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命令模式（Command Pattern）

定义

命令模式通过将操作包装成对象来实现操作的撤销、恢复上一个状态、重复执行命令等功能。例如，在编程中，这种模式常用于支持撤销（undo）和重做（redo）的功能，比如代码编辑器中的命令历史记录。

适用场景

角色说明

• Command（命令抽象类）：所有命令的基类，每个命令都必须实现execute方法。
• ConcreteCommand（具体命令实现类）：具体实现Command中的execute方法。
• Invoker（控制器）：负责执行命令的调用者，维护撤销栈和重做栈。
• Receiver（接收者）：命令操作的执行目标，维护相关状态。

代码示例

public abstract class Command {    public abstract void execute(Receiver receiver);    public abstract void undo();    public abstract void redo();    @Override    public abstract String toString();}

隐形咒命令

public class InvisibilityCommand extends Command {    private Receiver receiver;    @Override    public void execute(Receiver receiver) {        receiver.setVisibility(Visibility.INVISIBLE);        this.receiver = receiver;    }    @Override    public void undo() {        if (receiver != null) {            receiver.setVisibility(Visibility.VISIBLE);        }    }    @Override    public void redo() {        if (receiver != null) {            receiver.setVisibility(Visibility.INVISIBLE);        }    }    @Override    public String toString() {        return "invisible";    }}

缩小命令

public class ShrinkCommand extends Command {    private Size oldSize;    private Receiver receiver;    @Override    public void execute(Receiver receiver) {        oldSize = receiver.getSize();        receiver.getSize(Size.SMALL);        this.receiver = receiver;    }    @Override    public void undo() {        if (oldSize != null && receiver != null) {            Size temp = receiver.getSize();            receiver.setSize(oldSize);        }    }    @Override    public void redo() {        undo();    }    @Override    public String toString() {        return "small";    }}

接收者抽象类

public abstract class Receiver {    private Size size;    private Visibility visibility;    public Size getSize() {        return size;    }    public void setSize(Size size) {        this.size = size;    }    public Visibility getVisibility() {        return visibility;    }    public void setVisibility(Visibility visibility) {        this.visibility = visibility;    }    public abstract String toString();    public void printStatus() {        System.out.println(String.format("[size=%s] [visibility=%s]", getSize(), getVisibility()));    }}

妖怪接收者

public class GoblinReceiver extends Receiver {    public GoblinReceiver() {        setSize(Size.NORMAL);        setVisibility(Visibility.VISIBLE);    }    public String toString() {        return "GoblinReceiver";    }}

命令调用器

public class Invoker {    private Deque undoStack = new LinkedList<>();    private Deque redoStack = new LinkedList<>();    public void call(Command command, Receiver receiver) {        command.execute(receiver);        undoStack.offerLast(command);    }    public void undo() {        Command previousSpell = undoStack.pollLast();        if (previousSpell != null) {            redoStack.offerLast(previousSpell);            System.out.println(" undoes " + previousSpell + " command");            previousSpell.undo();        }    }    public void redo() {        Command previousSpell = redoStack.pollLast();        if (previousSpell != null) {            undoStack.offerLast(previousSpell);            System.out.println(" redoes " + previousSpell + " command");            previousSpell.redo();        }    }    @Override    public String toString() {        return "Invoker";    }}

测试类代码

public class App {    public static void main(String[] args) {        Invoker invoker = new Invoker();        GoblinReceiver goblinReceiver = new GoblinReceiver();        goblinReceiver.printStatus();        invoker.call(new ShrinkCommand(), goblinReceiver);        goblinReceiver.printStatus();        invoker.call(new InvisibilityCommand(), goblinReceiver);        goblinReceiver.printStatus();        invoker.undo();        goblinReceiver.printStatus();        invoker.undo();        goblinReceiver.printStatus();        invoker.redo();        goblinReceiver.printStatus();        invoker.redo();        goblinReceiver.printStatus();    }}

屏幕输出结果

接收者状态：[size=normal] [visibility=visible]Invoker在GoblinReceiver上执行命令 small接收者状态：[size=small] [visibility=visible]Invoker在GoblinReceiver上执行命令 invisible接收者状态：[size=small] [visibility=invisible]Invoker undoes invisible command接收者状态：[size=small] [visibility=visible]Invoker undoes small command接收者状态：[size=normal] [visibility=visible]Invoker redoes small command接收者状态：[size=small] [visibility=visible]Invoker redoes invisible command接收者状态：[size=small] [visibility=invisible]

优缺点分析

优点

缺点

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