Design Patterns 28 - 访问者模式

abstract class Visitor {
    public abstract void VisitConcreteElementA(ConcreteElementA concreteElementA);
    public abstract void VisitConcreteElementB(ConcreteElementB concreteElementB);    
}

class ConcreteVisitor1 : Visitor {
    public override void VisitConcreteElementA(ConcreteElementA concreteElementA) {
        // todo
    }
    public override void VisitConcreteElementB(ConcreteElementB concreteElementB) {
        // todo
    }
}

class ConcreteVisitor2 : Visitor {
    public override void VisitConcreteElementA(ConcreteElementA concreteElementA) {
        // todo
    }
    public override void VisitConcreteElementB(ConcreteElementB concreteElementB) {
        // todo
    }
}

abstract class Element {
    public abstract void Accept(Visitor visitor);
}

class ConcreteElementA : Element {
    public override void Accept(Visitor visitor) {
        visitor.VisitConcreteElementA(this);
    }
    public void OperationA {
        // todo
    }
}

class ConcreteElementB : Element {
    public override void Accept(Visitor visitor) {
        visitor.VisitConcreteElementB(this);
    }
    public void OperationB {
        // todo
    }
}

class ObjectStructure {
    private IList<Element> elements = new List<Element>();

    public void Attach(Element element) {
        elements.Add(element);
    }

    public void Detach(Element element) {
        elements.Remove(element);
    }

    public void Accept(Visitor visitor) {
        foreach(Element element in elements) {
            element.Accept(visitor);
        }
    }
}

public static void Main(stringp[] agrs) {
    ObjectStructure objectStructure = new ObjectStructure();
    objectStructure.Attach(new ConcreteElementA());
    objectStructure.Attach(new ConcreteElementB());

    ConcreteVisitor1 concreteVisitor1 = new ConcreteVisitor1();
    ConcreteVisitor2 concreteVisitor2 = new ConcreteVisitor2();

    objectStructure.Accept(concreteVisitor1);
    objectStructure.Accept(concreteVisitor2);
}