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") 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

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)

  ```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