如何为字段中的注释生成Dart代码？

debugcn 发表于 Dev

马塞尔

我正在使用编写Dart的代码生成器build_runner，但并未为字段中的注释调用构建器，尽管它确实适用于类中的注释。

是否可以在字段（或与此相关的任何位置）处调用生成器进行注释？

例如，将为以下文件调用构建器：

import 'package:my_annotation/my_annotation.dart';

part 'example.g.dart';

@MyAnnotation()
class Fruit {
  int number;
}

但不是这个：

import 'package:my_annotation/my_annotation.dart';

part 'example.g.dart';

class Fruit {
  @MyAnnotation()
  int number;
}

这是注释的定义：

class MyAnnotation {
  const MyAnnotation();
}

这就是生成器的定义方式。现在，只要调用它，它就会中止，从而导致显示错误消息。

library my_annotation_generator;

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

Builder generateAnnotation(BuilderOptions options) =>
    SharedPartBuilder([MyAnnotationGenerator()], 'my_annotation');

class MyAnnotationGenerator extends GeneratorForAnnotation<MyAnnotation> {
  @override
  generateForAnnotatedElement(Element element, ConstantReader annotation, _) {
    throw CodeGenError('Generating code for annotation is not implemented yet.');
}

这里的build.yaml配置：

targets:
  $default:
    builders:
      my_annotation_generator|my_annotation:
        enabled: true

builders:
  my_annotation:
    target: ":my_annotation_generator"
    import: "package:my_annotation/my_annotation.dart"
    builder_factories: ["generateAnnotation"]
    build_extensions: { ".dart": [".my_annotation.g.part"] }
    auto_apply: dependents
    build_to: cache
    applies_builders: ["source_gen|combining_builder"]

马塞尔

内建方法GeneratorForAnnotation使用LibraryElement的annotatedWith(...)方法，该方法仅检查顶级注释。要还检测字段上的注释，您需要编写一些自定义内容。

这是Generator我为我的项目写的：

abstract class GeneratorForAnnotatedField<AnnotationType> extends Generator {
  /// Returns the annotation of type [AnnotationType] of the given [element],
  /// or [null] if it doesn't have any.
  DartObject getAnnotation(Element element) {
    final annotations =
        TypeChecker.fromRuntime(AnnotationType).annotationsOf(element);
    if (annotations.isEmpty) {
      return null;
    }
    if (annotations.length > 1) {
      throw Exception(
          "You tried to add multiple @$AnnotationType() annotations to the "
          "same element (${element.name}), but that's not possible.");
    }
    return annotations.single;
  }

  @override
  String generate(LibraryReader library, BuildStep buildStep) {
    final values = <String>{};

    for (final element in library.allElements) {
      if (element is ClassElement && !element.isEnum) {
        for (final field in element.fields) {
          final annotation = getAnnotation(field);
          if (annotation != null) {
            values.add(generateForAnnotatedField(
              field,
              ConstantReader(annotation),
            ));
          }
        }
      }
    }

    return values.join('\n\n');
  }

  String generateForAnnotatedField(
      FieldElement field, ConstantReader annotation);
}

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