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

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.

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

• 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") 和 "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

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

• 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, 则可以用栈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)

      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

• 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