A SwiftUI wrapper around the new Sign in with Apple button that returns OAuthCredentials directly to login with Firebase

SignInWithAppleFirebaseButton.swift

import AuthenticationServices
import CryptoKit
import FirebaseAuth
import Foundation
import SwiftUI

// Adapted from https://firebase.google.com/docs/auth/ios/apple?authuser=0

@available(iOS 14.0, OSX 10.16, tvOS 14.0, *)
@available(watchOS, unavailable)
struct SignInWithAppleFirebaseButton: View {

    public typealias Body = SignInWithAppleButton

    private let label: SignInWithAppleButton.Label
    private let currentNonce: String
    private let completion: (Result<OAuthCredential, Error>) -> Void

    public init(_ label: SignInWithAppleButton.Label = .signIn, completion: @escaping (Result<OAuthCredential, Error>) -> Void) {
        self.label = label
        self.currentNonce = SignInWithAppleFirebaseButton.randomNonceString()
        self.completion = completion
    }

    public var body: SignInWithAppleButton {
        SignInWithAppleButton(label) { request in
            request.requestedScopes = [.fullName, .email]
            request.nonce = makeHashedNonce(currentNonce)
        } onCompletion: { result in
            switch result {
            case .success(let authResult):
                guard let appleCredential = authResult.credential as? ASAuthorizationAppleIDCredential else {
                    completion(.failure(AuthenticationError.incompatibleCredentials))
                    return
                }
                guard let appleIDToken = appleCredential.identityToken else {
                    completion(.failure(AuthenticationError.missingIdentityToken))
                    return
                }
                guard let idTokenString = String(data: appleIDToken, encoding: .utf8) else {
                    completion(.failure(AuthenticationError.invalidData(appleIDToken)))
                    return
                }
                // Initialize a Firebase credential.
                let credential = OAuthProvider.credential(
                    withProviderID: "apple.com",
                    idToken: idTokenString,
                    rawNonce: currentNonce)

                // Sign in with Firebase.
                completion(.success(credential))
            case .failure(let error):
                completion(.failure(error))
            }
        }
    }

    private func makeHashedNonce(_ input: String) -> String {
        let inputData = Data(input.utf8)
        let hashedData = SHA256.hash(data: inputData)
        let hashString = hashedData.compactMap {
            return String(format: "%02x", $0)
        }.joined()

        return hashString
    }

    // Adapted from https://auth0.com/docs/api-auth/tutorials/nonce#generate-a-cryptographically-random-nonce
    private static func randomNonceString(length: Int = 32) -> String {
        precondition(length > 0)
        let charset: Array<Character> =
            Array("0123456789ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz-._")
        var result = ""
        var remainingLength = length

        while remainingLength > 0 {
            let randoms: [UInt8] = (0 ..< 16).map { _ in
                var random: UInt8 = 0
                let errorCode = SecRandomCopyBytes(kSecRandomDefault, 1, &random)
                if errorCode != errSecSuccess {
                    fatalError("Unable to generate nonce. SecRandomCopyBytes failed with OSStatus \(errorCode)")
                }
                return random
            }

            randoms.forEach { random in
                if remainingLength == 0 {
                    return
                }

                if random < charset.count {
                    result.append(charset[Int(random)])
                    remainingLength -= 1
                }
            }
        }

        return result
    }

}

final class AuthenticationError: NSError {

    static let incompatibleCredentials = AuthenticationError(
        domain: "com.henrikpanhans.SignInWithAppleFirebaseButton",
        code: 1,
        description: "Incompatible credentials returned by Apple")

    static let missingIdentityToken = AuthenticationError(
        domain: "com.henrikpanhans.SignInWithAppleFirebaseButton",
        code: 2,
        description: "Unable to fetch identity token")

    static func invalidData(_ data: Data) -> AuthenticationError {
        AuthenticationError(
            domain: "com.henrikpanhans.SignInWithAppleFirebaseButton",
            code: 3,
            description: "Unable to serialize token string from data: \(data.debugDescription)")

    }

}

extension NSError {

    convenience init(domain: String, code: Int, description: String) {
        let dictionary = [NSLocalizedDescriptionKey: description]
        self.init(domain: domain, code: code, userInfo: dictionary)
    }

}