访问者しゃ模も式しき

访问者しゃ模も式しき是ぜ一种将算法与对象结构分离的软件设计模も式しき。

这个模も式しき的てき基本きほん想そう法ほう如下：首くび先さき我わが们拥有ゆう一いち个由许多对象构成的てき对象结构，这些对象的てき类都と拥有一いち个accept方法ほうほう用もちい来らい接受せつじゅ访问者しゃ对象；访问者しゃ是ぜ一いち个接口こう，它拥有ゆう一いち个visit方法ほうほう，这个方法ほうほう对访问到的てき对象结构中ちゅう不同ふどう类型的てき元素げんそ作出さくしゅつ不同ふどう的てき反はん应；在ざい对象结构的てき一いち次じ访问过程中ちゅう，我わが们遍历整个对象ぞう结构，对每一个元素都实施accept方法ほうほう，在ざい每まい一いち个元素的すてきaccept方法ほうほう中ちゅう回かい调访问者しゃ的てきvisit方法ほうほう，从而使し访问者しゃ得とく以处理り对象结构的てき每ごと一いち个元素げんそ。我わが们可以针对对象ぞう结构设计不同ふどう的てき实在的てき访问者しゃ类来完成かんせい不同ふどう的てき操作そうさ。

访问者しゃ模も式しき使し得え我わが们可以在传统的てき单分派ぶんぱ语言（如Smalltalk、Java和わC++）中ちゅう模も拟双そう分派ぶんぱ技わざ术。对于支持しじ多た分派ぶんぱ的てき语言（如CLOS），访问者しゃ模も式しき已やめ经内置おけ于语言げん特性とくせい之の中なか了りょう，从而不ふ再さい重要じゅうよう。

Python的てき例れい子こ[编辑]

"""
Visitor pattern example.
"""

from abc import ABCMeta, abstractmethod

NOT_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的てき例れい子こ[编辑]

 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[编辑]

这个例れい子こ是ぜHtmlparser计划（页面存そん档备份，存そん于互联网档案あん馆）里さと的てき一いち段だん示しめせ例れい。为获得とく一いち个Web页面的てき所有しょゆう内容ないよう，采さい用よう如下的てき方式ほうしき使用しよう类 TextExtractingVisitor:

Parser parser = new Parser("http://pageIwantToParse.com^{[永久えいきゅう失效しっこう連結れんけつ]}");
TextExtractingVisitor visitor = new TextExtractingVisitor();
parser.visitAllNodesWith(visitor);
System.out.println(visitor.getExtractedText());

参考さんこう条目じょうもく[编辑]