Add implementation

app/lib/main/plugin_initializations.dart

@@ -80,6 +80,8 @@ class PluginInitializations {
       'revenuecat_api_android_key': 'goog_EyqDtrZhkswSqMAcfqawHGAqZnX',
       'firebase_messaging_vapid_key':
           'BNT7Da6B6wi-mUBcGrt-9HxeIJZsPTsPpmR8cae_LhgJPcSFb5j0T8o-r-oFV1xAtXVXfRPIZlgUJR3tx8mLbbA',
+      'stripe_checkout_session_function_url':
+          'https://europe-west1-sharezone-c2bd8.cloudfunctions.net/createStripeCheckoutSession'
     });
 
     try {

app/lib/main/sharezone_bloc_providers.dart

@@ -112,7 +112,9 @@ import 'package:sharezone/util/notification_token_adder.dart';
 import 'package:sharezone/util/platform_information_manager/flutter_platform_information_retreiver.dart';
 import 'package:sharezone/util/platform_information_manager/get_platform_information_retreiver.dart';
 import 'package:sharezone_common/references.dart';
+import 'package:stripe_checkout_session/stripe_checkout_session.dart';
 import 'package:user/user.dart';
+import 'package:http/http.dart' as http;
 
 import '../homework/parent/src/homework_page_bloc.dart' as old;
 import '../notifications/is_firebase_messaging_supported.dart';
@@ -335,8 +337,15 @@ class _SharezoneBlocProvidersState extends State<SharezoneBlocProviders> {
       ),
       ChangeNotifierProvider(
         create: (context) => SharezonePlusPageController(
+          userId: UserId(api.uID),
           purchaseService: RevenueCatPurchaseService(),
           subscriptionService: subscriptionService,
+          stripeCheckoutSession: StripeCheckoutSession(
+            createCheckoutSessionFunctionUrl: widget
+                .blocDependencies.remoteConfiguration
+                .getString('stripe_checkout_session_function_url'),
+            client: http.Client(),
+          ),
         ),
       ),
       ChangeNotifierProvider(

app/lib/sharezone_plus/page/sharezone_plus_page_controller.dart

@@ -8,12 +8,13 @@
 
 import 'dart:async';
 
+import 'package:common_domain_models/common_domain_models.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:sharezone/sharezone_plus/subscription_service/revenue_cat_sharezone_plus_service.dart';
-import 'package:sharezone/sharezone_plus/subscription_service/stripe_sharezone_plus_service.dart';
 import 'package:sharezone/sharezone_plus/subscription_service/subscription_service.dart';
 import 'package:sharezone_utils/platform.dart';
+import 'package:stripe_checkout_session/stripe_checkout_session.dart';
 import 'package:url_launcher/url_launcher.dart';
 
 /// A fallback price if the price cannot be fetched from the backend.
@@ -28,22 +29,25 @@ class SharezonePlusPageController extends ChangeNotifier {
   // ignore: unused_field
   late SubscriptionService _subscriptionService;
 
-  late StripeSharezonePlusService _stripeService;
+  late StripeCheckoutSession _stripeCheckoutSession;
+  late UserId _userId;
 
   StreamSubscription<bool>? _hasPlusSubscription;
 
   SharezonePlusPageController({
     required RevenueCatPurchaseService purchaseService,
     required SubscriptionService subscriptionService,
-    required StripeSharezonePlusService stripeService,
+    required StripeCheckoutSession stripeCheckoutSession,
+    required UserId userId,
   }) {
     _purchaseService = purchaseService;
     _subscriptionService = subscriptionService;
-    _stripeService = stripeService;
+    _stripeCheckoutSession = stripeCheckoutSession;
+    _userId = userId;
 
     // Fake loading for development purposes.
     Future.delayed(const Duration(seconds: 1)).then((value) {
-      hasPlus = true;
+      hasPlus = false;
       price = fallbackPlusPrice;
       notifyListeners();
     });
@@ -66,20 +70,26 @@ class SharezonePlusPageController extends ChangeNotifier {
   String? price;
 
   Future<void> buySubscription() async {
-    hasPlus = true;
-
     if (PlatformCheck.isWeb) {
       await _buyOnWeb();
     }
 
+    hasPlus = true;
     notifyListeners();
   }
 
   Future<void> _buyOnWeb() async {
-    final checkoutUrl = await _stripeService.createCheckoutUrl();
+    final checkoutUrl = await _stripeCheckoutSession.create(
+      userId: '$_userId',
+    );
+
     await launchUrl(
       Uri.parse(checkoutUrl),
-      // TODO: Insert ticket for async link opening
+      // Since the request for creating the checkout session is asynchronous, we
+      // can't open the checkout in a new tab due to the browser security
+      // policy.
+      //
+      // See https://github.com/flutter/flutter/issues/78524.
       webOnlyWindowName: "_self",
     );
   }

app/pubspec.yaml

@@ -53,6 +53,8 @@ dependencies:
     path: ../lib/common_domain_models
   crash_analytics:
     path: ../lib/crash_analytics
+  stripe_checkout_session:
+    path: ../lib/sharezone_plus/stripe_checkout_session
   cupertino_icons: ^1.0.3
   date:
     path: ../lib/date
@@ -108,6 +110,7 @@ dependencies:
     path: ../lib/hausaufgabenheft_logik
   holidays:
     path: ../lib/holidays
+  http: ^0.13.5
   # Used so that we can select a minute interval (only either XX:00 or XX:30)
   # for the homework reminder time picker.
   interval_time_picker: ^2.0.0+5

lib/sharezone_plus/stripe_checkout_session/analysis_options.yaml

@@ -0,0 +1,9 @@
+# Copyright (c) 2023 Sharezone UG (haftungsbeschränkt)
+# Licensed under the EUPL-1.2-or-later.
+#
+# You may obtain a copy of the Licence at:
+# https://joinup.ec.europa.eu/software/page/eupl
+#
+# SPDX-License-Identifier: EUPL-1.2
+
+include: package:sharezone_lints/analysis_options.yaml

lib/sharezone_plus/stripe_checkout_session/lib/src/stripe_checkout_session.dart

@@ -0,0 +1,73 @@
+// Copyright (c) 2023 Sharezone UG (haftungsbeschränkt)
+// Licensed under the EUPL-1.2-or-later.
+//
+// You may obtain a copy of the Licence at:
+// https://joinup.ec.europa.eu/software/page/eupl
+//
+// SPDX-License-Identifier: EUPL-1.2
+
+import 'package:http/http.dart' as http;
+import 'dart:convert';
+import 'package:retry/retry.dart';
+
+class StripeCheckoutSession {
+  final String createCheckoutSessionFunctionUrl;
+  final http.Client client;
+
+  StripeCheckoutSession({
+    required this.client,
+    required this.createCheckoutSessionFunctionUrl,
+  });
+
+  /// Creates a new Stripe checkout session.
+  ///
+  /// Either [userId] or [buysFor] must be passed to identify the user. [userId]
+  /// is the ID of the user who is buying the subscription. [buysFor] is the ID
+  /// of the user for whom the subscription is bought and is used for the
+  /// "Parents buy for their children" feature.
+  ///
+  /// Returns the URL of the Stripe checkout session. The user must be
+  /// redirected to this URL to complete the payment.
+  Future<String> create({
+    String? userId,
+    String? buysFor,
+  }) async {
+    assert(
+      userId != null || buysFor != null,
+      'Either userId or buysFor must be passed',
+    );
+
+    return retry<String>(
+      () async {
+        final response = await client.post(
+          Uri.parse(createCheckoutSessionFunctionUrl),
+          headers: {
+            'Content-Type': 'application/json',
+          },
+          body: jsonEncode(
+            {
+              if (userId != null) 'userId': userId,
+              if (buysFor != null) 'buysFor': buysFor,
+            },
+          ),
+        );
+
+        if (response.statusCode == 200) {
+          var responseData = jsonDecode(response.body);
+          final String? url = responseData['url'];
+
+          if (url == null) {
+            throw Exception('Could not found url in response');
+          }
+
+          return url;
+        } else {
+          throw Exception(
+            'Request failed with status: ${response.statusCode} (${response.body})',
+          );
+        }
+      },
+      maxAttempts: 3,
+    );
+  }
+}

lib/sharezone_plus/stripe_checkout_session/lib/stripe_checkout_session.dart

@@ -0,0 +1,11 @@
+// Copyright (c) 2023 Sharezone UG (haftungsbeschränkt)
+// Licensed under the EUPL-1.2-or-later.
+//
+// You may obtain a copy of the Licence at:
+// https://joinup.ec.europa.eu/software/page/eupl
+//
+// SPDX-License-Identifier: EUPL-1.2
+
+library stripe_checkout_session;
+
+export 'src/stripe_checkout_session.dart';