[in_app_purchase] platform interface improvement

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart

@@ -24,15 +24,16 @@ abstract class InAppPurchasePlatform extends PlatformInterface {
   /// The instance of [InAppPurchasePlatform] to use.
   ///
   /// Defaults to `null`.
-  static InAppPurchasePlatform? get instance => _instance;
+  static InAppPurchasePlatform get instance => _instance;
 
-  static InAppPurchasePlatform? _instance;
+  // Should only be accessed after setter is called.
+  static late InAppPurchasePlatform _instance;
 
   /// Platform-specific plugins should set this with their own platform-specific
   /// class that extends [InAppPurchasePlatform] when they register themselves.
   // TODO(amirh): Extract common platform interface logic.
   // https://github.com/flutter/flutter/issues/43368
-  static void setInstance(InAppPurchasePlatform instance) {
+  static set instance(InAppPurchasePlatform instance) {
     PlatformInterface.verifyToken(instance, _token);
     _instance = instance;
   }

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart

@@ -2,18 +2,34 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// ignore: avoid_classes_with_only_static_members
+import 'package:in_app_purchase_platform_interface/in_app_purchase_platform_interface.dart';
+import 'package:plugin_platform_interface/plugin_platform_interface.dart';
+
 /// The interface that platform implementations must implement when they want to
 /// provide platform specific in_app_purchase features.
-abstract class InAppPurchasePlatformAddition {
+///
+/// Platform implementations should extend this class rather than implement it as `in_app_purchase`
+/// does not consider newly added methods to be breaking changes. Extending this class
+/// (using `extends`) ensures that the subclass will get the default implementation, while
+/// platform implementations that `implements` this interface will be broken by newly added
+/// [InAppPurchasePlatformAddition] methods.
+abstract class InAppPurchasePlatformAddition extends PlatformInterface {
+  /// Constructs a InAppPurchasePlatform.
+  InAppPurchasePlatformAddition() : super(token: _token);
+
+  static final Object _token = Object();
+
+  // Should only be accessed after setter is called.
+  static late InAppPurchasePlatformAddition _instance;
+
   /// The instance containing the platform-specific in_app_purchase
   /// functionality.
   ///
   /// To implement additional functionality extend
   /// [`InAppPurchasePlatformAddition`][3] with the platform-specific
   /// functionality, and when the plugin is registered, set the
   /// `InAppPurchasePlatformAddition.instance` with the new addition
-  /// implementationinstance.
+  /// implementation instance.
   ///
   /// Example implementation might look like this:
   /// ```dart
@@ -22,7 +38,7 @@ abstract class InAppPurchasePlatformAddition {
   /// }
   /// ```
   ///
-  /// The following snippit shows how to register the `InAppPurchaseMyPlatformAddition`:
+  /// The following snippet shows how to register the `InAppPurchaseMyPlatformAddition`:
   /// ```dart
   /// class InAppPurchaseMyPlatformPlugin {
   ///   static void registerWith(Registrar registrar) {
@@ -36,5 +52,13 @@ abstract class InAppPurchasePlatformAddition {
   ///   }
   /// }
   /// ```
-  static InAppPurchasePlatformAddition? instance;
+  static InAppPurchasePlatformAddition get instance => _instance;
+
+  /// Platform-specific plugins should set this with their own platform-specific
+  /// class that extends [InAppPurchasePlatformAddition] when they register themselves.
+  static set instance(InAppPurchasePlatformAddition instance) {
+    PlatformInterface.verifyToken(instance, _token);
+    assert(instance.runtimeType is! InAppPurchasePlatform);
+    _instance = instance;
+  }
 }

packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart

@@ -11,22 +11,18 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group('$InAppPurchasePlatform', () {
-    test('Default instance should return null', () {
-      expect(InAppPurchasePlatform.instance, null);
-    });
-
     test('Cannot be implemented with `implements`', () {
       expect(() {
-        InAppPurchasePlatform.setInstance(ImplementsInAppPurchasePlatform());
+        InAppPurchasePlatform.instance = ImplementsInAppPurchasePlatform();
       }, throwsNoSuchMethodError);
     });
 
     test('Can be extended', () {
-      InAppPurchasePlatform.setInstance(ExtendsInAppPurchasePlatform());
+      InAppPurchasePlatform.instance = ExtendsInAppPurchasePlatform();
     });
 
     test('Can be mocked with `implements`', () {
-      InAppPurchasePlatform.setInstance(MockInAppPurchasePlatform());
+      InAppPurchasePlatform.instance = MockInAppPurchasePlatform();
     });
 
     test(
@@ -124,6 +120,34 @@ void main() {
       );
     });
   });
+
+  group('$InAppPurchasePlatformAddition', () {
+    test('Cannot be implemented with `implements`', () {
+      expect(() {
+        InAppPurchasePlatformAddition.instance =
+            ImplementsInAppPurchasePlatformAddition();
+      }, throwsNoSuchMethodError);
+    });
+
+    test('InAppPurchasePlatformAddition Can be extended', () {
+      InAppPurchasePlatformAddition.instance =
+          ExtendsInAppPurchasePlatformAddition();
+    });
+
+    test('Can be mocked with `implements`', () {
+      InAppPurchasePlatformAddition.instance =
+          MockInAppPurchasePlatformAddition();
+    });
+
+    test('Provider can provide', () {
+      ImplementsInAppPurchasePlatformAdditionProvider.register();
+      final ImplementsInAppPurchasePlatformAdditionProvider provider =
+          ImplementsInAppPurchasePlatformAdditionProvider();
+      final InAppPurchasePlatformAddition addition =
+          provider.getPlatformAddition();
+      expect(addition.runtimeType, ExtendsInAppPurchasePlatformAddition);
+    });
+  });
 }
 
 class ImplementsInAppPurchasePlatform implements InAppPurchasePlatform {
@@ -143,3 +167,32 @@ class ExtendsInAppPurchasePlatform extends InAppPurchasePlatform {}
 class MockPurchaseParam extends Mock implements PurchaseParam {}
 
 class MockPurchaseDetails extends Mock implements PurchaseDetails {}
+
+class ImplementsInAppPurchasePlatformAddition
+    implements InAppPurchasePlatformAddition {
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+class MockInAppPurchasePlatformAddition extends Mock
+    with
+        // ignore: prefer_mixin
+        MockPlatformInterfaceMixin
+    implements
+        InAppPurchasePlatformAddition {}
+
+class ExtendsInAppPurchasePlatformAddition
+    extends InAppPurchasePlatformAddition {}
+
+class ImplementsInAppPurchasePlatformAdditionProvider
+    implements InAppPurchasePlatformAdditionProvider {
+  static void register() {
+    InAppPurchasePlatformAddition.instance =
+        ExtendsInAppPurchasePlatformAddition();
+  }
+
+  @override
+  T getPlatformAddition<T extends InAppPurchasePlatformAddition>() {
+    return InAppPurchasePlatformAddition.instance as T;
+  }
+}