Extending local authentication with async support
In this post, let’s take a look at how to extend the LocalAuthentication framework with an async way to perform local authentication.
The LocalAuthentication framework makes it easy to perform local authentication in many ways, e.g. with biometric authentication using TouchID & FaceID, which I wrote about here.
You can also let users use their passcode, Apple Watch etc. which gives you many options for your apps to authenticate the user.
You can use the LAContext class to achieve this. At the time of writing, it however only has completion block-based functions. 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, 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 {
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 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 this is a lot cleaner, and easier to use in other async functions. The approach can be used for all kind of functionality that not yet supports async/await.
I’m happy that Apple made it so easy, since it will take time to add support for async/await everywhere. This gives us easy ways to use completion-based functions with async/await.
Source Code
I have added this extension to my SwiftUIKit library. Feel free to try it out and let me know what you think!
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