Operation.swift

class AsyncOperation: Operation {
    var completion: () -> Void
    
    init(completion: @escaping () -> Void) {
        self.completion = completion
    }
    
    override func main() {
        let waitForFinish = DispatchGroup()
        waitForFinish.enter()
        
        if isCancelled { waitForFinish.leave(); return }

        CoolAsyncFunction {
            waitForFinish.leave()
        }

        waitForFinish.wait()
        
        completion()
        
        if self.isCancelled { return }
    }
}