ConcurrentOperation.swift

//
//  ConcurrentOperation.swift
//  Yevgeniy Prokoshev
//
//  Created by Yevgeniy Prokoshev on 14/08/2018.
//  Copyright © 2018 Yevgeniy Prokoshev. All rights reserved.
//

import Foundation

open class ConcurrentOperation: Operation {
    
    enum State: String {
        case Ready = "isReady"
        case Executing = "isExecuting"
        case Finished = "isFinished"
        
        fileprivate func associatedKey() -> String {
            switch self {
            case .Ready:
                return #keyPath(Operation.isReady)
            case .Executing:
                return #keyPath(Operation.isExecuting)
            case .Finished:
                return #keyPath(Operation.isFinished)
            }
        }
    }
    
    var state: State = .Ready {
        willSet {
            willChangeValue(forKey: newValue.rawValue)
            willChangeValue(forKey: state.rawValue)
        }
        didSet {
            didChangeValue(forKey: oldValue.rawValue)
            didChangeValue(forKey: state.rawValue)
        }
    }
    
    open override var isReady: Bool {
        get {
            return state == .Ready
        }
    }
    
    override open var isAsynchronous: Bool { return true }
    
    override open var isExecuting: Bool {
        get {
            return state == .Executing
        }
    }
    
    override open var isFinished: Bool {
        get {
            return state == .Finished
        }
    }
    
    override open func start() {
        guard !self.isCancelled else {
            self.completeOperation()
            return
        }
        self.state = .Executing
        self.executeOperation()
    }
    
    override open func main() {
        guard !self.isCancelled else {
            self.completeOperation()
            return
        }
         self.state = .Executing
        self.executeOperation()
    }
    
    func executeOperation() {
       assertionFailure("Override in subclass")
    }
    
    
    func completeOperation() {
        self.state = .Finished
    }
}