在飞镖，你可以使用反射在运行时检索元数据（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，这是怎么做到的呢？如果不是，是否有支持这个在以后的版本中达特什么计划？我主要指的是你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，的https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.d0rowtffuudf,它说：元数据与紧接在元数据中的节目构建体p的抽象语法树相关联，假设p不是自身的元数据或评论的元数据可以在运行时通过反射呼叫检索，所提供的注释的程序构建p是通过反射访问。

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.

对元数据的反光访问尚未实现为M3的​​释放。

感谢您。

推荐答案

样品code的理解。

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']

P.S。

如果飞镖都支持，我会建议使用此code通用的方法。

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;
}

和检索与泛型方法的元数据。

And retrieve metadata with generic method.

var goodbye = getAnnotation<Goodbye>(methodMirror);

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