fix: use secondary app if it is passed into FirebaseAuthUI

auth/src/main/java/com/firebase/ui/auth/AuthFlowController.kt

@@ -158,7 +158,11 @@ class AuthFlowController internal constructor(
      */
     fun createIntent(context: Context): Intent {
         checkNotDisposed()
-        return FirebaseAuthActivity.createIntent(context, configuration)
+        return FirebaseAuthActivity.createIntent(
+            context = context,
+            configuration = configuration,
+            authUI = authUI
+        )
     }
 
     /**

auth/src/main/java/com/firebase/ui/auth/FirebaseAuthActivity.kt

@@ -77,10 +77,10 @@ class FirebaseAuthActivity : ComponentActivity() {
         super.onCreate(savedInstanceState)
         enableEdgeToEdge()
 
-        // Extract configuration from cache using UUID key
-        val configKey = intent.getStringExtra(EXTRA_CONFIGURATION_KEY)
-        configuration = if (configKey != null) {
-            configurationCache.remove(configKey)
+        // Extract configuration and auth instance from cache using UUID key
+        val launchKey = intent.getStringExtra(EXTRA_CONFIGURATION_KEY)
+        configuration = if (launchKey != null) {
+            configurationCache.remove(launchKey)
         } else {
             null
         } ?: run {
@@ -90,7 +90,16 @@ class FirebaseAuthActivity : ComponentActivity() {
             return
         }
 
-        authUI = FirebaseAuthUI.getInstance()
+        authUI = if (launchKey != null) {
+            authUICache.remove(launchKey)
+        } else {
+            null
+        } ?: run {
+            // Missing auth instance, finish with error
+            setResult(RESULT_CANCELED)
+            finish()
+            return
+        }
 
         // Extract email link if present
         val emailLink = intent.getStringExtra(EmailLinkConstants.EXTRA_EMAIL_LINK)
@@ -191,14 +200,18 @@ class FirebaseAuthActivity : ComponentActivity() {
          */
         internal fun createIntent(
             context: Context,
-            configuration: AuthUIConfiguration
+            configuration: AuthUIConfiguration,
+            authUI: FirebaseAuthUI = FirebaseAuthUI.getInstance()
         ): Intent {
             val configKey = UUID.randomUUID().toString()
             configurationCache[configKey] = configuration
+            authUICache[configKey] = authUI
 
             return Intent(context, FirebaseAuthActivity::class.java).apply {
                 putExtra(EXTRA_CONFIGURATION_KEY, configKey)
             }
         }
+
+        private val authUICache = ConcurrentHashMap<String, FirebaseAuthUI>()
     }
 }

auth/src/test/java/com/firebase/ui/auth/FirebaseAuthActivityTest.kt

@@ -50,6 +50,7 @@ class FirebaseAuthActivityTest {
 
     private lateinit var applicationContext: Context
     private lateinit var authUI: FirebaseAuthUI
+    private lateinit var secondaryAuthUI: FirebaseAuthUI
     private lateinit var configuration: AuthUIConfiguration
 
     @Mock
@@ -79,8 +80,20 @@ class FirebaseAuthActivityTest {
                 .build()
         )
 
+        val secondaryApp = FirebaseApp.initializeApp(
+            applicationContext,
+            FirebaseOptions.Builder()
+                .setApiKey("fake-api-key-2")
+                .setApplicationId("fake-app-id-2")
+                .setProjectId("fake-project-id-2")
+                .build(),
+            "secondary"
+        )
+
         authUI = FirebaseAuthUI.getInstance()
         authUI.auth.useEmulator("127.0.0.1", 9099)
+        secondaryAuthUI = FirebaseAuthUI.getInstance(secondaryApp)
+        secondaryAuthUI.auth.useEmulator("127.0.0.1", 9099)
 
         configuration = AuthUIConfiguration(
             context = applicationContext,
@@ -180,6 +193,46 @@ class FirebaseAuthActivityTest {
         assertThat(activity.isFinishing).isFalse()
     }
 
+    @Test
+    fun `activity launched from secondary auth flow observes supplied authUI instead of default app`() =
+        runTest {
+            val controller = secondaryAuthUI.createAuthFlow(configuration)
+            val intent = controller.createIntent(applicationContext)
+            val activity = Robolectric.buildActivity(FirebaseAuthActivity::class.java, intent)
+                .create()
+                .start()
+                .resume()
+                .get()
+
+            `when`(mockFirebaseUser.uid).thenReturn("secondary-user-id")
+
+            authUI.updateAuthState(
+                AuthState.Success(
+                    result = null,
+                    user = mockFirebaseUser,
+                    isNewUser = false
+                )
+            )
+            shadowOf(Looper.getMainLooper()).idle()
+
+            assertThat(activity.isFinishing).isFalse()
+
+            secondaryAuthUI.updateAuthState(
+                AuthState.Success(
+                    result = null,
+                    user = mockFirebaseUser,
+                    isNewUser = false
+                )
+            )
+            shadowOf(Looper.getMainLooper()).idle()
+
+            assertThat(activity.isFinishing).isTrue()
+            val shadowActivity = shadowOf(activity)
+            assertThat(shadowActivity.resultCode).isEqualTo(Activity.RESULT_OK)
+            assertThat(shadowActivity.resultIntent.getStringExtra(FirebaseAuthActivity.EXTRA_USER_ID))
+                .isEqualTo("secondary-user-id")
+        }
+
     // =============================================================================================
     // Auth State Success Tests
     // =============================================================================================