Meta Command Pattern

"Allows you to create macros of commands so that you can execute multiple commands at once."

我們可以利用 Meta Command 一次執行很多個命令，延續上篇的範例中，還沒完成的 undo() 功能就是應用這個概念，undo就是 ctrl+z 回到上一步，假如現在執行 LightOffCommand()關燈，那undo後就會回到LightOnCommand()開燈的狀態。

實作未設定功能的 NoCommand() 以及 Undo 按鈕的 undoCommand()

前篇程式碼少了一個判斷，如果按下沒設定功能的按鈕會直接報錯XD
所以我們新增一個NoCommand()類別，在Remote()建構子先把所有按鈕都初始化成 NoCommand()，另外要把
undoCommand 也先初始化成沒有命令。
一開始可能會直觀地覺得，undo不就是取消現在的動作，以中文理解就是什麼都不做？
為什麼要先記住現在的命令？

其實Undo翻成中文應該要理解成回到上一步，假設現在把燈打開，我們取消動作後，若先前沒有undoCommand這個物件去記憶動作，燈就會保持在打開的狀態，下一次我們再按一下就會把燈關掉，而不是我們想的把燈打開。（完全像是在繞口令）

所以要修改過後的程式碼會變成以下：

首先新增一個類別 NoCommand()

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

public class NoCommand implements Command{
    public void execute(){
        System.out.println("The button hasn't been assign to any command.");
    }
    public void undo(){}
}

修改遙控器類別的初始化、新增Undo按鈕

public class Remote extends Invoker{
    ArrayList<Command> onCommands;
    ArrayList<Command> offCommands;
    Command undoCommand;
    
    public Remote(){
        NoCommand noCommand = new NoCommand();
        
        //先把八個按鈕包含undo都初始化成 noCommand
        onCommands = new ArrayList<Command>(Collections.nCopies(8, noCommand));
        offCommands = new ArrayList<Command>(Collections.nCopies(8, noCommand));
        undoCommand = new NoCommand();
    }

    public void setCommand(int btnVal, Command onCommand, Command offCommand){
        onCommands.set(btnVal - 1, onCommand);
        offCommands.set(btnVal - 1, offCommand);  
    }

    public void pressOnBtn(int btnVal){     
        System.out.print(this.toString(btnVal, "ON"));   
        onCommands.get(btnVal - 1).execute();   
        
        //要先讓undo按鈕知道現在執行的命令，如果使用者按下undo，就是執行跟undo記憶的相反的命令
        //若等下使用者按下undo，才會讓命令回到Off
        undoCommand = onCommands.get(btnVal - 1);
    }

    public void pressOffBtn(int btnVal){        
        System.out.print(this.toString(btnVal, "OFF"));  
        offCommands.get(btnVal - 1).execute();    
        //若等下使用者按下undo，才會讓命令回到On
        undoCommand = offCommands.get(btnVal - 1);
    }

    public void pressUndoBtn(){
        System.out.print(this.toString(0, "UNDO"));  
        onCommands.get(7).execute();   
        undoCommand.undo();
    }
    
    // 可以顯示目前所有按鈕的功能
    @Override
    public String toString() {
        StringBuffer stringBuf = new StringBuffer();
        stringBuf.append("\n_________________   Remote Control   _________________\n\n");
        for(int i = 0; i < onCommands.size() - 1; i++){
            stringBuf.append("[ SLOT "+ (i + 1) + "] " + onCommands.get(i).getClass().getName());
            stringBuf.append("    " + "[ SLOT " + (i + 1) + "] " + offCommands.get(i).getClass().getName() + "\n");
        }
        stringBuf.append("[ SLOT " + (onCommands.size()) + "] " + undoCommand.getClass().getName() + " -> " + "UNDO \n");
        return stringBuf.toString();
    }
    
    //可以顯示目前按下的按鈕
    public String toString(int btnVal, String func){
        return "Press " + func + " button " + btnVal + " -> ";
    }
}

以開關燈的命令為例，undo() 就是執行相反的令令

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 LightOffCommand implements Command{
    Light light;

    public LightOffCommand(Light light){
        this.light = light;
    }

    public void execute(){        
        light.off();
    }

    public void undo(){
        light.on();
    }
}

最後再測試看看：

public class CommandPattern {
    public static void main(String[] args) {
        Remote remote = new Remote();
        Light livingroomLight = new Light("living room");
        Light studioLight = new Light("Music Studio");

        Curtain bedroomCurtain = new Curtain("bedroom");
        Curtain livingroomCurtain = new Curtain("Livingroom");
        Fridge kitchenFridge = new Fridge("kitchen");

        CurtainOpenCommand livingroomCurtainOpen = new CurtainOpenCommand(livingroomCurtain);
        CurtainCloseCommand livingroomCurtainClose = new CurtainCloseCommand(livingroomCurtain);

        CurtainOpenCommand bedroomCurtainOpen = new CurtainOpenCommand(bedroomCurtain);
        CurtainCloseCommand bedroomCurtainClose = new CurtainCloseCommand(bedroomCurtain);

        LightOnCommand livingroomLightOnCommand = new LightOnCommand(livingroomLight);
        LightOffCommand livingroomLightOffCommand = new LightOffCommand(livingroomLight);

        LightOnCommand studioLightOnCommand = new LightOnCommand(studioLight);
        LightOffCommand studioLightOffCommand = new LightOffCommand(studioLight);

        FridgeOpenCommand kitchenFridgeOpenCommand = new FridgeOpenCommand(kitchenFridge);
        FridgeCloseCommand kitchenFridgeOffCommand = new FridgeCloseCommand(kitchenFridge);

        System.out.println(remote);

        remote.setCommand(1, livingroomCurtainOpen, livingroomCurtainClose);
        remote.setCommand(2, livingroomLightOnCommand, livingroomLightOffCommand);
        remote.setCommand(3, kitchenFridgeOpenCommand, kitchenFridgeOffCommand);
        remote.setCommand(4, bedroomCurtainOpen, bedroomCurtainClose);
        remote.setCommand(5, studioLightOnCommand, studioLightOffCommand);

        System.out.println(remote);

        remote.pressOffBtn(4);
        remote.pressOnBtn(5);       
        remote.pressUndoBtn();
        remote.pressOnBtn(5);
        remote.pressOnBtn(1);
        remote.pressOffBtn(7);
    }
}

輸出結果

參考資料：

1. 《深入淺出設計模式 (Head First Design Patterns) 》
2. 書中官方程式碼傳送門
3. 本篇完整程式碼傳送門

Disclaimer
因為讀的是原文版，所以難免會有翻譯詞不達意或是專有名詞上的差異，有錯誤的話歡迎在留言區一起交流！