设计模式（23）——Visitor（访问者）

浅浅的花香味﹌ 2022-08-08 12:42 226阅读 0赞

### 访问者模式是什么？ ###

访问者模式表示一个作用于某对象结构中的每个元素的操作，而不需要关注每个元素的类型。

![Image 1][]

### 访问者模式的适用范围： ###

想定义一个新的操作，而不想改变被操作对象的类型。

### 访问者模式的示例： ###

下面用在汽车的部件上添加打印操作的示例来说明访问者模式：

![Image 1][]

interface ICarElementVisitor {
        void visit(Wheel wheel);
        void visit(Engine engine);
        void visit(Body body);
        void visit(Car car);
    }
     
    interface ICarElement {
        void accept(ICarElementVisitor visitor); // CarElements have to provide accept().
    }
     
    class Wheel implements ICarElement {
        private String name;
     
        public Wheel(String name) {
            this.name = name;
        }
     
        public String getName() {
            return this.name;
        }
     
        public void accept(ICarElementVisitor visitor) {
            /*
             * accept(ICarElementVisitor) in Wheel implements
             * accept(ICarElementVisitor) in ICarElement, so the call
             * to accept is bound at run time. This can be considered
             * the first dispatch. However, the decision to call
             * visit(Wheel) (as opposed to visit(Engine) etc.) can be
             * made during compile time since 'this' is known at compile
             * time to be a Wheel. Moreover, each implementation of
             * ICarElementVisitor implements the visit(Wheel), which is
             * another decision that is made at run time. This can be
             * considered the second dispatch.
             */
            visitor.visit(this);
        }
    }
     
    class Engine implements ICarElement {
        public void accept(ICarElementVisitor visitor) {
            visitor.visit(this);
        }
    }
     
    class Body implements ICarElement {
        public void accept(ICarElementVisitor visitor) {
            visitor.visit(this);
        }
    }
     
    class Car implements ICarElement {
        ICarElement[] elements;
     
        public Car() {
            //create new Array of elements
            this.elements = new ICarElement[] { new Wheel("front left"),
                new Wheel("front right"), new Wheel("back left") ,
                new Wheel("back right"), new Body(), new Engine() };
        }
     
        public void accept(ICarElementVisitor visitor) {    
            for(ICarElement elem : elements) {
                elem.accept(visitor);
            }
            visitor.visit(this);    
        }
    }
     
    class CarElementPrintVisitor implements ICarElementVisitor {
        public void visit(Wheel wheel) {      
            System.out.println("Visiting " + wheel.getName() + " wheel");
        }
     
        public void visit(Engine engine) {
            System.out.println("Visiting engine");
        }
     
        public void visit(Body body) {
            System.out.println("Visiting body");
        }
     
        public void visit(Car car) {      
            System.out.println("Visiting car");
        }
    }
     
    class CarElementDoVisitor implements ICarElementVisitor {
        public void visit(Wheel wheel) {
            System.out.println("Kicking my " + wheel.getName() + " wheel");
        }
     
        public void visit(Engine engine) {
            System.out.println("Starting my engine");
        }
     
        public void visit(Body body) {
            System.out.println("Moving my body");
        }
     
        public void visit(Car car) {
            System.out.println("Starting my car");
        }
    }
     
    public class VisitorDemo {
        public static void main(String[] args) {
            ICarElement car = new Car();
            car.accept(new CarElementPrintVisitor());
            car.accept(new CarElementDoVisitor());
        }
    }

[Image 1]: