Simple starter snippet for custom async operation

AsyncOperation.swift

public class AsyncOperation: Operation {
    public enum OperationError: Error {
        case cancelled
    }
    
    public var isFailed: Bool = false
    private var _completion: (Error?) -> Void
    private var _execution: ((Error?) -> Void) -> Void
    private var _isFinished = false
    private var _isExecuting = false
    
    public init(execution: @escaping ((Error?) -> Void) -> Void,
                completion: @escaping (Error?) -> Void) {
        _execution = execution
        _completion = completion
        super.init()
    }
    
    public override var isFinished: Bool {
        return _isFinished
    }
    
    public override var isExecuting: Bool {
        return _isExecuting
    }
    
    public override func start() {
        willChangeValue(forKey: "isExecuting")
        _isExecuting = true
        didChangeValue(forKey: "isExecuting")
        _execution() { error in
            guard !self.isCancelled else {
                _completion(OperationError.cancelled)
                return
            }
            guard error == nil else {
                self.willChangeValue(forKey: "isFailed")
                self.isFailed = true
                self.didChangeValue(forKey: "isFailed")
                
                _completion(error)
                return
            }
            self.willChangeValue(forKey: "isExecuting")
            self.willChangeValue(forKey: "isFinished")
            self._isExecuting = false
            self._isFinished = true
            self.didChangeValue(forKey: "isExecuting")
            self.didChangeValue(forKey: "isFinished")
            
            _completion(nil)
        }
    }
}