Add ReadonlyMessageMixin and the hooks needed to support it.

The protoc plugin will use this mixin to implement a
getDefault() method for each message.

BUG=google#41

Review URL: https://chromiumcodereview.appspot.com//1228213004.

lib/protobuf.dart

@@ -24,6 +24,7 @@ part 'src/protobuf/generated_message.dart';
 part 'src/protobuf/generated_service.dart';
 part 'src/protobuf/pb_list.dart';
 part 'src/protobuf/protobuf_enum.dart';
+part 'src/protobuf/readonly_message.dart';
 part 'src/protobuf/rpc_client.dart';
 part 'src/protobuf/unknown_field_set.dart';
 part 'src/protobuf/utils.dart';

lib/src/protobuf/generated_message.dart

@@ -938,24 +938,34 @@ abstract class GeneratedMessage {
   /// default value if it is not set.
   getField(int tagNumber) {
     var value = _fieldValues[tagNumber];
+    if (value != null) return value;
+
     // Initialize the field.
-    if (value == null) {
-      MakeDefaultFunc makeDefaultFunc = info_.makeDefault(tagNumber);
-      if (makeDefaultFunc == null) {
-        makeDefaultFunc = _extensions[tagNumber].makeDefault;
-      }
-      value = makeDefaultFunc();
-      // TODO(antonm): ugly trick which should go away imho:
-      // right now getField for repeated fields returns a list
-      // which is implicitly added to the message.
-      // Should return immutable empty list instead, imho.
-      if (value is List) {
-        _fieldValues[tagNumber] = value;
-      }
+    MakeDefaultFunc makeDefaultFunc = info_.makeDefault(tagNumber);
+    if (makeDefaultFunc == null) {
+      makeDefaultFunc = _extensions[tagNumber].makeDefault;
+    }
+    value = makeDefaultFunc();
+    if (value is List) {
+      return _getDefaultRepeatedField(tagNumber, value);
     }
     return value;
   }
 
+  List _getDefaultRepeatedField(int tagNumber, List value) {
+    // Automatically save the repeated field so that changes won't be lost.
+    //
+    // TODO(skybrian) we could avoid this by generating another
+    // method for repeated fields:
+    //
+    //   msg.mutableFoo().add(123);
+    //
+    // Then msg.foo could return an immutable empty list by default.
+    // But it doesn't seem urgent or worth the migration.
+    _fieldValues[tagNumber] = value;
+    return value;
+  }
+
   /// Returns [:true:] if a value of [extension] is present.
   bool hasExtension(Extension extension) {
     _checkExtension(extension);

lib/src/protobuf/readonly_message.dart

@@ -0,0 +1,99 @@
+// Copyright (c) 2015, the Dart project authors.  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.
+
+part of protobuf;
+
+/// Modifies a GeneratedMessage so that it's read-only.
+abstract class ReadonlyMessageMixin {
+  static final _emptyUnknownFields = new _ReadonlyUnknownFieldSet();
+  static final _emptyList = new List.unmodifiable([]);
+
+  BuilderInfo get info_;
+
+  get unknownFields => _emptyUnknownFields;
+
+  List _getDefaultRepeatedField(int tagNumber, List value) => _emptyList;
+
+  void addExtension(Extension extension, var value) =>
+      _readonly("addExtension");
+
+  void clear() => _readonly("clear");
+
+  void clearExtension(Extension extension) => _readonly("clearExtension");
+
+  void clearField(int tagNumber) => _readonly("clearField");
+
+  void mergeFromBuffer(List<int> input,
+          [ExtensionRegistry extensionRegistry = ExtensionRegistry.EMPTY]) =>
+      _readonly("mergeFromBuffer");
+
+  void mergeFromCodedBufferReader(CodedBufferReader input,
+          [ExtensionRegistry extensionRegistry = ExtensionRegistry.EMPTY]) =>
+      _readonly("mergeFromCodedBufferReader");
+
+  void mergeFromJson(String data,
+          [ExtensionRegistry extensionRegistry = ExtensionRegistry.EMPTY]) =>
+      _readonly("mergeFromJson");
+
+  void mergeFromJsonMap(Map<String, dynamic> json,
+          [ExtensionRegistry extensionRegistry = ExtensionRegistry.EMPTY]) =>
+      _readonly("mergeFromJsonMap");
+
+  void mergeFromMessage(GeneratedMessage other) =>
+      _readonly("mergeFromMessage");
+
+  void mergeUnknownFields(UnknownFieldSet unknownFieldSet) =>
+      _readonly("mergeUnknownFields");
+
+  void setExtension(Extension extension, var value) =>
+      _readonly("setExtension");
+
+  void setField(int tagNumber, var value, [int fieldType = null]) =>
+      _readonly("setField");
+
+  void _readonly(String methodName) {
+    String messageType = info_.messageName;
+    throw new UnsupportedError(
+        "attempted to call $methodName on a read-only message ($messageType)");
+  }
+}
+
+class _ReadonlyUnknownFieldSet extends UnknownFieldSet {
+
+  @override
+  void clear() => _readonly("clear");
+
+  @override
+  void addField(int number, UnknownFieldSetField field) =>
+      _readonly("addField");
+
+  @override
+  void mergeField(int number, UnknownFieldSetField field) =>
+      _readonly("mergeField");
+
+  @override
+  bool mergeFieldFromBuffer(int tag, CodedBufferReader input) {
+    _readonly("mergeFieldFromBuffer");
+    return false; // not reached
+  }
+
+  @override
+  void mergeFromCodedBufferReader(CodedBufferReader input) =>
+      _readonly("mergeFromCodedBufferReader");
+
+  @override
+  void mergeFromUnknownFieldSet(UnknownFieldSet other) =>
+      _readonly("mergeFromUnknownFieldSet");
+
+  @override
+  UnknownFieldSetField _getField(int number) {
+    _readonly("a merge method");
+    return null; // not reached
+  }
+
+  void _readonly(String methodName) {
+    throw new UnsupportedError(
+        "attempted to call $methodName on a read-only UnknownFieldSet");
+  }
+}

lib/src/protobuf/unknown_field_set.dart

@@ -15,9 +15,9 @@ class UnknownFieldSet {
     mergeFromUnknownFieldSet(unknownFieldSet);
   }
 
-  UnknownFieldSet clone() => new UnknownFieldSet._clone(this);
+   UnknownFieldSet clone() => new UnknownFieldSet._clone(this);
 
-  Map<int, UnknownFieldSetField> asMap() => new Map.from(_fields);
+   Map<int, UnknownFieldSetField> asMap() => new Map.from(_fields);
 
   void clear() {
     _fields.clear();

test/readonly_message_test.dart

@@ -0,0 +1,50 @@
+#!/usr/bin/env dart
+// Copyright (c) 2015, the Dart project authors.  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.
+
+library readonly_message_test;
+
+import 'package:test/test.dart' show test, expect, throwsA, predicate;
+
+import 'package:protobuf/protobuf.dart'
+    show GeneratedMessage, ReadonlyMessageMixin, BuilderInfo;
+
+throwsError(Type expectedType, String expectedMessage) => throwsA(
+    predicate((x) {
+  expect(x.runtimeType, expectedType);
+  expect(x.message, expectedMessage);
+  return true;
+}));
+
+class Rec extends GeneratedMessage with ReadonlyMessageMixin {
+  @override
+  BuilderInfo info_ = new BuilderInfo("rec");
+}
+
+main() {
+  test("can write a read-only message", () {
+    expect(new Rec().writeToBuffer(), []);
+    expect(new Rec().writeToJson(), "{}");
+  });
+
+  test("can't merge to a read-only message", () {
+    expect(() => new Rec().mergeFromJson("{}"), throwsError(UnsupportedError,
+        "attempted to call mergeFromJson on a read-only message (rec)"));
+  });
+
+  test("can't set a field on a read-only message", () {
+    expect(() => new Rec().setField(123, 456), throwsError(UnsupportedError,
+        "attempted to call setField on a read-only message (rec)"));
+  });
+
+  test("can't clear a read-only message", () {
+    expect(() => new Rec().clear(), throwsError(UnsupportedError,
+        "attempted to call clear on a read-only message (rec)"));
+  });
+
+  test("can't modify unknown fields on a read-only message", () {
+    expect(() => new Rec().unknownFields.clear(), throwsError(UnsupportedError,
+        "attempted to call clear on a read-only UnknownFieldSet"));
+  });
+}

test/reserved_names_test.dart

@@ -6,7 +6,7 @@
 @TestOn("vm")
 library reserved_names_test;
 
-import 'package:test/test.dart' show test, expect, equals;
+import 'package:test/test.dart' show test, expect, equals, fail, TestOn;
 
 import 'package:protobuf/meta.dart' show GeneratedMessage_reservedNames;
 import 'package:protobuf/mixins_meta.dart' show findMixin;
@@ -29,6 +29,18 @@ void main() {
     expect(actual.toList()..sort(), equals(expected.toList()..sort()));
   });
 
+  test("ReadonlyMessageMixin doesn't add any reserved names", () {
+    var mixinNames =
+        findMemberNames('package:protobuf/protobuf.dart', #ReadonlyMessageMixin);
+    var reservedNames = new Set<String>.from(GeneratedMessage_reservedNames);
+    for (var name in mixinNames) {
+      if (name == "ReadonlyMessageMixin" || name == "unknownFields") continue;
+      if (!reservedNames.contains(name)) {
+        fail("name from ReadonlyMessageMixin is not reserved: ${name}");
+      }
+    }
+  });
+
   test('PbMapMixin reserved names are up to date', () {
     var meta = findMixin("PbMapMixin");
     var actual = new Set<String>.from(meta.findReservedNames());