design-patterns - 扩建工厂

我有两节课

class A {
    String _id;

    static final Map<String, A> _instances = <String, A>{};

    factory A.getInstance(String id){
        if(!_instances.containsKey(id)){
            _instances[id] = new A._internal(id);
        }
        return _instances[id];
    }

    A._internal(this._id);
}

class B extends A {
    factory B.getInstance(int id){
        return new A.getInstance(id.toString());
    }
}

我知道这不是一个好习惯，因为A类不是B类的子类型。有什么方法可以实现？

最佳答案

您必须通过实现所有方法或使用A来实现noSuchMethod(invocation)和委托(delegate)方法调用。

对于给定的A类:

class A {
  static final Map<String, A> _instances = <String, A>{};

  String _id;

  A._internal(this._id);
  factory A.getInstance(String id) =>
      _instances.putIfAbsent(id, () => new A._internal(id));

  String get id => _id;
  String get name => "name_${_id}";
}

您的B类可能类似于:

class B implements A {
  final A a;

  B(int id) : a = new A.getInstance(id.toString());

  String get id => a.id;
  String get name => a.name;
}

或使用noSuchMethod:

import 'dart:mirrors';
class B implements A {
  final A a;

  B(int id) : a = new A.getInstance(id.toString());

  noSuchMethod(Invocation invocation) => reflect(a).delegate(invocation);
}

关于design-patterns - 扩建工厂，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/17535308/