sdk/lib/core/expando.dart

// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of dart.core;

/// An [Expando] allows adding new properties to objects.
///
/// Does not work on numbers, strings, booleans or `null`.
///
/// An `Expando` does not hold on to the added property value after an object
/// becomes inaccessible.
///
/// Since you can always create a new number that is identical to an existing
/// number, it means that an expando property on a number could never be
/// released. To avoid this, expando properties cannot be added to numbers.
/// The same argument applies to strings, booleans and `null`, which also have
/// literals that evaluate to identical values when they occur more than once.
///
/// There is no restriction on other classes, even for compile time constant
/// objects. Be careful if adding expando properties to compile time constants,
/// since they will stay alive forever.
class Expando<T extends Object> {
  /// The name of the this [Expando] as passed to the constructor. If
  /// no name was passed to the constructor, the name is `null`.
  final String? name;

  /// Creates a new [Expando]. The optional name is only used for
  /// debugging purposes and creating two different [Expando]s with the
  /// same name yields two [Expando]s that work on different properties
  /// of the objects they are used on.
  external Expando([String? name]);

  /// Expando toString method override.
  String toString() => "Expando:$name";

  /// Gets the value of this [Expando]'s property on the given
  /// object. If the object hasn't been expanded, the method returns
  /// `null`.
  ///
  /// The object must not be a number, a string or a boolean.
  external T? operator [](Object object);

  /// Sets the value of this [Expando]'s property on the given
  /// object. Properties can effectively be removed again by setting
  /// their value to `null`.
  ///
  /// The object must not be a number, a string or a boolean.
  external void operator []=(Object object, T? value);
}