Add BuiltListAsyncDeserializer.

Author: davidmorgan

CHANGELOG.md

@@ -22,6 +22,9 @@
 - Add support for polymorphism to `StandardJsonPlugin`. It will now specify
   type names as needed via a `discriminator` field, which by defualt is
   called `$`. This can be changed in the `StandardJsonPlugin` constructor.
+- Add `BuiltListAsyncDeserializer`. It provides a way to deserialize large
+  responses without blocking, provided the top level serialized type is
+  `BuiltList`.
 
 ## 4.4.1
 

built_value/lib/async_serializer.dart

@@ -0,0 +1,25 @@
+// Copyright (c) 2017, Google Inc. Please see the AUTHORS file for details.
+// All rights reserved. Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:built_value/serializer.dart';
+
+/// Deserializer for `BuiltList` that runs asynchronously.
+///
+/// If you need to deserialize large payloads without blocking, arrange that
+/// the top level serialized object is a `BuiltList`. Then use this class to
+/// deserialize to a [Stream] of objects.
+class BuiltListAsyncDeserializer {
+  Stream<Object> deserialize(Serializers serializers, Iterable serialized,
+      {FullType specifiedType: FullType.unspecified}) async* {
+    final elementType = specifiedType.parameters.isEmpty
+        ? FullType.unspecified
+        : specifiedType.parameters[0];
+
+    for (final item in serialized) {
+      yield serializers.deserialize(item, specifiedType: elementType);
+    }
+  }
+}

built_value/test/built_list_async_deserializer_test.dart

@@ -0,0 +1,28 @@
+// Copyright (c) 2017, Google Inc. Please see the AUTHORS file for details.
+// All rights reserved. Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+import 'package:built_collection/built_collection.dart';
+import 'package:built_value/async_serializer.dart';
+import 'package:built_value/serializer.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('BuiltList', () {
+    final data = new BuiltList<int>([1, 2, 3]);
+    final specifiedType =
+        const FullType(BuiltList, const [const FullType(int)]);
+    final serializers = (new Serializers().toBuilder()
+          ..addBuilderFactory(specifiedType, () => new ListBuilder<int>()))
+        .build();
+    final serialized = [1, 2, 3];
+
+    test('can be deserialized asynchronously', () async {
+      final deserialized = await new BuiltListAsyncDeserializer()
+          .deserialize(serializers, serialized, specifiedType: specifiedType)
+          .toList();
+
+      expect(deserialized, data);
+    });
+  });
+}