Android 进阶之设计模式 访问者模式

概述

访问者模式是一个个人认为比较绕的设计模式，它有两个比较常用的用途

第一：给某个已有的对象在运行时时期添加某个方法，这个方法就是在Visitor类中实现的方法。
声明我需要添加哪个方法，以及实现这个方法需要谁的帮助。这里Leader 要给老板作报告，但是他不会抓了个雇员从参数里面扔进来。

public interface Visitor {
    public void doAsTheBossOrder(Employee me);
}

public class Leader implements Visitor{
    public void doAsTheBossOrder(Employee me) {
        me.operation1();
        me.operation2();
        me.operation3();
        reportToBoss();
    }
    public void reportToBoss() {
        System.out.println("这是我办的");
    }
}

public abstract class Employee {
    public abstract void accept(Visitor leader);
    public void operation1() {
        System.out.println("施展我的技能1");
    }
    public void operation2() {
        System.out.println("施展我的技能2");
    }
    public void operation3() {
        System.out.println("施展我的技能3");
    }
}

public class Engeener extends Employee{
    public void accept(Visitor leader) {
        leader.doAsTheBossOrder(this);
    }
}

public class Client {
    public static void main(String args[]) {
        Employee engeener = new Engeener();
        Visitor leader = new Leader();
        engeener.accept(leader);
    }
}

我们从上面例子可以看出Engeener类并没有doAsTheBossOrder的方法，但是调用engeener.accept(leader)的时候实际上运行的就是doAsTheBossOrder方法，这相当于在运行时的时候Engeener临时添加了一个doAsTheBossOrder方法。这个和装饰模式有存在很大的差异，虽然二者都是通过组合对象来实现的，但是装饰模式是一种静态添加的，添加后装饰类就拥有了这个方法，而访问者模式中是动态添加的，并没有在类代码中实际添加该方法。

遍历不用结构的元素：

public interface Visitor {
    public void visit(CommentEmployee employee);
    public void visit(Leader leader);
}

public class ConcreVisitor implements Visitor {

    public void visit(CommentEmployee employee) {
        System.out.println(this.getCommonEmployeeInfo(employee));
    }

    public void visit(Leader leader) {
        System.out.println(this.getLeaderInfo(leader));
    }

    private String getBasicInfo(Employee employee) {
        return "Id :" +employee.getId() + "Name :"+employee.getName()
                +"Sex :"+((employee.getSex() == 0)? "MAN":"LAYDY")
                +"Salary :" + employee.getSalary();
    }

    private String getLeaderInfo(Leader leader) {
        String basicInfo = this.getBasicInfo(leader);
        String otherInfo = "Performance :"+leader.getPerformence();
        return basicInfo + otherInfo;
    }

    private String getCommonEmployeeInfo(CommentEmployee employee) {
        String basicInfo = this.getBasicInfo(employee);
        String otherInfo = "Job :"+employee.getJob();
        return basicInfo + otherInfo;
    }
}

public abstract class Employee {

    private long id;
    private String name;
    private int sex;
    private int salary;

    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getSex() {
        return sex;
    }
    public void setSex(int sex) {
        this.sex = sex;
    }
    public int getSalary() {
        return salary;
    }
    public void setSalary(int salary) {
        this.salary = salary;
    }
    public abstract void accept(Visitor visitor);
}

public class CommentEmployee extends Employee {
    private String job;
    public String getJob() {
        return job;
    }
    public void setJob(String job) {
        this.job = job;
    }
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

public class Leader extends Employee {

    private String performence;
    public String getPerformence() {
        return performence;
    }
    public void setPerformence(String performence) {
        this.performence = performence;
    }
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

import java.awt.List;
import java.util.ArrayList;

public class ObjectStruct {

    private ArrayList<Employee> memberlist = new ArrayList<Employee>();

    public void addMember(Employee employee) {
        memberlist.add(employee);
    }

    public void handleRequest() {
        for(Employee em: memberlist) {
            Visitor visitor = new ConcreVisitor();
            em.accept(visitor);
        }
    }
}

public class Client {

    public static void main(String args[]) {
        ObjectStruct objectStruce = new ObjectStruct();
        CommentEmployee em1 = new CommentEmployee();
        em1.setId(1);
        em1.setName("jimmy1");
        em1.setSex(0);
        em1.setSalary(10000);
        em1.setJob("码农");
        objectStruce.addMember(em1);

        Leader em2 = new Leader();
        em2.setId(2);
        em2.setName("hello ketty");
        em2.setSex(3);
        em2.setSalary(40000);
        em2.setPerformence("Well done");
        objectStruce.addMember(em2);

        objectStruce.handleRequest();
    }
}