Extending local authentication with async support

Apr 29, 2022 · Follow on Twitter and Mastodon swiftauthenticationasync-await

In this post, let’s take a look at how to extend the LocalAuthentication framework with an async way to perform local authentication.

Apple’s LocalAuthentication framework makes it easy to perform local authentication for a user in many ways, e.g. with biometric authentication using TouchID or FaceID, which I wrote about in this post a while back. You can also let users use their passcode, Apple Watch etc.

To achieve this, you use the LAContext class, which however only offers a completion block-based way to perform authentication. This means that your code will look something like this:

let policy = LAPolicy.deviceOwnerAuthenticationWithBiometrics
let reason = "The app needs your biometric information to unlock this feature."
let result = LAContext().evaluatePolicy(policy, localizedReason: reason) { result, error in
    // Handle the result or error

This code is still asynchronous, but doesn’t use the new async capabilities of Swift, which means that it does’t play all that way with other async functions.

Apple will probably extend LAContext with more async capabilities soon enough, but until they do, you can easily do it yourself by extending LAContext with a throwing async function:

@available(iOS 15.0, macOS 12.0, *)
extension LAContext {
    func evaluatePolicy(_ policy: LAPolicy, localizedReason reason: String) async throws -> Bool {
        return try await withCheckedThrowingContinuation { cont in
            LAContext().evaluatePolicy(policy, localizedReason: reason) { result, error in
                if let error = error { return cont.resume(throwing: error) }
                cont.resume(returning: result)

By wrapping the code in a withCheckedThrowingContinuation, you can create a throwing async function that can be called like this:

let context = LAContext()
let policy = LAPolicy.deviceOwnerAuthenticationWithBiometrics
let result = try await context.evaluatePolicy(.policy, reason: "...")

I think that this is a lot cleaner, and easier to use in other async functions.

This approach can be used for all kind of functionality that is not yet extended to support async/await. I’m really glad that Apple made it so easy, since it will take time for them to add this support everywhere.

Source code

I have added this extension to my SwiftKit library. Feel free to try it out and let me know what you think!

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