[in_app_purchase] platform interface improvement (flutter#3821)

Adds some improvements to the platform interface. 

1. Make `InAppPurchasePlatformAddition` a subclass of `PlatformInterface` to take advantage of the existing token checking logic. (I also consider `InAppPurchasePlatformAddition` a `PlatformInterface` because it works in a similar way.
2. Make the `instance` variable `late` as we should never access it before setter.
3. Add tests for `InAppPurchasePlatformAddition`

part of flutter#78525

Author: Chris Yang

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:
 

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;
 

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

@@ -23,27 +23,28 @@ abstract class InAppPurchasePlatform extends PlatformInterface {
 
   /// The instance of [InAppPurchasePlatform] to use.
   ///
-  /// Defaults to `null`.
-  static InAppPurchasePlatform? get instance => _instance;
-
-  static InAppPurchasePlatform? _instance;
+  /// Must be set before accessing.
+  static InAppPurchasePlatform get instance => _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;
   }
 
+  // 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

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

@@ -2,18 +2,33 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:in_app_purchase_platform_interface/in_app_purchase_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.
+/// 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 set
+/// the [InAppPurchasePlatformAddition.instance] to an instance of that class.
+///
+/// All the APIs added by [InAppPurchasePlatformAddition] implementations will be accessed from
+/// [InAppPurchasePlatformAdditionProvider.getPlatformAddition] by the client APPs.
+/// To avoid clients directly calling [InAppPurchasePlatform] APIs,
+/// an [InAppPurchasePlatformAddition] implementation should not be a type of [InAppPurchasePlatform].
 abstract class InAppPurchasePlatformAddition {
+  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
   /// `InAppPurchasePlatformAddition.instance` with the new addition
-  /// implementationinstance.
+  /// implementation instance.
   ///
   /// Example implementation might look like this:
   /// ```dart
@@ -22,7 +37,7 @@ 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 +51,13 @@ abstract class InAppPurchasePlatformAddition {
   ///   }
   /// }
   /// ```
-  static InAppPurchasePlatformAddition? instance;
+  static InAppPurchasePlatformAddition? get instance => _instance;
+
+  /// Sets the instance to a desired [InAppPurchasePlatformAddition] implementation.
+  ///
+  /// The `instance` should not be a type of [InAppPurchasePlatform].
+  static set instance(InAppPurchasePlatformAddition? instance) {
+    assert(instance is! InAppPurchasePlatform);
+    _instance = instance;
+  }
 }

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?>();
 }

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;
 

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 +120,50 @@ void main() {
       );
     });
   });
+
+  group('$InAppPurchasePlatformAddition', () {
+    setUp(() {
+      InAppPurchasePlatformAddition.instance = null;
+    });
+
+    test('Cannot be implemented with `implements`', () {
+      expect(InAppPurchasePlatformAddition.instance, isNull);
+    });
+
+    test('Can be implemented.', () {
+      InAppPurchasePlatformAddition.instance =
+          ImplementsInAppPurchasePlatformAddition();
+    });
+
+    test('InAppPurchasePlatformAddition Can be extended', () {
+      InAppPurchasePlatformAddition.instance =
+          ExtendsInAppPurchasePlatformAddition();
+    });
+
+    test('Can not be a `InAppPurchasePlatform`', () {
+      expect(
+          () => InAppPurchasePlatformAddition.instance =
+              ExtendsInAppPurchasePlatformAdditionIsPlatformInterface(),
+          throwsAssertionError);
+    });
+
+    test('Provider can provide', () {
+      ImplementsInAppPurchasePlatformAdditionProvider.register();
+      final ImplementsInAppPurchasePlatformAdditionProvider provider =
+          ImplementsInAppPurchasePlatformAdditionProvider();
+      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);
+    });
+  });
 }
 
 class ImplementsInAppPurchasePlatform implements InAppPurchasePlatform {
@@ -143,3 +183,29 @@ 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 ExtendsInAppPurchasePlatformAddition
+    extends InAppPurchasePlatformAddition {}
+
+class ImplementsInAppPurchasePlatformAdditionProvider
+    implements InAppPurchasePlatformAdditionProvider {
+  static void register() {
+    InAppPurchasePlatformAddition.instance =
+        ExtendsInAppPurchasePlatformAddition();
+  }
+
+  @override
+  T getPlatformAddition<T extends InAppPurchasePlatformAddition?>() {
+    return InAppPurchasePlatformAddition.instance as T;
+  }
+}
+
+class ExtendsInAppPurchasePlatformAdditionIsPlatformInterface
+    extends InAppPurchasePlatform
+    implements ExtendsInAppPurchasePlatformAddition {}