chore: update example app on how to embed SwiftUI auth in UIKit

samples/swiftui/FirebaseSwiftUISample/FirebaseSwiftUISample/Application/ContentView.swift

@@ -118,6 +118,30 @@ struct ContentView: View {
           }
         }
         .tint(Color(.label))
+        NavigationLink {
+          UIKitEmbeddingExample()
+            .navigationTitle("Embedding in UIKit")
+        } label: {
+          VStack(alignment: .leading, spacing: 16) {
+            Text("UIKit embedding example")
+              .font(.headline)
+              .fontWeight(.bold)
+            Text("How to host FirebaseSwiftUI inside a UIKit view controller")
+            Text(
+              "• Inline authentication surface\n• UIHostingController inside UIKit\n• No auth sheet toggle required"
+            )
+            .font(.caption)
+            .foregroundColor(.secondary)
+          }
+          .multilineTextAlignment(.leading)
+          .padding()
+          .frame(maxWidth: .infinity, alignment: .leading)
+          .background {
+            RoundedRectangle(cornerRadius: 16)
+              .fill(Color(UIColor.secondarySystemBackground))
+          }
+        }
+        .tint(Color(.label))
       }
       .padding()
       .navigationTitle("FirebaseUI Demo")

samples/swiftui/FirebaseSwiftUISample/FirebaseSwiftUISample/Examples/UIKitEmbeddingExample.swift

@@ -0,0 +1,187 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import FirebaseAppleSwiftUI
+import FirebaseAuth
+import FirebaseAuthSwiftUI
+import FirebaseGoogleSwiftUI
+import SwiftUI
+import UIKit
+
+struct UIKitEmbeddingExample: View {
+  private let authService: AuthService
+
+  init() {
+    authService = AuthService()
+      .withAppleSignIn()
+      .withGoogleSignIn()
+      .withEmailSignIn()
+  }
+
+  var body: some View {
+    ScrollView {
+      VStack(alignment: .leading, spacing: 20) {
+        Text("Embed FirebaseSwiftUI inside any UIKit container")
+          .font(.title2)
+          .fontWeight(.bold)
+
+        Text(
+          "This example creates a UIKit view controller, mounts a SwiftUI screen with UIHostingController, and uses AuthPickerView for the unauthenticated flow."
+        )
+        .foregroundStyle(.secondary)
+
+        FirebaseAuthUIKitContainer()
+          .frame(minHeight: 620)
+      }
+      .padding()
+    }
+    .background(Color(UIColor.systemGroupedBackground))
+    .environment(authService)
+  }
+}
+
+private struct FirebaseAuthUIKitContainer: UIViewControllerRepresentable {
+  @Environment(AuthService.self) private var authService
+
+  func makeUIViewController(context: Context) -> EmbeddedAuthViewController {
+    let viewController = EmbeddedAuthViewController()
+    viewController.update(authService: authService)
+    return viewController
+  }
+
+  func updateUIViewController(_ uiViewController: EmbeddedAuthViewController, context: Context) {
+    uiViewController.update(authService: authService)
+  }
+}
+
+@MainActor
+private final class EmbeddedAuthViewController: UIViewController {
+  private var hostingController: UIHostingController<AnyView>?
+
+  override func viewDidLoad() {
+    super.viewDidLoad()
+    view.backgroundColor = .clear
+  }
+
+  func update(authService: AuthService) {
+    let rootView = AnyView(EmbeddedAuthView().environment(authService))
+
+    if let hostingController {
+      hostingController.rootView = rootView
+      return
+    }
+
+    let hostingController = UIHostingController(rootView: rootView)
+    hostingController.view.translatesAutoresizingMaskIntoConstraints = false
+    hostingController.view.backgroundColor = .clear
+
+    addChild(hostingController)
+    view.addSubview(hostingController.view)
+    NSLayoutConstraint.activate([
+      hostingController.view.topAnchor.constraint(equalTo: view.topAnchor),
+      hostingController.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+      hostingController.view.trailingAnchor.constraint(equalTo: view.trailingAnchor),
+      hostingController.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
+    ])
+    hostingController.didMove(toParent: self)
+
+    self.hostingController = hostingController
+  }
+}
+
+private struct EmbeddedAuthView: View {
+  @Environment(AuthService.self) private var authService
+
+  var body: some View {
+    AuthPickerView {
+      authenticatedApp
+    }
+    .onChange(of: authService.authenticationState) { _, newValue in
+      if newValue != .authenticating {
+        authService.isPresented = newValue == .unauthenticated
+      }
+    }
+  }
+
+  private var authenticatedApp: some View {
+    VStack(spacing: 24) {
+      VStack(alignment: .leading, spacing: 8) {
+        Text("UIKit-hosted auth flow")
+          .font(.headline)
+          .fontWeight(.semibold)
+
+        Text("This SwiftUI view is rendered by a UIKit UIViewController.")
+          .font(.subheadline)
+          .foregroundStyle(.secondary)
+      }
+      .frame(maxWidth: .infinity, alignment: .leading)
+
+      if authService.authenticationState == .unauthenticated {
+        VStack(spacing: 16) {
+          Text("Not Authenticated")
+            .font(.title3)
+            .fontWeight(.semibold)
+
+          Text(
+            "AuthPickerView handles the sign-in UI. This UIKit-hosted screen just decides what to show before and after authentication."
+          )
+          .multilineTextAlignment(.center)
+          .foregroundStyle(.secondary)
+
+          Button("Authenticate") {
+            authService.isPresented = true
+          }
+          .buttonStyle(.borderedProminent)
+        }
+        .frame(maxWidth: .infinity)
+        .padding(.vertical, 32)
+      } else {
+        VStack(spacing: 20) {
+          Image(systemName: "person.crop.circle.badge.checkmark")
+            .font(.system(size: 56))
+            .foregroundStyle(.green)
+
+          Text(authService.currentUser?.email ?? "Signed in")
+            .font(.title3)
+            .fontWeight(.semibold)
+
+          Text("Firebase Auth is now authenticated. From here, UIKit or SwiftUI can take over the rest of your app flow.")
+            .multilineTextAlignment(.center)
+            .foregroundStyle(.secondary)
+
+          Button("Manage Account") {
+            authService.isPresented = true
+          }
+          .buttonStyle(.bordered)
+
+          Button("Sign Out") {
+            Task {
+              try? await authService.signOut()
+            }
+          }
+          .buttonStyle(.borderedProminent)
+        }
+        .frame(maxWidth: .infinity)
+        .padding(.top, 24)
+      }
+    }
+    .padding(24)
+    .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .top)
+    .background(
+      RoundedRectangle(cornerRadius: 24, style: .continuous)
+        .fill(Color(UIColor.secondarySystemGroupedBackground))
+    )
+  }
+
+}