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设计模式学习专栏五--------命令模式

设计模式学习专栏五--------命令模式

作者: 你的头发真的好长 | 来源:发表于2019-03-11 18:05 被阅读0次

    设计模式学习专栏五--------命令模式

    场景


    设计一个智能遥控器,遥控器上有7个插槽 , 每个插槽(某设备具体供应商)对应两个按钮 on , off ,以及一个全局的撤销操作undo.

    image

    较差的实现方式

    判断每个插槽对应的具体厂商是谁, 然后做出对应的动作

    if(slot1 == Light){
        light.on()
    }else if(slot1 == Hottub){
        hottub.on()
    }else if(slot1 == TV){
        tv.on();
    }
    ...
    
    • 出现的问题
      • 遥控器和具体的设备厂商耦合到了一起 , 遥控器需要 认识 某个插槽当前对应设备的厂商
      • 当加入/删除 新的设备厂商时, 当前代码都需要进行改动

    如何解决


    对象村餐厅的例子

    image
    • 一张订单封装了 准备餐点的请求
    • 女招待的工作是接收订单 , 然后调用订单的OrderUp()方法 , 女招待不需要担心订单的内容是什么, 或者由谁来准备餐点 , 她只需要指导,订单由一个OrderUp()方法可以调用即可.
    • 快餐厨师具有准备餐点的知识 . 他只要看到订单就知道如何准备餐点 . 厨师和女招侍之间从不需要直接沟通.

    把餐厅想象成设计模式的一种模型 , 这个模型允许通过"封装请求的命令"(订单)将 "发出请求的对象"(女招待, "订单来啦")"接收与执行这些请求的对象"(厨师) 分隔开 .

    从餐厅到命令模式

    image

    命令模式总览


    定义: 将请求封装成对象(订单),将发出请求的对象(服务员/按钮)和执行请求的对象(厨师/具体供应商)解耦 , 也可以支持撤销操作

    • 类图
    image
    • 模式的理解

      • 角色

        • 封装请求的命令对象(订单) : 一个命令对象通过在特定接收者上绑定一组动作来封装一个请求 receiver.action1(), 该对象之暴露一个execute()方法, 当此方法被调用时, 接收者就会进行对应的动作 . 从女招待的角度来看, 它不需要知道具体哪个 接收者进行了什么动作,只知道如果调用execute()方法 , 请求的目的就能达到了
        • 发出请求的对象(女招待) : ①负责接收命令对象 ②在合适的时候调用命令对象的execute()方法
        • 执行请求的对象(厨师) : 真正执行请求的接收者
      • 细节

        • "撤销操作" 只需在Invocker中定义变量Command undoCommand保存撤销操作即可 . 如果要联系撤销, 则可以用的形式存储执行过的命令

          public class LightOnCommand implements Command {
            Light light;
          
            public LightOnCommand(Light light) {
                this.light = light;
            }
          
            public void execute() {
                light.on();
            }
          
            public void undo() {
                light.off();
            }
          }
          
          public class RemoteControlWithUndo {
            Command[] onCommands;
            Command[] offCommands;
            Command undoCommand;    //记录上一次命令
           
            public RemoteControlWithUndo() {
                onCommands = new Command[7];
                offCommands = new Command[7];
           
                Command noCommand = new NoCommand();
                for(int i=0;i<7;i++) {
                    onCommands[i] = noCommand;
                    offCommands[i] = noCommand;
                }
                undoCommand = noCommand;
            }
            
            public void setCommand(int slot, Command onCommand, Command offCommand) {
                onCommands[slot] = onCommand;
                offCommands[slot] = offCommand;
            }
           
            public void onButtonWasPushed(int slot) {
                onCommands[slot].execute();
                undoCommand = onCommands[slot]; //记录上一次命令
            }
           
            public void offButtonWasPushed(int slot) {
                offCommands[slot].execute();
                undoCommand = offCommands[slot];    //记录上一次命令
            }
           
            public void undoButtonWasPushed() {
                undoCommand.undo(); //执行撤销命令
            }
          }
          
    - "Party模式" :  封装一次请求中的批量操作 (按下一个按钮,同时弄暗灯光 , 打开音响和电视,设置好DVD)
    
      ```java
      public class MacroCommand implements Command {
        Command[] commands;
       
        public MacroCommand(Command[] commands) {
            this.commands = commands;
        }
       
        public void execute() {
            for (int i = 0; i < commands.length; i++) {
                commands[i].execute();
            }
        }
      }
      ```
    
    • 使用场景

      • 队列请求 (日志安排 , 线程池 , 工作队列)
        • 以线程池为例 , 将线程需要执行的动作封装进Runnable对象中
        • 线程池 以 阻塞队列存储Runnable对象
        • 当线程空闲时 , 从队列中取出Runnable对象 , 并执行Runnable中重写的Run()方法
        • 总结 : 即线程池中的线程不需要知道具体的执行者是谁,干了什么,只需要在空闲时从队列中取出命令对象Runnable并调用run()方法即可
      • 日志请求
        • 当每个命令被执行时, 会被储存在磁盘中
        • 在系统死机后, 重新加载存储的命令,并以准备的次序执行

    案例代码部分

    image
    • 命令对象Command

      public interface Command {
        public void execute();
        public void undo();
      }
      
    • 具体命令对象--开灯

      public class LightOnCommand implements Command {
        Light light;
        int level;
        public LightOnCommand(Light light) {
            this.light = light;
        }
       
        public void execute() {
              level = light.getLevel();
            light.on();
        }
       
        public void undo() {
            light.dim(level);
        }
      }
      
    • 具体命令对象 --关灯

      public class LightOffCommand implements Command {
        Light light;
        int level;
        public LightOffCommand(Light light) {
            this.light = light;
        }
       
        public void execute() {
              level = light.getLevel();
            light.off();
        }
       
        public void undo() {
            light.dim(level);
        }
      }
      
    • 设备厂商 -- Receiver

      public class Light {
        String location;
        int level;
      
        public Light(String location) {
            this.location = location;
        }
      
        public void on() {
            level = 100;
            System.out.println("Light is on");
        }
      
        public void off() {
            level = 0;
            System.out.println("Light is off");
        }
      
        public void dim(int level) {
            this.level = level;
            if (level == 0) {
                off();
            }
            else {
                System.out.println("Light is dimmed to " + level + "%");
            }
        }
      
        public int getLevel() {
            return level;
        }
      }
      
    • 调用者Invoker --远程遥控器

      public class RemoteControlWithUndo {
        Command[] onCommands;
        Command[] offCommands;
        Command undoCommand;
       
        public RemoteControlWithUndo() {
            onCommands = new Command[7];
            offCommands = new Command[7];
       
            Command noCommand = new NoCommand();
            for(int i=0;i<7;i++) {
                onCommands[i] = noCommand;
                offCommands[i] = noCommand;
            }
            undoCommand = noCommand;
        }
        
        public void setCommand(int slot, Command onCommand, Command offCommand) {
            onCommands[slot] = onCommand;
            offCommands[slot] = offCommand;
        }
       
        public void onButtonWasPushed(int slot) {
            onCommands[slot].execute();
            undoCommand = onCommands[slot];
        }
       
        public void offButtonWasPushed(int slot) {
            offCommands[slot].execute();
            undoCommand = offCommands[slot];
        }
       
        public void undoButtonWasPushed() {
            undoCommand.undo();
        }
      }
      
    • 主程序

      public class RemoteLoader {
      
        public static void main(String[] args) {
            RemoteControlWithUndo remoteControl = new RemoteControlWithUndo();
      
            Light livingRoomLight = new Light("Living Room");
      
            LightOnCommand livingRoomLightOn =
                    new LightOnCommand(livingRoomLight);
            LightOffCommand livingRoomLightOff =
                    new LightOffCommand(livingRoomLight);
      
            remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff);
      
            remoteControl.onButtonWasPushed(0);
            remoteControl.offButtonWasPushed(0);
            System.out.println(remoteControl);
            remoteControl.undoButtonWasPushed();    //撤销操作
            System.out.println(remoteControl);      //此时应该是开灯状态
        }
      }
      
    • 输出结果

      Light is on
      Light is off
      
      ------ Remote Control -------
      [slot 0] headfirst.designpatterns.command.undo.LightOnCommand    headfirst.designpatterns.command.undo.LightOffCommand
      [slot 1] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 2] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 3] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 4] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 5] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 6] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [undo] headfirst.designpatterns.command.undo.LightOffCommand
      
      Light is dimmed to 100%       //撤销操作 ==> 开灯状态
      
      ------ Remote Control -------
      [slot 0] headfirst.designpatterns.command.undo.LightOnCommand    headfirst.designpatterns.command.undo.LightOffCommand
      [slot 1] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 2] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 3] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 4] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 5] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [slot 6] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand
      [undo] headfirst.designpatterns.command.undo.LightOffCommand
      

    参考

    ​ 书籍: HeadFirst设计模式

    ​ 代码参考地址: 我就是那个地址

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