add a simple observable macro example

Author: jakemac53

analysis_options.yaml

@@ -3,4 +3,5 @@ analyzer:
     - accepted/2.3/spread-collections/examples/**
     # TODO: remove these when the analyzer supports macros
     - working/macros/example/lib/data_class.dart
+    - working/macros/example/lib/observable.dart
     - working/macros/example/bin/user_main.dart

working/macros/example/bin/run.dart

@@ -19,10 +19,14 @@ void main() async {
       File(dartToolDir.uri.resolve('bootstrap.dart').toFilePath());
   log('Bootstrapping macro program (${bootstrapFile.path}).');
   var dataClassUri = Uri.parse('package:macro_proposal/data_class.dart');
+  var observableUri = Uri.parse('package:macro_proposal/observable.dart');
   var bootstrapContent = bootstrapMacroIsolate({
     dataClassUri.toString(): {
       'DataClass': [''],
-    }
+    },
+    observableUri.toString(): {
+      'Observable': [''],
+    },
   }, SerializationMode.byteDataClient);
   bootstrapFile.writeAsStringSync(bootstrapContent);
   var bootstrapKernelFile =
@@ -75,9 +79,12 @@ void main() async {
     '--source',
     Platform.script.resolve('user_main.dart').toFilePath(),
     '--packages-file=.dart_tool/package_config.json',
+    '--enable-experiment=macros',
     '--precompiled-macro-format=kernel',
     '--precompiled-macro',
     '$dataClassUri;${bootstrapKernelFile.path}',
+    '--precompiled-macro',
+    '$observableUri;${bootstrapKernelFile.path}',
     '--macro-serialization-mode=bytedata',
     '--input-linked',
     bootstrapKernelFile.path,

working/macros/example/bin/user_main.dart

@@ -5,6 +5,7 @@
 import 'dart:math';
 
 import 'package:macro_proposal/data_class.dart';
+import 'package:macro_proposal/observable.dart';
 
 void main() {
   var rand = Random();
@@ -21,6 +22,11 @@ void main() {
   var phoenix =
       joe.copyWith(name: 'Phoenix', age: rand.nextInt(100), reports: [joe]);
   print(phoenix);
+
+  var observableUser = ObservableUser(age: 10, name: 'Georgio');
+  observableUser
+    ..age = 11
+    ..name = 'Greg';
 }
 
 @DataClass()
@@ -34,3 +40,17 @@ class User {
 class Manager extends User {
   final List<User> reports;
 }
+
+class ObservableUser {
+  @Observable()
+  int _age;
+
+  @Observable()
+  String _name;
+
+  ObservableUser({
+    required int age,
+    required String name,
+  })  : _age = age,
+        _name = name;
+}

working/macros/example/lib/observable.dart

@@ -0,0 +1,42 @@
+// Copyright (c) 2022, 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.
+
+// There is no public API exposed yet, the in progress api lives here.
+import 'dart:async';
+
+import 'package:_fe_analyzer_shared/src/macros/api.dart';
+
+macro class Observable implements FieldDeclarationsMacro {
+  const Observable();
+
+  @override
+  Future<void> buildDeclarationsForField(
+      FieldDeclaration field, ClassMemberDeclarationBuilder builder) async {
+    final name = field.identifier.name;
+    if (!name.startsWith('_')) {
+      throw ArgumentError(
+          '@observable can only annotate private fields, and it will create '
+          'public getters and setters for them, but the public field '
+          '$name was annotated.');
+    }
+    var publicName = name.substring(1);
+    var getter = DeclarationCode.fromParts(
+        [field.type.code, ' get $publicName => ', field.identifier, ';']);
+    builder.declareInClass(getter);
+
+    var print =
+            // ignore: deprecated_member_use
+            await builder.resolveIdentifier(Uri.parse('dart:core'), 'print');
+    var setter = DeclarationCode.fromParts([
+      'set $publicName(',
+      field.type.code,
+      ' val) {\n',
+      print,
+      "('Setting $publicName to \${val}');\n",
+      field.identifier,
+      ' = val;\n}',
+    ]);
+    builder.declareInClass(setter);
+  }
+}

working/macros/example/pubspec.yaml

@@ -1,7 +1,7 @@
 name: macro_proposal
 publish_to: none
 environment:
-  sdk: ">=2.17.0-0 <3.0.0"
+  sdk: ">=2.18.0-0 <3.0.0"
 dev_dependencies:
   args: ^2.3.0
   dart_style: ^2.2.1