这篇文章里面有对build_runner 的详细介绍 》 https://juejin.cn/post/6844903878392053774

1. 新建一个plugin，名字叫code_generate

2. 在code_generate项目下

pubspec.yaml文件中添加source_gen

dependencies:
  flutter:
    sdk: flutter
  source_gen: '>=0.8.0'

创建builder.dart文件

import 'package:code_generate/api_generator.dart';
import 'package:source_gen/source_gen.dart';
import 'package:build/build.dart';

Builder apiServiceBuilder(BuilderOptions options) => LibraryBuilder(ApiServiceGenerator());

创建api_service.dart文件

const Object apiService = ApiService();

class ApiService {
  final String method;

  const ApiService({this.method = ""});
}

创建api_generator.dart文件

import 'package:analyzer/dart/element/element.dart';
import 'package:code_generate/api_service.dart';
import 'package:source_gen/source_gen.dart';
import 'package:build/build.dart';

class ApiServiceGenerator extends GeneratorForAnnotation<ApiService> {
  @override
  generateForAnnotatedElement(Element element, ConstantReader annotation, BuildStep buildStep) {
    // String className = element.displayName;
    // String path = buildStep.inputId.path;
    // String name = annotation.peek('name').stringValue;
    // $path  $name   $className
    return '''
    ///我是ApiServiceGenerator 》》 
    ''';
  }
}

3.新建build.yaml文件

targets:
  $default:
    builders:
      mark_builder:
        enabled: true

builders:
  mark_builder:
    import: 'package:code_generate/builder.dart'
    builder_factories: ['apiServiceBuilder']
    build_extensions: { '.dart': ['.dsm.dart'] }
    auto_apply: root_package
    build_to: source

4. 在主项目中的pubspec.yaml文件中引入这个插件并加入build_runner库

dependencies:
  code_generate:
    path: code_generate/


dev_dependencies:
  flutter_test:
    sdk: flutter
  build_runner: '>=0.9.1'  #需要引用该库

在使用的时候

@apiService
abstract class MyService {
  void requestLogin(String params);
}

运行
flutter packages pub run build_runner build