AsynchronousOperation.swift

//  Copyright © 2017 Jumprope. All rights reserved.

import Foundation

// From: https://stackoverflow.com/questions/32322386/how-to-download-multiple-files-sequentially-using-nsurlsession-downloadtask-in-s

// Asynchronous operation base class
//
// This is abstract to class performs all of the necessary KVN of `isFinished` and
// `isExecuting` for a concurrent `Operation` subclass. You can subclass this and
// implement asynchronous operations. All you must do is:
//
// - override `main()` with the tasks that initiate the asynchronous task;
//
// - call `completeOperation()` function when the asynchronous task is done;
//
// - optionally, periodically check `self.cancelled` status, performing any clean-up
//   necessary and then ensuring that `completeOperation()` is called; or
//   override `cancel` method, calling `super.cancel()` and then cleaning-up
//   and ensuring `completeOperation()` is called.

public class AsynchronousOperation: Operation {

  override public var isAsynchronous: Bool { return true }

  private let stateLock = NSLock()

  private var _executing: Bool = false
  override private(set) public var isExecuting: Bool {
    get {
      return stateLock.withCriticalScope { _executing }
    }
    set {
      willChangeValue(forKey: "isExecuting")
      stateLock.withCriticalScope { _executing = newValue }
      didChangeValue(forKey: "isExecuting")
    }
  }

  private var _finished: Bool = false
  override private(set) public var isFinished: Bool {
    get {
      return stateLock.withCriticalScope { _finished }
    }
    set {
      willChangeValue(forKey: "isFinished")
      stateLock.withCriticalScope { _finished = newValue }
      didChangeValue(forKey: "isFinished")
    }
  }

  // Complete the operation
  //
  // This will result in the appropriate KVN of isFinished and isExecuting

  public func completeOperation() {
    if isExecuting {
      isExecuting = false
    }

    if !isFinished {
      isFinished = true
    }
  }

  override public func start() {
    if isCancelled {
      isFinished = true
      return
    }

    isExecuting = true

    main()
  }
}