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

example-throws-in-callback.swift

import Foundation

// 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) {
    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))
}