operationObserver.swift

import Foundation

class OperationObserver: NSObject {
    init(operation: AsyncOperation) {
        super.init()
        operation.addObserver(self, forKeyPath: "finished", options: .new, context: nil)
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        guard let key = keyPath else {
            return
        }
        
        switch key {
        case "finished":
            print("done")
        default:
            print("doing")
        }
    }
}