feat: re-evaluate payment readiness when the app is resumed

pay/lib/src/widgets/pay_button.dart

@@ -123,8 +123,8 @@ abstract class PayButton extends StatefulWidget {
 /// button loads. If the payment provider is available for a given user, the
 /// [_payButton] is added to the tree. Otherwise, if set, the replacement widget
 /// in [childOnError] is shown.
-class _PayButtonState extends State<PayButton> {
-  late final Future<bool> _userCanPayFuture;
+class _PayButtonState extends State<PayButton> with WidgetsBindingObserver {
+  late Future<bool> _userCanPayFuture;
 
   /// A method to initialize payment result streams
   ///
@@ -146,6 +146,7 @@ class _PayButtonState extends State<PayButton> {
   @override
   void initState() {
     super.initState();
+    WidgetsBinding.instance.addObserver(this);
     if (!widget._isPlatformSupported) return;
 
     _userCanPayFuture = _userCanPay();
@@ -155,6 +156,23 @@ class _PayButtonState extends State<PayButton> {
     }
   }
 
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.resumed) {
+      if (widget._isPlatformSupported) {
+        setState(() {
+          _userCanPayFuture = _userCanPay();
+        });
+      }
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     if (!widget._isPlatformSupported) {

pay/test/pay_button_test.dart

@@ -0,0 +1,77 @@
+// Copyright 2024 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
+//
+//     https://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 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pay/pay.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  const channel = MethodChannel('plugins.flutter.io/pay');
+  const providerGooglePay = PayProvider.google_pay;
+  const payConfigString =
+      '{"provider": "google_pay", "data": {}}';
+  final paymentConfiguration =
+      PaymentConfiguration.fromJsonString(payConfigString);
+
+  testWidgets('PayButton re-evaluates readiness when app is resumed',
+      (WidgetTester tester) async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.android;
+    final log = <MethodCall>[];
+    var canPayResponse = true;
+
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(channel, (MethodCall methodCall) async {
+      log.add(methodCall);
+      if (methodCall.method == 'userCanPay') {
+        return canPayResponse;
+      }
+      return null;
+    });
+
+    // Pump the GooglePayButton widget.
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: GooglePayButton(
+            paymentConfiguration: paymentConfiguration,
+            paymentItems: const [],
+            onPaymentResult: (_) {},
+          ),
+        ),
+      ),
+    );
+
+    // Initial build/initState should call userCanPay.
+    await tester.pumpAndSettle();
+    expect(log.length, 1);
+    expect(log.first.method, 'userCanPay');
+
+    // Simulate resuming the app.
+    tester.binding.handleAppLifecycleStateChanged(AppLifecycleState.resumed);
+    await tester.pumpAndSettle();
+
+    // Verify that userCanPay was called a second time.
+    expect(log.length, 2);
+    expect(log[1].method, 'userCanPay');
+
+    // Clean up.
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(channel, null);
+    debugDefaultTargetPlatformOverride = null;
+  });
+}