設(shè)計模式學(xué)習(xí)專欄五--------命令模式

場景

設(shè)計一個智能遙控器,遙控器上有7個插槽 , 每個插槽(某設(shè)備具體供應(yīng)商)對應(yīng)兩個按鈕 on , off ,以及一個全局的撤銷操作undo.

image

較差的實(shí)現(xiàn)方式

判斷每個插槽對應(yīng)的具體廠商是誰, 然后做出對應(yīng)的動作

if(slot1 == Light){
    light.on()
}else if(slot1 == Hottub){
    hottub.on()
}else if(slot1 == TV){
    tv.on();
}
...

出現(xiàn)的問題
- 遙控器和具體的設(shè)備廠商耦合到了一起 , 遙控器需要認(rèn)識某個插槽當(dāng)前對應(yīng)設(shè)備的廠商
- 當(dāng)加入/刪除新的設(shè)備廠商時, 當(dāng)前代碼都需要進(jìn)行改動

如何解決

對象村餐廳的例子

image

一張訂單封裝了準(zhǔn)備餐點(diǎn)的請求
女招待的工作是接收訂單 , 然后調(diào)用訂單的OrderUp()方法 , 女招待不需要擔(dān)心訂單的內(nèi)容是什么, 或者由誰來準(zhǔn)備餐點(diǎn) , 她只需要指導(dǎo),訂單由一個OrderUp()方法可以調(diào)用即可.
快餐廚師具有準(zhǔn)備餐點(diǎn)的知識 . 他只要看到訂單就知道如何準(zhǔn)備餐點(diǎn) . 廚師和女招侍之間從不需要直接溝通.

把餐廳想象成設(shè)計模式的一種模型 , 這個模型允許通過"封裝請求的命令"(訂單)將 "發(fā)出請求的對象"(女招待, "訂單來啦") 和 "接收與執(zhí)行這些請求的對象"(廚師) 分隔開 .

從餐廳到命令模式

image

命令模式總覽

定義: 將請求封裝成對象(訂單),將發(fā)出請求的對象(服務(wù)員/按鈕)和執(zhí)行請求的對象(廚師/具體供應(yīng)商)解耦 , 也可以支持撤銷操作

類圖

image

模式的理解

角色
- 封裝請求的命令對象(訂單) : 一個命令對象通過在特定接收者上綁定一組動作來封裝一個請求 receiver.action1(), 該對象之暴露一個execute()方法, 當(dāng)此方法被調(diào)用時, 接收者就會進(jìn)行對應(yīng)的動作 . 從女招待的角度來看, 它不需要知道具體哪個接收者進(jìn)行了什么動作,只知道如果調(diào)用execute()方法 , 請求的目的就能達(dá)到了
- 發(fā)出請求的對象(女招待) : ①負(fù)責(zé)接收命令對象 ②在合適的時候調(diào)用命令對象的execute()方法
- 執(zhí)行請求的對象(廚師) : 真正執(zhí)行請求的接收者

細(xì)節(jié)

"撤銷操作" 只需在Invocker中定義變量Command undoCommand保存撤銷操作即可 . 如果要聯(lián)系撤銷, 則可以用棧的形式存儲執(zhí)行過的命令

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;    //記錄上一次命令
 
  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]; //記錄上一次命令
  }
 
  public void offButtonWasPushed(int slot) {
      offCommands[slot].execute();
      undoCommand = offCommands[slot];    //記錄上一次命令
  }
 
  public void undoButtonWasPushed() {
      undoCommand.undo(); //執(zhí)行撤銷命令
  }
}

- "Party模式" :  封裝一次請求中的批量操作 (按下一個按鈕,同時弄暗燈光 , 打開音響和電視,設(shè)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();
        }
    }
  }
  ```

使用場景
- 隊列請求 (日志安排 , 線程池 , 工作隊列)
  - 以線程池為例 , 將線程需要執(zhí)行的動作封裝進(jìn)Runnable對象中
  - 線程池以阻塞隊列存儲Runnable對象
  - 當(dāng)線程空閑時 , 從隊列中取出Runnable對象 , 并執(zhí)行Runnable中重寫的Run()方法
  - 總結(jié) : 即線程池中的線程不需要知道具體的執(zhí)行者是誰,干了什么,只需要在空閑時從隊列中取出命令對象Runnable并調(diào)用run()方法即可
- 日志請求
  - 當(dāng)每個命令被執(zhí)行時, 會被儲存在磁盤中
  - 在系統(tǒng)死機(jī)后, 重新加載存儲的命令,并以準(zhǔn)備的次序執(zhí)行

案例代碼部分

image

命令對象Command

public interface Command {
  public void execute();
  public void undo();
}

具體命令對象--開燈

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);
  }
}

具體命令對象 --關(guān)燈

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);
  }
}

設(shè)備廠商 -- 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;
  }
}

調(diào)用者Invoker --遠(yuǎn)程遙控器

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();    //撤銷操作
      System.out.println(remoteControl);      //此時應(yīng)該是開燈狀態(tài)
  }
}

輸出結(jié)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%       //撤銷操作 ==> 開燈狀態(tài)

------ 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

參考

? 書籍: HeadFirst設(shè)計模式

? 代碼參考地址: 我就是那個地址