An (NS)Operation wrapping a Promise

PromiseOperation.swift

import Foundation
import PromiseKit

class PromiseOperation<T>: Operation {
    var result: T? = nil
    var error: Error?
    var promise: Promise<T>

    override var isExecuting: Bool {
        get {return _isExecuting}
        set {
            willChangeValue(forKey: "isExecuting")
            _isExecuting = newValue
            didChangeValue(forKey: "isExecuting")
        }
    }
    private var _isExecuting = false

    override var isFinished: Bool {
        get {return _isFinished}
        set {
            willChangeValue(forKey: "isFinished")
            _isFinished = newValue
            didChangeValue(forKey: "isFinished")
        }
    }
    private var _isFinished = false

    init(promise: Promise<T>) {
        self.promise = promise
    }

    override func start() {
        Log()
        self.isExecuting = true
        self.isFinished = false
        promise.done { result -> Void in
            self.result = result
            self.error = nil
        }.ensure {
            self.isExecuting = false
            self.isFinished = true
        }.catch { error in
            self.result = nil
            self.error = error
        }
    }
}