fix(auth): pre-fill email when "Continue as" button is tapped (#2423)

auth/src/main/java/com/firebase/ui/auth/ui/method_picker/AuthMethodPicker.kt

@@ -82,6 +82,9 @@ class MethodPickerTermsConfiguration(
  * @param providers The list of providers to display.
  * @param logo An optional logo to display.
  * @param onProviderSelected A callback when a provider is selected.
+ * @param onContinueAsSelected A callback when the "Continue as..." button is selected, with the
+ * provider and saved identifier (email, phone number, etc.). Falls back to [onProviderSelected]
+ * if not provided.
  * @param customLayout An optional custom layout composable for the provider buttons.
  * @param termsOfServiceUrl The URL for the Terms of Service.
  * @param privacyPolicyUrl The URL for the Privacy Policy.
@@ -97,12 +100,15 @@ fun AuthMethodPicker(
     providers: List<AuthProvider>,
     logo: AuthUIAsset? = null,
     onProviderSelected: (AuthProvider) -> Unit,
+    onContinueAsSelected: ((AuthProvider, String?) -> Unit)? = null,
     termsOfServiceUrl: String? = null,
     privacyPolicyUrl: String? = null,
     lastSignInPreference: SignInPreferenceManager.SignInPreference? = null,
     customLayout: (@Composable (List<AuthProvider>, (AuthProvider) -> Unit) -> Unit)? = null,
     termsConfiguration: MethodPickerTermsConfiguration? = null,
 ) {
+    val continueAsHandler: (AuthProvider, String?) -> Unit =
+        onContinueAsSelected ?: { provider, _ -> onProviderSelected(provider) }
     val context = LocalContext.current
     val inPreview = LocalInspectionMode.current
     val stringProvider = LocalAuthUIStringProvider.current
@@ -148,7 +154,7 @@ fun AuthMethodPicker(
                                     provider = lastProvider,
                                     identifier = preference.identifier,
                                     enabled = providerButtonsEnabled,
-                                    onClick = { onProviderSelected(lastProvider) }
+                                    onClick = { continueAsHandler(lastProvider, preference.identifier) }
                                 )
                                 Spacer(modifier = Modifier.height(24.dp))
 

auth/src/main/java/com/firebase/ui/auth/ui/screens/FirebaseAuthScreen.kt

@@ -147,6 +147,7 @@ fun FirebaseAuthScreen(
     val pendingReauthState = remember { mutableStateOf<AuthState.ReauthenticationRequired?>(null) }
     val pendingReauthOperation = remember { mutableStateOf<(suspend (android.content.Context) -> Unit)?>(null) }
     val emailLinkFromDifferentDevice = remember { mutableStateOf<String?>(null) }
+    val prefillEmail = remember { mutableStateOf<String?>(null) }
     val lastSignInPreference =
         remember { mutableStateOf<SignInPreferenceManager.SignInPreference?>(null) }
     val startRoute = remember(configuration.providers, configuration.isProviderChoiceAlwaysShown) {
@@ -218,7 +219,14 @@ fun FirebaseAuthScreen(
                             lastSignInPreference = lastSignInPreference.value,
                             customLayout = customMethodPickerLayout,
                             termsConfiguration = customMethodPickerTermsConfiguration,
-                            onProviderSelected = onProviderSelected,
+                            onProviderSelected = { provider ->
+                                prefillEmail.value = null
+                                onProviderSelected(provider)
+                            },
+                            onContinueAsSelected = { provider, identifier ->
+                                prefillEmail.value = identifier
+                                onProviderSelected(provider)
+                            },
                         )
                     }
                 }
@@ -228,6 +236,7 @@ fun FirebaseAuthScreen(
                         context = context,
                         configuration = configuration,
                         authUI = authUI,
+                        prefillEmail = prefillEmail.value,
                         credentialForLinking = pendingLinkingCredential.value,
                         emailLinkFromDifferentDevice = emailLinkFromDifferentDevice.value,
                         onContinueWithProvider = continueWithProvider,

auth/src/main/java/com/firebase/ui/auth/ui/screens/email/EmailAuthScreen.kt

@@ -129,6 +129,7 @@ fun EmailAuthScreen(
     context: Context,
     configuration: AuthUIConfiguration,
     authUI: FirebaseAuthUI,
+    prefillEmail: String? = null,
     credentialForLinking: AuthCredential? = null,
     emailLinkFromDifferentDevice: String? = null,
     onContinueWithProvider: (String) -> Unit = {},
@@ -150,7 +151,7 @@ fun EmailAuthScreen(
     }
     val mode = rememberSaveable { mutableStateOf(initialMode) }
     val displayNameValue = rememberSaveable { mutableStateOf("") }
-    val emailTextValue = rememberSaveable { mutableStateOf("") }
+    val emailTextValue = rememberSaveable { mutableStateOf(prefillEmail ?: "") }
     val passwordTextValue = rememberSaveable { mutableStateOf("") }
     val confirmPasswordTextValue = rememberSaveable { mutableStateOf("") }