From 674bdddef029e849e60122f0de13e99fe52f0ae7 Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Thu, 22 Apr 2021 10:19:19 -0700
Subject: [PATCH 1/9] platform interface improvement

---
 .../lib/src/in_app_purchase_platform.dart     |  7 ++-
 .../in_app_purchase_platform_addition.dart    | 35 +++++++++--
 .../test/in_app_purchase_platform_test.dart   | 60 +++++++++++++++++--
 3 files changed, 88 insertions(+), 14 deletions(-)

diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart
index f8dc4c998494..e3a99f077d8d 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart
+++ b/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;
   }
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index 5c41f138ecea..835519b2695c 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -2,10 +2,27 @@
 // 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.
   ///
@@ -13,7 +30,7 @@ abstract class InAppPurchasePlatformAddition {
   /// [`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 +39,7 @@ 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 +53,13 @@ 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;
+  }
 }
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
index d5c1ae5fc127..73dec75df43c 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
@@ -11,22 +11,19 @@ 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 +121,31 @@ 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 +165,29 @@ 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;
+  }
+}

From 875d3f1274fda7b16d0418e07cbdc27593624b7b Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Thu, 22 Apr 2021 10:27:25 -0700
Subject: [PATCH 2/9] format

---
 .../in_app_purchase_platform_addition.dart    |  1 -
 .../test/in_app_purchase_platform_test.dart   | 31 +++++++++++--------
 2 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index 835519b2695c..77660faf4776 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -14,7 +14,6 @@ import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 /// 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);
 
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
index 73dec75df43c..7b885a3066e7 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
@@ -11,7 +11,6 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group('$InAppPurchasePlatform', () {
-
     test('Cannot be implemented with `implements`', () {
       expect(() {
         InAppPurchasePlatform.instance = ImplementsInAppPurchasePlatform();
@@ -123,28 +122,31 @@ void main() {
   });
 
   group('$InAppPurchasePlatformAddition', () {
-
     test('Cannot be implemented with `implements`', () {
       expect(() {
-        InAppPurchasePlatformAddition.instance = ImplementsInAppPurchasePlatformAddition();
+        InAppPurchasePlatformAddition.instance =
+            ImplementsInAppPurchasePlatformAddition();
       }, throwsNoSuchMethodError);
     });
 
     test('InAppPurchasePlatformAddition Can be extended', () {
-      InAppPurchasePlatformAddition.instance = ExtendsInAppPurchasePlatformAddition();
+      InAppPurchasePlatformAddition.instance =
+          ExtendsInAppPurchasePlatformAddition();
     });
 
     test('Can be mocked with `implements`', () {
-      InAppPurchasePlatformAddition.instance = MockInAppPurchasePlatformAddition();
+      InAppPurchasePlatformAddition.instance =
+          MockInAppPurchasePlatformAddition();
     });
 
     test('Provider can provide', () {
       ImplementsInAppPurchasePlatformAdditionProvider.register();
-      final ImplementsInAppPurchasePlatformAdditionProvider provider = ImplementsInAppPurchasePlatformAdditionProvider();
-      final InAppPurchasePlatformAddition addition = provider.getPlatformAddition();
+      final ImplementsInAppPurchasePlatformAdditionProvider provider =
+          ImplementsInAppPurchasePlatformAdditionProvider();
+      final InAppPurchasePlatformAddition addition =
+          provider.getPlatformAddition();
       expect(addition.runtimeType, ExtendsInAppPurchasePlatformAddition);
     });
-
   });
 }
 
@@ -166,7 +168,8 @@ class MockPurchaseParam extends Mock implements PurchaseParam {}
 
 class MockPurchaseDetails extends Mock implements PurchaseDetails {}
 
-class ImplementsInAppPurchasePlatformAddition implements InAppPurchasePlatformAddition {
+class ImplementsInAppPurchasePlatformAddition
+    implements InAppPurchasePlatformAddition {
   @override
   dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
@@ -178,12 +181,14 @@ class MockInAppPurchasePlatformAddition extends Mock
     implements
         InAppPurchasePlatformAddition {}
 
-class ExtendsInAppPurchasePlatformAddition extends InAppPurchasePlatformAddition {}
-
-class ImplementsInAppPurchasePlatformAdditionProvider implements InAppPurchasePlatformAdditionProvider {
+class ExtendsInAppPurchasePlatformAddition
+    extends InAppPurchasePlatformAddition {}
 
+class ImplementsInAppPurchasePlatformAdditionProvider
+    implements InAppPurchasePlatformAdditionProvider {
   static void register() {
-    InAppPurchasePlatformAddition.instance = ExtendsInAppPurchasePlatformAddition();
+    InAppPurchasePlatformAddition.instance =
+        ExtendsInAppPurchasePlatformAddition();
   }
 
   @override

From 06daea2115fb9d96da99b5403c07191488d8ef14 Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Fri, 23 Apr 2021 12:30:09 -0700
Subject: [PATCH 3/9] review

---
 .../in_app_purchase/CHANGELOG.md              |  2 +-
 .../src/in_app_purchase/product_details.dart  |  2 +-
 .../lib/src/in_app_purchase_platform.dart     | 10 ++++-----
 .../in_app_purchase_platform_addition.dart    | 17 +++++++++------
 ...p_purchase_platform_addition_provider.dart |  8 +++----
 .../src/types/product_details_response.dart   |  2 +-
 .../test/in_app_purchase_platform_test.dart   | 21 +++++++++++++++++--
 7 files changed, 42 insertions(+), 20 deletions(-)

diff --git a/packages/in_app_purchase/in_app_purchase/CHANGELOG.md b/packages/in_app_purchase/in_app_purchase/CHANGELOG.md
index 5e7f54560b8c..45b8b7661203 100644
--- a/packages/in_app_purchase/in_app_purchase/CHANGELOG.md
+++ b/packages/in_app_purchase/in_app_purchase/CHANGELOG.md
@@ -354,7 +354,7 @@ Beta release.
 
 * Ability to list products, load previous purchases, and make purchases.
 * Simplified Dart API that's been unified for ease of use.
-* Platform specific APIs more directly exposing `StoreKit` and `BillingClient`.
+* Platform-specific APIs more directly exposing `StoreKit` and `BillingClient`.
 
 Includes:
 
diff --git a/packages/in_app_purchase/in_app_purchase/lib/src/in_app_purchase/product_details.dart b/packages/in_app_purchase/in_app_purchase/lib/src/in_app_purchase/product_details.dart
index ccdec42b7303..4ba61305e445 100644
--- a/packages/in_app_purchase/in_app_purchase/lib/src/in_app_purchase/product_details.dart
+++ b/packages/in_app_purchase/in_app_purchase/lib/src/in_app_purchase/product_details.dart
@@ -90,7 +90,7 @@ class ProductDetailsResponse {
 
   /// The list of identifiers that are in the `identifiers` of [InAppPurchaseConnection.queryProductDetails] but failed to be fetched.
   ///
-  /// There's multiple platform specific reasons that product information could fail to be fetched,
+  /// There's multiple platform-specific reasons that product information could fail to be fetched,
   /// ranging from products not being correctly configured in the storefront to the queried IDs not existing.
   final List<String> notFoundIDs;
 
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart
index e3a99f077d8d..eac4a0712078 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart
@@ -23,12 +23,9 @@ abstract class InAppPurchasePlatform extends PlatformInterface {
 
   /// The instance of [InAppPurchasePlatform] to use.
   ///
-  /// Defaults to `null`.
+  /// Must be set before accessing.
   static InAppPurchasePlatform get instance => _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.
@@ -38,13 +35,16 @@ abstract class InAppPurchasePlatform extends PlatformInterface {
     _instance = instance;
   }
 
+  // Should only be accessed after setter is called.
+  static late InAppPurchasePlatform _instance;
+
   /// Listen to this broadcast stream to get real time update for purchases.
   ///
   /// This stream will never close as long as the app is active.
   ///
   /// Purchase updates can happen in several situations:
   /// * When a purchase is triggered by user in the app.
-  /// * When a purchase is triggered by user from the platform specific store front.
+  /// * When a purchase is triggered by user from the platform-specific store front.
   /// * When a purchase is restored on the device by the user in the app.
   /// * If a purchase is not completed ([completePurchase] is not called on the
   ///   purchase object) from the last app session. Purchase updates will happen
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index 77660faf4776..7785c0c3a289 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -6,7 +6,7 @@ import 'package:in_app_purchase_platform_interface/in_app_purchase_platform_inte
 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.
+/// provide platform-specific in_app_purchase features.
 ///
 /// 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
@@ -20,11 +20,13 @@ abstract class InAppPurchasePlatformAddition extends PlatformInterface {
   static final Object _token = Object();
 
   // Should only be accessed after setter is called.
-  static late InAppPurchasePlatformAddition _instance;
+  static InAppPurchasePlatformAddition? _instance;
 
   /// The instance containing the platform-specific in_app_purchase
   /// functionality.
   ///
+  /// Returns `null` by default.
+  ///
   /// To implement additional functionality extend
   /// [`InAppPurchasePlatformAddition`][3] with the platform-specific
   /// functionality, and when the plugin is registered, set the
@@ -52,13 +54,16 @@ abstract class InAppPurchasePlatformAddition extends PlatformInterface {
   ///   }
   /// }
   /// ```
-  static InAppPurchasePlatformAddition get instance => _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);
+  static set instance(InAppPurchasePlatformAddition? instance) {
     _instance = instance;
+    if (_instance == null) {
+      return;
+    }
+    PlatformInterface.verifyToken(instance!, _token);
+    assert(instance.runtimeType is! InAppPurchasePlatform);
   }
 }
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition_provider.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition_provider.dart
index d981f73b4019..642bbb419c6e 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition_provider.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition_provider.dart
@@ -5,13 +5,13 @@
 import 'package:in_app_purchase_platform_interface/src/in_app_purchase_platform_addition.dart';
 
 /// The [InAppPurchasePlatformAdditionProvider] is responsible for providing
-/// a platform specific [InAppPurchasePlatformAddition].
+/// a platform-specific [InAppPurchasePlatformAddition].
 ///
-/// [InAppPurchasePlatformAddition] implementation contain platform specific
+/// [InAppPurchasePlatformAddition] implementation contain platform-specific
 /// features that are not available from the platform idiomatic
 /// [InAppPurchasePlatform] API.
 abstract class InAppPurchasePlatformAdditionProvider {
-  /// Provides a platform specific implementation of the [InAppPurchasePlatformAddition]
+  /// Provides a platform-specific implementation of the [InAppPurchasePlatformAddition]
   /// class.
-  T getPlatformAddition<T extends InAppPurchasePlatformAddition>();
+  T getPlatformAddition<T extends InAppPurchasePlatformAddition?>();
 }
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/product_details_response.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/product_details_response.dart
index 868f9428add2..d8331d6560d8 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/product_details_response.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/product_details_response.dart
@@ -18,7 +18,7 @@ class ProductDetailsResponse {
 
   /// The list of identifiers that are in the `identifiers` of [InAppPurchasePlatform.queryProductDetails] but failed to be fetched.
   ///
-  /// There's multiple platform specific reasons that product information could fail to be fetched,
+  /// There's multiple platform-specific reasons that product information could fail to be fetched,
   /// ranging from products not being correctly configured in the storefront to the queried IDs not existing.
   final List<String> notFoundIDs;
 
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
index 7b885a3066e7..750599170990 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
@@ -122,6 +122,15 @@ void main() {
   });
 
   group('$InAppPurchasePlatformAddition', () {
+
+    setUp(() {
+      InAppPurchasePlatformAddition.instance = null;
+    });
+
+    test('Cannot be implemented with `implements`', () {
+      expect(InAppPurchasePlatformAddition.instance, isNull);
+    });
+
     test('Cannot be implemented with `implements`', () {
       expect(() {
         InAppPurchasePlatformAddition.instance =
@@ -143,10 +152,18 @@ void main() {
       ImplementsInAppPurchasePlatformAdditionProvider.register();
       final ImplementsInAppPurchasePlatformAdditionProvider provider =
           ImplementsInAppPurchasePlatformAdditionProvider();
-      final InAppPurchasePlatformAddition addition =
+      final InAppPurchasePlatformAddition? addition =
           provider.getPlatformAddition();
       expect(addition.runtimeType, ExtendsInAppPurchasePlatformAddition);
     });
+
+    test('Provider can provide `null`', () {
+      final ImplementsInAppPurchasePlatformAdditionProvider provider =
+          ImplementsInAppPurchasePlatformAdditionProvider();
+      final InAppPurchasePlatformAddition? addition =
+          provider.getPlatformAddition();
+      expect(addition, isNull);
+    });
   });
 }
 
@@ -192,7 +209,7 @@ class ImplementsInAppPurchasePlatformAdditionProvider
   }
 
   @override
-  T getPlatformAddition<T extends InAppPurchasePlatformAddition>() {
+  T getPlatformAddition<T extends InAppPurchasePlatformAddition?>() {
     return InAppPurchasePlatformAddition.instance as T;
   }
 }

From 7f1653f98b16e2bc467a353d0ebafc3d53c05190 Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Mon, 3 May 2021 10:28:56 -0700
Subject: [PATCH 4/9] review

---
 .../in_app_purchase_platform_addition.dart    | 34 +++++--------------
 .../test/in_app_purchase_platform_test.dart   |  9 ++---
 2 files changed, 11 insertions(+), 32 deletions(-)

diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index 7785c0c3a289..2d113ef4fc53 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -8,20 +8,13 @@ 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.
 ///
-/// 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 InAppPurchasePlatformAddition? _instance;
-
+/// Platforms that wants to introduce platform-specific public APIs should create
+/// a class that either extend or implements [InAppPurchasePlatformAddition]. Then replace
+/// the [InAppPurchasePlatformAddition.instance] with an instance of that class.
+///
+/// We highly recommand against to have [InAppPurchasePlatformAddition] and [InAppPurchasePlatform]
+/// being the same class.
+abstract class InAppPurchasePlatformAddition {
   /// The instance containing the platform-specific in_app_purchase
   /// functionality.
   ///
@@ -54,16 +47,5 @@ abstract class InAppPurchasePlatformAddition extends PlatformInterface {
   ///   }
   /// }
   /// ```
-  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) {
-    _instance = instance;
-    if (_instance == null) {
-      return;
-    }
-    PlatformInterface.verifyToken(instance!, _token);
-    assert(instance.runtimeType is! InAppPurchasePlatform);
-  }
+  static InAppPurchasePlatformAddition? instance;
 }
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
index 750599170990..d83b301d08ed 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
@@ -122,7 +122,6 @@ void main() {
   });
 
   group('$InAppPurchasePlatformAddition', () {
-
     setUp(() {
       InAppPurchasePlatformAddition.instance = null;
     });
@@ -131,11 +130,9 @@ void main() {
       expect(InAppPurchasePlatformAddition.instance, isNull);
     });
 
-    test('Cannot be implemented with `implements`', () {
-      expect(() {
-        InAppPurchasePlatformAddition.instance =
-            ImplementsInAppPurchasePlatformAddition();
-      }, throwsNoSuchMethodError);
+    test('Can be implemented.', () {
+      InAppPurchasePlatformAddition.instance =
+          ImplementsInAppPurchasePlatformAddition();
     });
 
     test('InAppPurchasePlatformAddition Can be extended', () {

From 0f3d0b9728c1220370e3b50c70ac619fbd836dd3 Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Mon, 3 May 2021 10:31:57 -0700
Subject: [PATCH 5/9] update docs

---
 .../lib/src/in_app_purchase_platform_addition.dart            | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index 2d113ef4fc53..cbf38f2ff916 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -12,7 +12,9 @@ import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 /// a class that either extend or implements [InAppPurchasePlatformAddition]. Then replace
 /// the [InAppPurchasePlatformAddition.instance] with an instance of that class.
 ///
-/// We highly recommand against to have [InAppPurchasePlatformAddition] and [InAppPurchasePlatform]
+/// All the APIs added by [InAppPurchasePlatformAddition] implementers will be accessed from
+/// [InAppPurchasePlatformAdditionProvider.getPlatformAddition] by the client APPs.
+/// To avoid client APPs to directly call the [InAppPurchasePlatform] APIs, we highly recommand to not have [InAppPurchasePlatformAddition] and [InAppPurchasePlatform]
 /// being the same class.
 abstract class InAppPurchasePlatformAddition {
   /// The instance containing the platform-specific in_app_purchase

From 899bafb0da6b33565fe6f1dac9a24f2c1840b793 Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Mon, 3 May 2021 10:40:11 -0700
Subject: [PATCH 6/9] asert and test

---
 .../src/in_app_purchase_platform_addition.dart   | 16 +++++++++++++---
 .../test/in_app_purchase_platform_test.dart      | 11 +++++++++++
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index cbf38f2ff916..1c1798a8d857 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -14,9 +14,11 @@ import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 ///
 /// All the APIs added by [InAppPurchasePlatformAddition] implementers will be accessed from
 /// [InAppPurchasePlatformAdditionProvider.getPlatformAddition] by the client APPs.
-/// To avoid client APPs to directly call the [InAppPurchasePlatform] APIs, we highly recommand to not have [InAppPurchasePlatformAddition] and [InAppPurchasePlatform]
-/// being the same class.
+/// To avoid client APPs to directly call the [InAppPurchasePlatform] APIs, an [InAppPurchasePlatformAddition] implementation must not
+/// be a type of [InAppPurchasePlatform].
 abstract class InAppPurchasePlatformAddition {
+  static InAppPurchasePlatformAddition? _instance;
+
   /// The instance containing the platform-specific in_app_purchase
   /// functionality.
   ///
@@ -49,5 +51,13 @@ abstract class InAppPurchasePlatformAddition {
   ///   }
   /// }
   /// ```
-  static InAppPurchasePlatformAddition? instance;
+  static InAppPurchasePlatformAddition? get instance => _instance;
+
+  /// Sets the instance to a desired [InAppPurchasePlatformAddition] implementation.
+  ///
+  /// The `instance` must not be a type of [InAppPurchasePlatform].
+  static set instance(InAppPurchasePlatformAddition? instance) {
+    assert(!(instance is InAppPurchasePlatform));
+    _instance = instance;
+  }
 }
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
index d83b301d08ed..f0c7f84d4a26 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
@@ -145,6 +145,13 @@ void main() {
           MockInAppPurchasePlatformAddition();
     });
 
+    test('Can not be a `InAppPurchasePlatform`', () {
+      expect(
+          () => InAppPurchasePlatformAddition.instance =
+              ExtendsInAppPurchasePlatformAdditionIsPlatformInterface(),
+          throwsAssertionError);
+    });
+
     test('Provider can provide', () {
       ImplementsInAppPurchasePlatformAdditionProvider.register();
       final ImplementsInAppPurchasePlatformAdditionProvider provider =
@@ -210,3 +217,7 @@ class ImplementsInAppPurchasePlatformAdditionProvider
     return InAppPurchasePlatformAddition.instance as T;
   }
 }
+
+class ExtendsInAppPurchasePlatformAdditionIsPlatformInterface
+    extends InAppPurchasePlatform
+    implements ExtendsInAppPurchasePlatformAddition {}

From 8a7341dcb15bc1d0bb9f81fe65e927f553dc1a93 Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Mon, 3 May 2021 10:40:54 -0700
Subject: [PATCH 7/9] tupo

---
 .../lib/src/in_app_purchase_platform_addition.dart              | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index 1c1798a8d857..8c00f8a2fd68 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -53,7 +53,7 @@ abstract class InAppPurchasePlatformAddition {
   /// ```
   static InAppPurchasePlatformAddition? get instance => _instance;
 
-  /// Sets the instance to a desired [InAppPurchasePlatformAddition] implementation.
+  /// Set the instance to a desired [InAppPurchasePlatformAddition] implementation.
   ///
   /// The `instance` must not be a type of [InAppPurchasePlatform].
   static set instance(InAppPurchasePlatformAddition? instance) {

From a697f099a8c4d009778f118cff7f066b65c9b281 Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Tue, 4 May 2021 14:22:34 -0700
Subject: [PATCH 8/9] fix analyzer errors

---
 .../lib/src/in_app_purchase_platform_addition.dart            | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index 8c00f8a2fd68..5d9dac210e93 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -3,8 +3,8 @@
 // found in the LICENSE file.
 
 import 'package:in_app_purchase_platform_interface/in_app_purchase_platform_interface.dart';
-import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 
+// ignore: avoid_classes_with_only_static_members
 /// The interface that platform implementations must implement when they want to
 /// provide platform-specific in_app_purchase features.
 ///
@@ -57,7 +57,7 @@ abstract class InAppPurchasePlatformAddition {
   ///
   /// The `instance` must not be a type of [InAppPurchasePlatform].
   static set instance(InAppPurchasePlatformAddition? instance) {
-    assert(!(instance is InAppPurchasePlatform));
+    assert(instance is! InAppPurchasePlatform);
     _instance = instance;
   }
 }

From b8774b6a2eeba82851c8bd02e5d062276b75bad7 Mon Sep 17 00:00:00 2001
From: Chris Yang <ychris@google.com>
Date: Wed, 5 May 2021 11:18:22 -0700
Subject: [PATCH 9/9] review

---
 .../lib/src/in_app_purchase_platform_addition.dart | 14 +++++++-------
 .../lib/src/types/product_details_response.dart    |  2 +-
 .../test/in_app_purchase_platform_test.dart        | 12 ------------
 3 files changed, 8 insertions(+), 20 deletions(-)

diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
index 5d9dac210e93..746675549295 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform_addition.dart
@@ -9,13 +9,13 @@ import 'package:in_app_purchase_platform_interface/in_app_purchase_platform_inte
 /// provide platform-specific in_app_purchase features.
 ///
 /// Platforms that wants to introduce platform-specific public APIs should create
-/// a class that either extend or implements [InAppPurchasePlatformAddition]. Then replace
-/// the [InAppPurchasePlatformAddition.instance] with an instance of that class.
+/// a class that either extend or implements [InAppPurchasePlatformAddition]. Then set
+/// the [InAppPurchasePlatformAddition.instance] to an instance of that class.
 ///
-/// All the APIs added by [InAppPurchasePlatformAddition] implementers will be accessed from
+/// All the APIs added by [InAppPurchasePlatformAddition] implementations will be accessed from
 /// [InAppPurchasePlatformAdditionProvider.getPlatformAddition] by the client APPs.
-/// To avoid client APPs to directly call the [InAppPurchasePlatform] APIs, an [InAppPurchasePlatformAddition] implementation must not
-/// be a type of [InAppPurchasePlatform].
+/// To avoid clients directly calling [InAppPurchasePlatform] APIs,
+/// an [InAppPurchasePlatformAddition] implementation should not be a type of [InAppPurchasePlatform].
 abstract class InAppPurchasePlatformAddition {
   static InAppPurchasePlatformAddition? _instance;
 
@@ -53,9 +53,9 @@ abstract class InAppPurchasePlatformAddition {
   /// ```
   static InAppPurchasePlatformAddition? get instance => _instance;
 
-  /// Set the instance to a desired [InAppPurchasePlatformAddition] implementation.
+  /// Sets the instance to a desired [InAppPurchasePlatformAddition] implementation.
   ///
-  /// The `instance` must not be a type of [InAppPurchasePlatform].
+  /// The `instance` should not be a type of [InAppPurchasePlatform].
   static set instance(InAppPurchasePlatformAddition? instance) {
     assert(instance is! InAppPurchasePlatform);
     _instance = instance;
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/product_details_response.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/product_details_response.dart
index d8331d6560d8..11b244a84ae3 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/product_details_response.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/product_details_response.dart
@@ -18,7 +18,7 @@ class ProductDetailsResponse {
 
   /// The list of identifiers that are in the `identifiers` of [InAppPurchasePlatform.queryProductDetails] but failed to be fetched.
   ///
-  /// There's multiple platform-specific reasons that product information could fail to be fetched,
+  /// There are multiple platform-specific reasons that product information could fail to be fetched,
   /// ranging from products not being correctly configured in the storefront to the queried IDs not existing.
   final List<String> notFoundIDs;
 
diff --git a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
index f0c7f84d4a26..9c0f2dc00020 100644
--- a/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
+++ b/packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart
@@ -140,11 +140,6 @@ void main() {
           ExtendsInAppPurchasePlatformAddition();
     });
 
-    test('Can be mocked with `implements`', () {
-      InAppPurchasePlatformAddition.instance =
-          MockInAppPurchasePlatformAddition();
-    });
-
     test('Can not be a `InAppPurchasePlatform`', () {
       expect(
           () => InAppPurchasePlatformAddition.instance =
@@ -195,13 +190,6 @@ class ImplementsInAppPurchasePlatformAddition
   dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
 
-class MockInAppPurchasePlatformAddition extends Mock
-    with
-        // ignore: prefer_mixin
-        MockPlatformInterfaceMixin
-    implements
-        InAppPurchasePlatformAddition {}
-
 class ExtendsInAppPurchasePlatformAddition
     extends InAppPurchasePlatformAddition {}