design-patterns - 扩建工厂

Question

我有两个类(class)

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的做法，因为 A 类不是 B 类的子类型。有什么办法可以做到这一点吗？

Answer 1

您必须实现 A 并通过实现所有方法或使用noSuchMethod(invocation) 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 类可能如下所示:

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);
}