Get the most out of Sign in with Apple

// Configure request, setup delegates and perform authorization request

    @objc func handleAuthorizationButtonPress() {
        let request = ASAuthorizationAppleIDProvider().createRequest()
        request.requestedScopes = [.fullName, .email]
        
        request.nonce = myNonceString()
        request.state = myStateString()
            
        let controller = ASAuthorizationController(authorizationRequests: [request])
            
        controller.delegate = self
        controller.presentationContextProvider = self
            
        controller.performRequests()
    }

// ASAuthorizationControllerDelegate

func authorizationController(controller: ASAuthorizationController, didCompleteWithAuthorization authorization: ASAuthorization) {
        if let credential = authorization.credential as? ASAuthorizationAppleIDCredential {
            let userIdentifier = credential.user
            let fullName = credential.fullName
            let email = credential.email
            let realUserStatus = credential.realUserStatus
            
            let state = credential.state
            let identityToken = credential.identityToken
            let authorizationCode = credential.authorizationCode
            
            // Securely store the userIdentifier locally
            self.saveUserIdentifier(userIdentifier)
            
            // Create a session with your server and verify the information
            self.createSession(identityToken: identityToken, authorizationCode: authorizationCode)
    }
}

// Getting a credential state
        
        let provider = ASAuthorizationAppleIDProvider()
        
        provider.getCredentialState(forUserID: getStoredUserIdentifier()) { 
                                                        (credentialState, error) in
            switch(credentialState) {
            case .authorized:
                // Sign in with Apple credential Valid
            case .revoked:
                // Sign in with Apple credential Revoked, Sign out
            case .notFound:
                // Credential was not found, fallback to login screen
            case .transferred:
                // Application was recently transferred, refresh User Identifier
            @unknown default:
                break
            }
        }

// Migrating a user identifier

        let request = ASAuthorizationAppleIDProvider().createRequest()
        request.requestedScopes = [.fullName, .email]
            
        request.user = getStoredUserIdentifier()
        
        request.nonce = myNonceString()
        request.state = myStateString()
            
        let controller = ASAuthorizationController(authorizationRequests: [request])
            
        controller.delegate = self
        controller.presentationContextProvider = self
            
        controller.performRequests()

// SwiftUI example:

SignInWithAppleButton(.signIn) {
    onRequest: { (request) in
        request.requestedScopes = [.fullName, .email]
        request.nonce = myNonceString()
        request.state = myStateString()
    }
    onCompletion: { (result) in
        switch result {
        case .success(let authorization):
            // Handle Authorization
        case .failure(let error)
            // Handle Failure
        }
    }
}.signInWithAppleButtonStyle(.black)

enum VerificationResult : Int { case success; case failure; case twoFactorAuthRequired;

override func convertAccountToSignInWithAppleWithoutUserInteraction(
    for serviceIdentifier: ASCredentialServiceIdentifier, 
    existingCredential: ASPasswordCredential
) {
    verifyCredential(existingCredential) { (result: VerificationResult) in
        switch result {
        case .failure:
            self.extensionContext.cancelRequest(withError: 
                ASExtensionError(.failed))
        case .success:
          self.extensionContext.getSignInWithAppleAuthorizationWithState(state: myStateString(),
                                                                         nonce: myNonceString(),      
                                                                         {…}        
        case .twoFactorAuthRequired:
            self.extensionContext.cancelRequest(withError: 
                ASExtensionError(.userInteractionRequired))
    }
}