FieldValue now internally stores the platform-dependent value.

Refactor tests and code around the new API.

packages/cloud_firestore/cloud_firestore/lib/cloud_firestore.dart

@@ -12,14 +12,7 @@ import 'package:cloud_firestore_platform_interface/cloud_firestore_platform_inte
 import 'package:firebase_core/firebase_core.dart';
 
 export 'package:cloud_firestore_platform_interface/cloud_firestore_platform_interface.dart'
-    show
-        FieldPath,
-        FieldValuePlatform,
-        Blob,
-        GeoPoint,
-        Timestamp,
-        Source,
-        DocumentChangeType;
+    show FieldPath, Blob, GeoPoint, Timestamp, Source, DocumentChangeType;
 
 part 'src/collection_reference.dart';
 part 'src/document_change.dart';

packages/cloud_firestore/cloud_firestore/lib/src/field_value.dart

@@ -10,39 +10,43 @@ part of cloud_firestore;
 /// This class serves as a static factory for FieldValuePlatform instances, but also
 /// as a Facade for the Fieldvalue type, so plugin users don't need to worry about
 /// the actual internal implementation of their Fieldvalues, after they're created.
-class FieldValue {
-  static final platform.FieldValueFactoryPlatform _delegate =
+class FieldValue extends platform.FieldValuePlatform {
+  static final platform.FieldValueFactoryPlatform _factory =
       platform.FieldValueFactoryPlatform.instance;
 
+  FieldValue._(platform.FieldValuePlatform delegate) : super(delegate) {
+    platform.FieldValuePlatform.verifyExtends(delegate);
+  }
+
   /// Returns a special value that tells the server to union the given elements
   /// with any array value that already exists on the server.
   ///
   /// Each specified element that doesn't already exist in the array will be
   /// added to the end. If the field being modified is not already an array it
   /// will be overwritten with an array containing exactly the specified
   /// elements.
-  static platform.FieldValuePlatform arrayUnion(List<dynamic> elements) =>
-      _delegate.arrayUnion(elements);
+  static FieldValue arrayUnion(List<dynamic> elements) =>
+      FieldValue._(_factory.arrayUnion(elements));
 
   /// Returns a special value that tells the server to remove the given
   /// elements from any array value that already exists on the server.
   ///
   /// All instances of each element specified will be removed from the array.
   /// If the field being modified is not already an array it will be overwritten
   /// with an empty array.
-  static platform.FieldValuePlatform arrayRemove(List<dynamic> elements) =>
-      _delegate.arrayRemove(elements);
+  static FieldValue arrayRemove(List<dynamic> elements) =>
+      FieldValue._(_factory.arrayRemove(elements));
 
   /// Returns a sentinel for use with update() to mark a field for deletion.
-  static platform.FieldValuePlatform delete() => _delegate.delete();
+  static FieldValue delete() => FieldValue._(_factory.delete());
 
   /// Returns a sentinel for use with set() or update() to include a
   /// server-generated timestamp in the written data.
-  static platform.FieldValuePlatform serverTimestamp() =>
-      _delegate.serverTimestamp();
+  static FieldValue serverTimestamp() =>
+      FieldValue._(_factory.serverTimestamp());
 
   /// Returns a special value for use with set() or update() that tells the
   /// server to increment the field’s current value by the given value.
-  static platform.FieldValuePlatform increment(num value) =>
-      _delegate.increment(value);
+  static FieldValue increment(num value) =>
+      FieldValue._(_factory.increment(value));
 }

packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/method_channel/method_channel_firestore.dart

@@ -59,7 +59,7 @@ class MethodChannelFirestore extends FirestorePlatform {
   static bool _initialized = false;
 
   /// The [MethodChannel] used to communicate with the native plugin
-  static const MethodChannel channel = MethodChannel(
+  static MethodChannel channel = MethodChannel(
     'plugins.flutter.io/cloud_firestore',
     StandardMethodCodec(FirestoreMessageCodec()),
   );

packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/method_channel/utils/firestore_message_codec.dart

@@ -69,11 +69,12 @@ class FirestoreMessageCodec extends StandardMessageCodec {
       buffer.putUint8(_kBlob);
       writeSize(buffer, value.bytes.length);
       buffer.putUint8List(value.bytes);
-    } else if (value is MethodChannelFieldValue) {
-      final int code = _kFieldValueCodes[value.type];
+    } else if (value is FieldValuePlatform) {
+      MethodChannelFieldValue delegate = FieldValuePlatform.getDelegate(value);
+      final int code = _kFieldValueCodes[delegate.type];
       assert(code != null);
       buffer.putUint8(code);
-      if (value.value != null) writeValue(buffer, value.value);
+      if (delegate.value != null) writeValue(buffer, delegate.value);
     } else if (value is FieldPath) {
       final int code = _kFieldPathCodes[value.type];
       assert(code != null);
@@ -107,24 +108,16 @@ class FirestoreMessageCodec extends StandardMessageCodec {
         final int length = readSize(buffer);
         final List<int> bytes = buffer.getUint8List(length);
         return Blob(bytes);
+      case _kDocumentId:
+        return FieldPath.documentId;
       case _kArrayUnion:
-        final List<dynamic> value = readValue(buffer);
-        return FieldValueFactoryPlatform.instance.arrayUnion(value);
       case _kArrayRemove:
-        final List<dynamic> value = readValue(buffer);
-        return FieldValueFactoryPlatform.instance.arrayRemove(value);
       case _kDelete:
-        return FieldValueFactoryPlatform.instance.delete();
       case _kServerTimestamp:
-        return FieldValueFactoryPlatform.instance.serverTimestamp();
       case _kIncrementDouble:
-        final double value = readValue(buffer);
-        return FieldValueFactoryPlatform.instance.increment(value);
       case _kIncrementInteger:
-        final int value = readValue(buffer);
-        return FieldValueFactoryPlatform.instance.increment(value);
-      case _kDocumentId:
-        return FieldPath.documentId;
+      // These cases are only needed on tests, and therefore handled
+      // by [TestFirestoreMessageCodec], a subclass of this codec.
       default:
         return super.readValueOfType(type, buffer);
     }

packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/platform_interface/field_value_factory.dart

@@ -86,9 +86,26 @@ abstract class FieldValueFactoryPlatform extends PlatformInterface {
 /// never "peek" inside subclasses of this.
 ///
 /// Just treat it as a "black box".
-abstract class FieldValuePlatform extends PlatformInterface {
+class FieldValuePlatform extends PlatformInterface {
   static final Object _token = Object();
 
   /// Constructor
-  FieldValuePlatform() : super(token: _token);
+  FieldValuePlatform([this._delegate]) : super(token: _token);
+
+  final FieldValuePlatform _delegate;
+
+  /// Throws an [AssertionError] if [instance] does not extend
+  /// [FieldValuePlatform].
+  ///
+  /// This is used by the app-facing [FieldValue] to ensure that
+  /// the object in which it's going to delegate calls has been
+  /// constructed properly.
+  static verifyExtends(FieldValuePlatform instance) {
+    PlatformInterface.verifyToken(instance, _token);
+  }
+
+  /// Used by platform implementers to obtain a value suitable for being passed
+  /// through to the underlying implementation.
+  static dynamic getDelegate(FieldValuePlatform fieldValue) =>
+      fieldValue._delegate;
 }

packages/cloud_firestore/cloud_firestore_platform_interface/test/document_reference_test.dart

@@ -5,6 +5,8 @@
 import 'package:cloud_firestore_platform_interface/cloud_firestore_platform_interface.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import 'test_common.dart';
+
 const _kCollectionId = "test";
 const _kDocumentId = "document";
 
@@ -14,7 +16,8 @@ class TestDocumentReference extends DocumentReferencePlatform {
 }
 
 void main() {
-  TestWidgetsFlutterBinding.ensureInitialized();
+  initializeMethodChannel();
+
   group("$DocumentReferencePlatform()", () {
     test("Parent", () {
       final document = TestDocumentReference._();

packages/cloud_firestore/cloud_firestore_platform_interface/test/method_channel_cloud_firestore_test.dart

@@ -14,10 +14,13 @@ import 'package:cloud_firestore_platform_interface/src/method_channel/method_cha
 import 'package:cloud_firestore_platform_interface/src/method_channel/method_channel_field_value.dart';
 import 'package:cloud_firestore_platform_interface/src/method_channel/utils/firestore_message_codec.dart';
 
+import 'test_common.dart';
+import 'test_firestore_message_codec.dart';
+
 void main() {
-  TestWidgetsFlutterBinding.ensureInitialized();
+  initializeMethodChannel();
 
-  group('$MethodChannelFirestore()', () {
+  group('MethodChannelFirestore()', () {
     int mockHandleId = 0;
     FirebaseApp app;
     MethodChannelFirestore firestore;
@@ -1159,18 +1162,50 @@ void main() {
       });
 
       test('encode and decode FieldValue', () {
+        const MessageCodec<dynamic> decoder = TestFirestoreMessageCodec();
+
         _checkEncodeDecode<dynamic>(
-            codec, FieldValueFactoryPlatform.instance.arrayUnion(<int>[123]));
+          codec,
+          FieldValuePlatform(
+            FieldValueFactoryPlatform.instance.arrayUnion(<int>[123]),
+          ),
+          decodingCodec: decoder,
+        );
         _checkEncodeDecode<dynamic>(
-            codec, FieldValueFactoryPlatform.instance.arrayRemove(<int>[123]));
+          codec,
+          FieldValuePlatform(
+            FieldValueFactoryPlatform.instance.arrayRemove(<int>[123]),
+          ),
+          decodingCodec: decoder,
+        );
         _checkEncodeDecode<dynamic>(
-            codec, FieldValueFactoryPlatform.instance.delete());
+          codec,
+          FieldValuePlatform(
+            FieldValueFactoryPlatform.instance.delete(),
+          ),
+          decodingCodec: decoder,
+        );
         _checkEncodeDecode<dynamic>(
-            codec, FieldValueFactoryPlatform.instance.serverTimestamp());
+          codec,
+          FieldValuePlatform(
+            FieldValueFactoryPlatform.instance.serverTimestamp(),
+          ),
+          decodingCodec: decoder,
+        );
         _checkEncodeDecode<dynamic>(
-            codec, FieldValueFactoryPlatform.instance.increment(1.0));
+          codec,
+          FieldValuePlatform(
+            FieldValueFactoryPlatform.instance.increment(1.0),
+          ),
+          decodingCodec: decoder,
+        );
         _checkEncodeDecode<dynamic>(
-            codec, FieldValueFactoryPlatform.instance.increment(1));
+          codec,
+          FieldValuePlatform(
+            FieldValueFactoryPlatform.instance.increment(1),
+          ),
+          decodingCodec: decoder,
+        );
       });
 
       test('encode and decode FieldPath', () {
@@ -1381,9 +1416,15 @@ void main() {
   });
 }
 
-void _checkEncodeDecode<T>(MessageCodec<T> codec, T message) {
+void _checkEncodeDecode<T>(
+  MessageCodec<T> codec,
+  T message, {
+  MessageCodec<T> decodingCodec,
+}) {
+  MessageCodec<T> decoder = decodingCodec ?? codec;
+
   final ByteData encoded = codec.encodeMessage(message);
-  final T decoded = codec.decodeMessage(encoded);
+  final T decoded = decoder.decodeMessage(encoded);
   if (message == null) {
     expect(encoded, isNull);
     expect(decoded, isNull);
@@ -1404,8 +1445,8 @@ bool _deepEquals(dynamic valueA, dynamic valueB) {
   if (valueA is List) return valueB is List && _deepEqualsList(valueA, valueB);
   if (valueA is Map) return valueB is Map && _deepEqualsMap(valueA, valueB);
   if (valueA is double && valueA.isNaN) return valueB is double && valueB.isNaN;
-  if (valueA is MethodChannelFieldValue) {
-    return valueB is MethodChannelFieldValue &&
+  if (valueA is FieldValuePlatform) {
+    return valueB is FieldValuePlatform &&
         _deepEqualsFieldValue(valueA, valueB);
   }
   if (valueA is FieldPath) {
@@ -1456,10 +1497,10 @@ bool _deepEqualsMap(
   return true;
 }
 
-bool _deepEqualsFieldValue(
-  MethodChannelFieldValue valueA,
-  MethodChannelFieldValue valueB,
-) {
+bool _deepEqualsFieldValue(FieldValuePlatform a, FieldValuePlatform b) {
+  MethodChannelFieldValue valueA = FieldValuePlatform.getDelegate(a);
+  MethodChannelFieldValue valueB = FieldValuePlatform.getDelegate(b);
+
   if (valueA.type != valueB.type) return false;
   if (valueA.value == null) return valueB.value == null;
   if (valueA.value is List) return _deepEqualsList(valueA.value, valueB.value);

packages/cloud_firestore/cloud_firestore_platform_interface/test/method_channel_collection_reference_test.dart

@@ -9,11 +9,13 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:cloud_firestore_platform_interface/src/method_channel/method_channel_firestore.dart';
 import 'package:cloud_firestore_platform_interface/src/method_channel/method_channel_collection_reference.dart';
 
+import 'test_common.dart';
+
 const _kCollectionId = "test";
 const _kDocumentId = "document";
 
 void main() {
-  TestWidgetsFlutterBinding.ensureInitialized();
+  initializeMethodChannel();
 
   group("$MethodChannelCollectionReference", () {
     MethodChannelCollectionReference _testCollection;

packages/cloud_firestore/cloud_firestore_platform_interface/test/method_channel_document_reference.dart

@@ -13,7 +13,7 @@ import 'package:cloud_firestore_platform_interface/src/method_channel/method_cha
 import 'test_common.dart';
 
 void main() {
-  TestWidgetsFlutterBinding.ensureInitialized();
+  initializeMethodChannel();
 
   group("$MethodChannelDocumentReference()", () {
     MethodChannelDocumentReference _documentReference;

packages/cloud_firestore/cloud_firestore_platform_interface/test/query_test.dart

@@ -7,6 +7,8 @@ import 'package:flutter_test/flutter_test.dart';
 
 import 'package:cloud_firestore_platform_interface/src/method_channel/method_channel_query.dart';
 
+import 'test_common.dart';
+
 const _kQueryPath = "test/collection";
 
 class TestQuery extends MethodChannelQuery {
@@ -17,7 +19,8 @@ class TestQuery extends MethodChannelQuery {
 }
 
 void main() {
-  TestWidgetsFlutterBinding.ensureInitialized();
+  initializeMethodChannel();
+
   group("$QueryPlatform()", () {
     test("parameters", () {
       _hasDefaultParameters(TestQuery._().parameters);

packages/cloud_firestore/cloud_firestore_platform_interface/test/test_common.dart

@@ -7,12 +7,23 @@ import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import 'package:cloud_firestore_platform_interface/src/method_channel/method_channel_firestore.dart';
+import 'test_firestore_message_codec.dart';
 
 typedef MethodCallCallback = dynamic Function(MethodCall methodCall);
 
 const kCollectionId = "test";
 const kDocumentId = "document";
 
+void initializeMethodChannel() {
+  // Install the Codec that is able to decode FieldValues.
+  MethodChannelFirestore.channel = MethodChannel(
+    'plugins.flutter.io/cloud_firestore',
+    StandardMethodCodec(TestFirestoreMessageCodec()),
+  );
+
+  TestWidgetsFlutterBinding.ensureInitialized();
+}
+
 void handleMethodCall(MethodCallCallback methodCallCallback) =>
     MethodChannelFirestore.channel.setMockMethodCallHandler((call) async {
       expect(