fix(ui_auth): automatically upgrade anonymous accounts

packages/firebase_ui_auth/example/lib/main.dart

@@ -33,6 +33,10 @@ Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
   await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);
 
+  if (FirebaseAuth.instance.currentUser == null) {
+    await FirebaseAuth.instance.signInAnonymously();
+  }
+
   FirebaseUIAuth.configureProviders([
     EmailAuthProvider(),
     emailLinkProviderConfig,
@@ -66,7 +70,7 @@ class FirebaseAuthUIExample extends StatelessWidget {
   String get initialRoute {
     final auth = FirebaseAuth.instance;
 
-    if (auth.currentUser == null) {
+    if (auth.currentUser == null || auth.currentUser!.isAnonymous) {
       return '/';
     }
 
@@ -139,6 +143,13 @@ class FirebaseAuthUIExample extends StatelessWidget {
                   Navigator.pushReplacementNamed(context, '/profile');
                 }
               }),
+              AuthStateChangeAction<CredentialLinked>((context, state) {
+                if (!state.user.emailVerified) {
+                  Navigator.pushNamed(context, '/verify-email');
+                } else {
+                  Navigator.pushReplacementNamed(context, '/profile');
+                }
+              }),
               mfaAction,
               EmailLinkSignInAction((context) {
                 Navigator.pushReplacementNamed(context, '/email-link-sign-in');

packages/firebase_ui_auth/lib/src/auth_flow.dart

@@ -126,7 +126,7 @@ class AuthFlow<T extends AuthProvider> extends ValueNotifier<AuthState>
 
   @override
   void onCredentialLinked(AuthCredential credential) {
-    value = CredentialLinked(credential);
+    value = CredentialLinked(credential, auth.currentUser!);
   }
 
   @override

packages/firebase_ui_auth/lib/src/auth_state.dart

@@ -107,8 +107,11 @@ class CredentialLinked extends AuthState {
   /// A credential that was linked with the currently signed in user account.
   final AuthCredential credential;
 
+  /// An instance of the [User] the credential was associated with.
+  final User user;
+
   /// {@macro ui.auth.auth_state.credential_linked}
-  CredentialLinked(this.credential);
+  CredentialLinked(this.credential, this.user);
 }
 
 /// {@template ui.auth.auth_state.auth_failed}

packages/firebase_ui_auth/lib/src/providers/auth_provider.dart

@@ -109,8 +109,12 @@ abstract class AuthProvider<T extends AuthListener, K extends AuthCredential> {
   /// {@macro ui.auth.auth_provider}
   AuthProvider();
 
+  /// Indicates whether the user should be upgraded and new credential should be
+  /// linked.
+  bool get shouldUpgradeAnonymous => auth.currentUser?.isAnonymous ?? false;
+
   /// Signs the user in with the provided [AuthCredential].
-  void signInWithCredential(AuthCredential credential) {
+  void signInWithCredential(K credential) {
     authListener.onBeforeSignIn();
     auth
         .signInWithCredential(credential)
@@ -120,8 +124,9 @@ abstract class AuthProvider<T extends AuthListener, K extends AuthCredential> {
 
   /// Links a provided [AuthCredential] with the currently signed in user
   /// account.
-  void linkWithCredential(AuthCredential credential) {
+  void linkWithCredential(K credential) {
     authListener.onCredentialReceived(credential);
+
     try {
       final user = auth.currentUser!;
       user
@@ -175,6 +180,11 @@ abstract class AuthProvider<T extends AuthListener, K extends AuthCredential> {
       // Only email provider has a different action for sign in and sign up
       // and implements it's own sign up logic.
       case AuthAction.signUp:
+        if (shouldUpgradeAnonymous) {
+          linkWithCredential(credential);
+          break;
+        }
+
         signInWithCredential(credential);
         break;
       case AuthAction.none:

packages/firebase_ui_auth/lib/src/providers/email_auth_provider.dart

@@ -55,10 +55,23 @@ class EmailAuthProvider
     fba.EmailAuthCredential credential,
     AuthAction action,
   ) {
-    if (action == AuthAction.signUp) {
-      signUpWithCredential(credential);
-    } else {
-      super.onCredentialReceived(credential, action);
+    switch (action) {
+      case AuthAction.signIn:
+        signInWithCredential(credential);
+        break;
+      case AuthAction.signUp:
+        if (shouldUpgradeAnonymous) {
+          return linkWithCredential(credential);
+        }
+
+        signUpWithCredential(credential);
+        break;
+      case AuthAction.link:
+        linkWithCredential(credential);
+        break;
+      case AuthAction.none:
+        super.onCredentialReceived(credential, action);
+        break;
     }
   }
 }