Dart build_runner无法生成文件，请求排查问题原因

算力阿峰

2026-6-23

问题

我在名为temp的包中配置了如下build.yaml文件：

builders:
  my_package:
    import: "package: my_package/my_package.dart"
    builder_factories: ["myBuilder"]
    build_extensions: {'.dart': ['.g.dart']}
    auto_apply: none
    build_to: source

已确认pubspec.yaml中正确导入了my_package，my_package包的lib/my_package.dart文件中实现了对应的Builder并导出：

library my_package;

import 'package:build/build.dart';

Builder myBuilder(BuilderOptions options) {
  return SimpleBuilder();
}

class SimpleBuilder extends Builder {
  SimpleBuilder();

  @override
  Map<String, List<String>> get buildExtensions => {
        '.dart': ['.g.dart']
      };

  @override
  Future<void> build(BuildStep buildStep) async {
    log.info('SimpleBuilder: Processing ${buildStep.inputId.path}');

    final outputId = buildStep.inputId.changeExtension('.g.dart');
    final content = 'Generated content for ${buildStep.inputId.path}';

    log.info('SimpleBuilder: Writing to ${outputId.path}');

    await buildStep.writeAsString(outputId, content);
  }
}

temp包中存在lib/filename.dart文件，执行命令：

dart run build_runner build --verbose

预期生成lib/filename.g.dart文件，但无任何输出，日志显示：

dart run build_runner build --verbose
Resolving dependencies in `/my_drive/temp`... 
Downloading packages... 
Got dependencies in `/my_drive/temp`.
Building package executable... 
Built build_runner:build_runner.
[INFO] Entrypoint:Generating build script...
[INFO] Entrypoint:Generating build script completed, took 157ms

[INFO] Bootstrap:Precompiling build script......
[INFO] Bootstrap:Precompiling build script... completed, took 2.0s

[FINE] Bootstrap:Core package locations file does not exist
[INFO] BuildDefinition:Initializing inputs
[INFO] BuildDefinition:Building new asset graph...
[INFO] BuildDefinition:Building new asset graph completed, took 513ms

[INFO] BuildDefinition:Checking for unexpected pre-existing outputs....
[INFO] BuildDefinition:Checking for unexpected pre-existing outputs. completed, took 0ms

[INFO] Build:Running build...
[INFO] Build:Running build completed, took 3ms

[INFO] Build:Caching finalized dependency graph...
[INFO] Build:Caching finalized dependency graph completed, took 34ms

[INFO] Build:Succeeded after 41ms with 0 outputs (0 actions)

日志中也未出现自定义的打印信息。修改导入路径或builder_factories时会报错，说明配置路径是正确的，请问问题出在哪里？

解决方案

核心问题由两个配置错误导致，逐一修正即可：

build.yaml中import路径的空格问题
配置里的import: "package: my_package/my_package.dart"中，package:后多了一个空格，正确路径应为package:my_package/my_package.dart。这个空格会干扰Builder的匹配逻辑，导致无法正常触发。
auto_apply: none未指定触发范围
当auto_apply设为none时，build_runner不会自动将Builder应用到任何文件，必须显式指定处理目标：
1. 若要让Builder自动处理当前包的所有Dart文件，将auto_apply改为root_package；
2. 若需保留auto_apply: none，需在build.yaml中添加targets配置指定处理范围，示例：
```
targets:
  $default:
    builders:
      my_package:
        enabled: true
        generate_for:
          - lib/**.dart
```

修正后重新执行dart run build_runner build --verbose，即可看到自定义日志并生成对应的.g.dart文件。

内容的提问来源于stack exchange，提问作者anonymous-dev