{ $0.element.identifier == action.identifier }
            .first
            .apply { self.queue.removeAtIndex($0.index) }
        
        queue.insert(action, atIndex: 0)
    }
    
    func addAction(action: PriorityAction<T>) {
        queue.append(action)
    }

    func start() {
        
        guard self.queue.count > 0 && inProgress == false else { return }
        
        inProgress = true
        let action = self.queue.removeFirst().action
            action
            .apply()
            .startOn(QueueScheduler(name: "PriorityEngine.Scheduler"))
            .materialize()
            .startWithNext(handleEvent)
    }
    
    private func handleEvent(event: Event<[T], ActionError<Error>>) {
        
        switch event {
        case .Next(_): break
        default:
            inProgress = false
            start()
        }
    }
}