In Dart, can you retrieve metadata (e.g., annotations) at runtime using reflection?

Issue

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.

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.

Reflective access to metadata is not yet implemented as of the M3 release.“

Thank you.

Solution

Sample code for understanding.

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'";
}

Output:

'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.

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;
}

And retrieve metadata with generic method.

var goodbye = getAnnotation<Goodbye>(methodMirror);

Answered By – mezoni

Answer Checked By – Pedro (FlutterFixes Volunteer)