在Dart中,您可以使用反射在运行时检索元数据(例如,注释)吗？

Question

如果是这样,这是如何实现的？如果没有,是否有任何计划在未来的Dart版本中支持这一点？我主要是指你自己创建的自定义注释.

在此文档链接https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.d0rowtffuudf中,它说:"元数据与程序构造p的抽象语法树相关联紧接着元数据,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,前提是注释的程序结构p可以通过反射访问.

从M3版本开始,尚未实现对元数据的反射访问."

谢谢.

Answer 1

用于理解的示例代码.

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']

PS

如果Dart支持我建议使用此代码的泛型方法.

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.

var goodbye = getAnnotation<Goodbye>(methodMirror);