在 Dart 中，您能否在运行时使用反射检索元数据(例如，注释)? [英] In Dart, can you retrieve metadata (e.g., annotations) at runtime using reflection?

问题描述

如果是这样，这是如何实现的?如果没有，是否有计划在未来的 Dart 版本中支持这一点?我主要指的是您自己创建的自定义注释.

If so, how is this accomplished? If not, are there any plans to support this in future Dart releases? I'm mostly referring to your own created custom annotations.

在此文档链接中，https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.d0rowtffuudf，它说:元数据与紧跟在元数据，假设 p 本身不是元数据或注释.元数据可以在运行时通过反射调用检索，前提是带注释的程序结构 p 可通过反射访问.

In this documentation link, https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.d0rowtffuudf, it says: "Metadata is associated with the abstract syntax tree of the program construct p that immediately follows the metadata, assuming p is not itself metadata or a comment . Metadata can be retrieved at runtime via a reflective call, provided the annotated program construct p is accessible via reflection.

在 M3 版本中尚未实现对元数据的反射访问."

谢谢.

推荐答案

示例代码以供理解.

import "dart:mirrors";

void main() {
  var object = new Class1();
  var classMirror = reflectClass(object.runtimeType);
  // Retrieve 'HelloMetadata' for 'object'
  HelloMetadata hello = getAnnotation(classMirror, HelloMetadata);
  print("'HelloMetadata' for object: $hello");

  // Retrieve 'Goodbye' for 'object.method'
  var methodMirror = (reflect(object.method) as ClosureMirror).function;
  Goodbye goodbye = getAnnotation(methodMirror, Goodbye);
  print("'Goodbye' for object: $goodbye");

  // Retrieve all 'Goodbye' for 'object.method'
  List<Goodbye> goodbyes = getAnnotations(methodMirror, Goodbye);
  print("'Goodbye's for object.method': $goodbyes");

  // Retrieve all metadata for 'object.method'
  List all = getAnnotations(methodMirror);
  print("'Metadata for object.method': $all");
}

Object getAnnotation(DeclarationMirror declaration, Type annotation) {
  for (var instance in declaration.metadata) {
    if (instance.hasReflectee) {
      var reflectee = instance.reflectee;
      if (reflectee.runtimeType == annotation) {
        return reflectee;
      }
    }
  }

  return null;
}

List getAnnotations(DeclarationMirror declaration, [Type annotation]) {
  var result = [];
  for (var instance in declaration.metadata) {
    if (instance.hasReflectee) {
      var reflectee = instance.reflectee;
      if (annotation == null) {
        result.add(reflectee);
      } else if (reflectee.runtimeType == annotation) {
        result.add(reflectee);
      }
    }
  }

  return result;
}

@HelloMetadata("Class1")
class Class1 {
  @HelloMetadata("method")
  @Goodbye("method")
  @Goodbye("Class1")
  void method() {
  }
}

class HelloMetadata {
  final String text;
  const HelloMetadata(this.text);
  String toString() => "Hello '$text'";
}

class Goodbye {
  final String text;
  const Goodbye(this.text);
  String toString() => "Goodbye '$text'";
}

输出:

'HelloMetadata' for object: Hello 'Class1'
'Goodbye' for object: Goodbye 'method'
'Goodbye's for object.method': [Goodbye 'method', Goodbye 'Class1']
'Metadata for object.method': [Hello 'method', Goodbye 'method', Goodbye 'Class1']

附言

如果 Dart 支持通用方法，我会推荐使用此代码.

If Dart had supported the generic methods that I would recommend to use this code.

T getAnnotation<T>(DeclarationMirror declaration) {
  for (var instance in declaration.metadata) {
    if (instance.hasReflectee) {
      var reflectee = instance.reflectee;
      if (reflectee.runtimeType == T) {
        return reflectee;
      }
    }
  }

  return null;
}

并使用通用方法检索元数据.

And retrieve metadata with generic method.

var goodbye = getAnnotation<Goodbye>(methodMirror);

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