Apply feedback from PR

Author: mvanbeusekom

packages/in_app_purchase/in_app_purchase_platform_interface/CHANGELOG.md

@@ -1,3 +1,3 @@
 ## 1.0.0
 
-- Initial open-source release.
+* Initial open-source release.

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

@@ -21,26 +21,13 @@ abstract class InAppPurchasePlatform extends PlatformInterface {
 
   static final Object _token = Object();
 
-  static InAppPurchasePlatform? _instance;
-
-  /// The default instance of [InAppPurchasePlatform] to use.
-  static InAppPurchasePlatform get instance {
-    final InAppPurchasePlatform? platform = _instance;
-    if (platform == null) {
-      throw UnimplementedError(
-          'No platform specific implementation set. Please make sure you set the `instance` with a valid platform specific implementation of the `InAppPurchasePlatform` class.');
-    }
-
-    return platform;
-  }
-
-  /// 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 set instance(InAppPurchasePlatform instance) {
+  //// Ensures that implementers are using `extends` rather than
+  /// `implements` and throws [AssertionError] if not.
+  ///
+  /// This is implemented as a static method so that it cannot be overridden
+  /// with `noSuchMethod`.
+  static void verifyToken(InAppPurchasePlatform instance) {
     PlatformInterface.verifyToken(instance, _token);
-    _instance = instance;
   }
 
   /// Listen to this broadcast stream to get real time update for purchases.

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/in_app_purchase_error.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'in_app_purchase_source.dart';
-
 /// Captures an error from the underlying purchase platform.
 ///
 /// The error can happen during the purchase, restoring a purchase, or querying product.
@@ -20,7 +18,7 @@ class IAPError {
       this.details});
 
   /// Which source is the error on.
-  final IAPSource source;
+  final String source;
 
   /// The error code.
   final String code;

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/in_app_purchase_source.dart

@@ -23,8 +23,9 @@ enum PurchaseStatus {
 
   /// The purchase has been restored to the device.
   ///
-  /// You should validate the receipt and if valid deliver the content. Once the
+  /// You should validate the purchase and if valid deliver the content. Once the
   /// content has been delivered or if the receipt is invalid you should finish
-  /// the purchase by calling the `finishPurchase` method.
+  /// the purchase by calling the `finishPurchase` method. More information on
+  /// verifying purchases can be found [here](https://pub.dev/packages/in_app_purchase#loading-previous-purchases).
   restored,
 }

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/purchase_status.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'in_app_purchase_source.dart';
-
 /// Represents the data that is used to verify purchases.
 ///
 /// The property [source] helps you to determine the method to verify purchases.
@@ -44,5 +42,5 @@ class PurchaseVerificationData {
   final String serverVerificationData;
 
   /// Indicates the source of the purchase.
-  final IAPSource source;
+  final String source;
 }

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/purchase_verification_data.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 export 'in_app_purchase_error.dart';
-export 'in_app_purchase_source.dart';
 export 'product_details.dart';
 export 'product_details_response.dart';
 export 'purchase_details.dart';

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

@@ -18,4 +18,4 @@ dev_dependencies:
 
 environment:
   sdk: ">=2.12.0 <3.0.0"
-  flutter: ">=1.22.0"
+  flutter: ">=1.22.0"

packages/in_app_purchase/in_app_purchase_platform_interface/pubspec.yaml

@@ -11,23 +11,21 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group('$InAppPurchasePlatform', () {
-    test('default instance is null and throws unimplemented exception', () {
-      expect(() => InAppPurchasePlatform.instance, throwsUnimplementedError);
-    });
-
     test('Cannot be implemented with `implements`', () {
+      final InAppPurchasePlatform instance = ImplementsInAppPurchasePlatform();
       expect(() {
-        InAppPurchasePlatform.instance = ImplementsInAppPurchasePlatform();
+        InAppPurchasePlatform.verifyToken(instance);
       }, throwsNoSuchMethodError);
     });
 
     test('Can be extended', () {
-      InAppPurchasePlatform.instance = ExtendsInAppPurchasePlatform();
+      final InAppPurchasePlatform instance = ExtendsInAppPurchasePlatform();
+      InAppPurchasePlatform.verifyToken(instance);
     });
 
     test('Can be mocked with `implements`', () {
       final MockInAppPurchasePlatform mock = MockInAppPurchasePlatform();
-      InAppPurchasePlatform.instance = mock;
+      InAppPurchasePlatform.verifyToken(mock);
     });
 
     test(