访问者模式

Python的例子

"""Visitor pattern example."""from abc import ABCMeta, abstractmethodNOT_IMPLEMENTED = "You should implement this."class CarElement:
    __metaclass__ = ABCMeta
    @abstractmethod
    def accept(self, visitor):
        raise NotImplementedError(NOT_IMPLEMENTED)class Body(CarElement):
    def accept(self, visitor):
        visitor.visitBody(self)class Engine(CarElement):
    def accept(self, visitor):
        visitor.visitEngine(self)class Wheel(CarElement):
    def __init__(self, name):
        self.name = name
    def accept(self, visitor):
        visitor.visitWheel(self)class Car(CarElement):
    def __init__(self):
        self.elements = [
            Wheel("front left"), Wheel("front right"),
            Wheel("back left"), Wheel("back right"),
            Body(), Engine()
        ]

    def accept(self, visitor):
        for element in self.elements:
            element.accept(visitor)
        visitor.visitCar(self)class CarElementVisitor:
    __metaclass__ = ABCMeta
    @abstractmethod
    def visitBody(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitEngine(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitWheel(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitCar(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)class CarElementDoVisitor(CarElementVisitor):
    def visitBody(self, body):
        print("Moving my body.")
    def visitCar(self, car):
        print("Starting my car.")
    def visitWheel(self, wheel):
        print("Kicking my {} wheel.".format(wheel.name))
    def visitEngine(self, engine):
        print("Starting my engine.")class CarElementPrintVisitor(CarElementVisitor):
    def visitBody(self, body):
        print("Visiting body.")
    def visitCar(self, car):
        print("Visiting car.")
    def visitWheel(self, wheel):
        print("Visiting {} wheel.".format(wheel.name))
    def visitEngine(self, engine):
        print("Visiting engine.")car = Car()car.accept(CarElementPrintVisitor())car.accept(CarElementDoVisitor())

Java的例子

interface Visitor {
     void visit(Wheel wheel);
     void visit(Engine engine);
     void visit(Body body);
     void visit(Car car);
 }

 class Wheel {
     private String name;
     Wheel(String name) {
         this.name = name;
     }
     String getName() {
         return this.name;
     }
     void accept(Visitor visitor) {
         visitor.visit(this);
     }
 }
  
 class Engine {
     void accept(Visitor visitor) {
         visitor.visit(this);
     }
 }

 class Body {
     void accept(Visitor visitor) {
         visitor.visit(this);
     }
 }

 class Car {
     private Engine  engine = new Engine();
     private Body    body   = new Body();
     private Wheel[] wheels 
         = { new Wheel("front left"), new Wheel("front right"),
             new Wheel("back left") , new Wheel("back right")  };
     void accept(Visitor visitor) {
         visitor.visit(this);
         engine.accept(visitor);
         body.accept(visitor);
         for (int i = 0; i < wheels.length; ++ i)
             wheels[i].accept(visitor);
     }
 }

 class PrintVisitor implements Visitor {
     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");
     }
 }

 public class VisitorDemo {
     static public void main(String[] args) {
         Car car = new Car();
         Visitor visitor = new PrintVisitor();
         car.accept(visitor);
     }
 }

一个实际的例子

这个例子是Htmlparser计划里的一段示例。为获得一个Web页面的所有内容，采用如下的方式使用类TextExtractingVisitor:
Parser parser = new Parser("http://pageIwantToParse.com");
TextExtractingVisitor visitor = new TextExtractingVisitor();
parser.visitAllNodesWith(visitor);
System.out.println(visitor.getExtractedText());