feat(ui_auth): Add "Go Back" button to EmailLinkSignInView (#63)

* Add "Go Back" button to EmailLinkSignInView

* Protect against going back if this is the root view

* Only check Navigator.canPop() on state init

* Add unit tests

* Add license header and fix formatting

* Update packages/firebase_ui_auth/test/views/email_link_sign_in_view_test.dart

* fix formatting

---------

Co-authored-by: Andrei Lesnitsky <[email protected]>

Author: rstieger

Diff for: packages/firebase_ui_auth/lib/src/views/email_link_sign_in_view.dart

@@ -39,6 +39,8 @@ class EmailLinkSignInView extends StatefulWidget {
 
 class _EmailLinkSignInViewState extends State<EmailLinkSignInView> {
   final emailCtrl = TextEditingController();
+  late final canPop = Navigator.canPop(context);
+
   @override
   Widget build(BuildContext context) {
     final l = FirebaseUILocalizations.labelsOf(context);
@@ -82,6 +84,16 @@ class _EmailLinkSignInViewState extends State<EmailLinkSignInView> {
                 },
               ),
             ],
+            if (canPop) ...[
+              const SizedBox(height: 8),
+              UniversalButton(
+                text: l.goBackButtonLabel,
+                variant: ButtonVariant.text,
+                onPressed: () {
+                  Navigator.of(context).pop();
+                },
+              ),
+            ],
             const SizedBox(height: 8),
             if (state is AuthFailed) ErrorText(exception: state.exception),
           ],

Diff for: packages/firebase_ui_auth/test/views/email_link_sign_in_view_test.dart

@@ -0,0 +1,72 @@
+// Copyright 2023, the Chromium project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:firebase_auth/firebase_auth.dart';
+import 'package:firebase_ui_auth/firebase_ui_auth.dart';
+import 'package:firebase_ui_localizations/firebase_ui_localizations.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../test_utils.dart';
+
+void main() {
+  const labels = DefaultLocalizations();
+  late MockAuth auth;
+  late MockDynamicLinks dynamicLinks;
+  late EmailLinkAuthProvider emailLinkProvider;
+
+  setUp(() {
+    auth = MockAuth();
+    dynamicLinks = MockDynamicLinks();
+    final actionCodeSettings = ActionCodeSettings(
+      url: 'https://example.com',
+    );
+    emailLinkProvider = EmailLinkAuthProvider(
+      actionCodeSettings: actionCodeSettings,
+      dynamicLinks: dynamicLinks,
+    );
+  });
+
+  /// If EmailLinkSignInView is the root view, there should be
+  /// no option to go back.
+  testWidgets('no go back option if root view', (tester) async {
+    await tester.pumpWidget(TestMaterialApp(
+      child: EmailLinkSignInView(
+        provider: emailLinkProvider,
+        auth: auth,
+      ),
+    ));
+
+    final button = find.text(labels.goBackButtonLabel);
+    expect(button, findsNothing);
+  });
+
+  /// If EmailLinkSignInView is pushed from another view, there
+  /// should be a button allowing a user to go back.
+  testWidgets('show go back option if not root', (tester) async {
+    await tester.pumpWidget(
+      TestMaterialApp(
+        child: Builder(
+          builder: (context) => TextButton(
+            child: const Text("Push"),
+            onPressed: () => Navigator.push(
+              context,
+              MaterialPageRoute(
+                builder: (context) => Scaffold(
+                  body: EmailLinkSignInView(
+                      provider: emailLinkProvider, auth: auth),
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.textContaining("Push"));
+    await tester.pumpAndSettle();
+    final button = find.text(labels.goBackButtonLabel);
+    expect(button, findsOneWidget);
+  });
+}