An example of swift async callbacks that either throw or return a result

example-throws-in-callback-that-works-in-playgrounds.swift

import Foundation
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true

// either throws an Error or returns a String result
typealias ThrowsCallback = () throws -> (String)

enum AsyncError: Error {
    case kaboom
}

func asyncActionWithCallback(_ someData: String, callback: @escaping (ThrowsCallback) -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
        if someData == "please error" {
            callback({ throw AsyncError.kaboom })
        } else {
            callback({ return "you provided \(someData)" })
        }
    }
}

asyncActionWithCallback("Hello") { result in
    let value = try? result()
    print(String(describing: value))
}

asyncActionWithCallback("please error") { result in
    let value = try? result()
    print(String(describing: value))
    PlaygroundPage.current.finishExecution()
}