設計模式（23）——Visitor（訪問者）

訪問者模式是什麼？

訪問者模式表示一個作用於某對象結構中的每個元素的操作，而不需要關注每個元素的類型。

訪問者模式的適用範圍：

想定義一個新的操作，而不想改變被操作對象的類型。

訪問者模式的示例：

下面用在汽車的部件上添加打印操作的示例來說明訪問者模式：

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());
    }
}