Subclass of NSOperation to make it asynchronous in Swift 3

AsynchronousOperation.swift

//
//  AsynchronousOperation.swift
//
//  Created by Vasily Ulianov on 09.02.17.
//  Copyright © 2017 Vasily Ulianov. All rights reserved.
//

import Foundation

/// Subclass of `Operation` that add support of asynchronous operations.
/// ## How to use:
/// 1. Call `super.main()` when override `main` method, call `super.start()` when override `start` method.
/// 2. When operation is finished or cancelled set `self.state = .finished`
class AsynchronousOperation: Operation {
	override var isAsynchronous: Bool { return true }
	override var isExecuting: Bool { return state == .executing }
	override var isFinished: Bool { return state == .finished }
	
	var state = State.ready {
		willSet {
			willChangeValue(forKey: state.keyPath)
			willChangeValue(forKey: newValue.keyPath)
		}
		didSet {
			didChangeValue(forKey: state.keyPath)
			didChangeValue(forKey: oldValue.keyPath)
		}
	}
	
	enum State: String {
		case ready = "Ready"
		case executing = "Executing"
		case finished = "Finished"
		fileprivate var keyPath: String { return "is" + self.rawValue }
	}
	
	override func start() {
		if self.isCancelled {
			state = .finished
		} else {
			state = .ready
			main()
		}
	}
	
	override func main() {
		if self.isCancelled {
			state = .finished
		} else {
			state = .executing
		}
	}
}

very useful, thanks!

works like a charm! thanks

Awesome Dood!

There is an issue in the steps that you provided for using this code. I made the same mistake, which can be found in my question over on Stack Overflow. According to Apple's documentation, you should not call super at any time.

Cool! and thanks @nickkohrn for your valuable comment!

@nickkohrn, maybe you don't need to call super if you're directly subclassing Operation. But if you subclass AsynchronousOperation you have to call super, because there are some work is done inside my implementation of that methods. For example state = .executing won't be executed and your implementation will become buggy.

state is not thread-safe, setting state might cause TSAN issues.

that if else in main() though... 4 lines of code waisted

How I could use AsynchronousOperation in the API call?

ApiClient looks like this:

internal class ApiClient : NSObject {

    /// Data task method to invoke back-end endpoints. Errors will be returned as ApiClientError.
    internal func send(_ request: URLRequest, completion: @escaping (Data?, Error?) -> Void)
}

that if else in main() though... 4 lines of code waisted

Why? You need to flag operation as executing before performing any actions, also operation maybe cancelled even before starting.

Awesome thanks Sorix. I made one example project to understand also.

If you are looking for threadsafety solution, consider this:
https://github.com/richardtin/Advanced-NSOperations/blob/672023bc066eae6c3a9b2ed985b8c40a4e604a77/Earthquakes/Operations/Operation.swift