Command命令模式

简介：

简单来说，就像我们的复制、删除、插入等等都是命令，我们将命令封装为一个对象，并且支持撤销，将一系列命令串成一条链或者与链表结合使用，可以实现一系列的do和undo

模式类图：

command: 抽象的命令类
X_Command、Y_Command、Z_Command: 具体的命令

使用场景：

我现在能想到的一般是在画图软件，办公软件，开发工具（IDE）当中比较常见的，毕竟作为一个码农，复制粘贴用的多了:)

模式实例：

比如我对一串字符串进行一系列的操作，我这个例子可能不太准确，角色划分不太明显，参考：https://www.cnblogs.com/meet/p/5116430.html
https://blog.csdn.net/qq_22764659/article/details/81335701

1、Content被操作的对象

package com.mashibing.command;
public class Content {
    String msg = "hello everybody ";;
}

2、Command抽象命令

package com.mashibing.command;
public abstract class Command {
    public abstract void doit();
    public abstract void undo();
}

3、InsertCommand插入命令：

package com.mashibing.command;
public class InsertCommand extends Command{
    Content c;
    String strToInsert="http://www.mashibing.com";
    public InsertCommand(Content c) {
        this.c = c;
    }
    @Override
 public void doit() {
        c.msg = c.msg + strToInsert;
    }
    @Override
 public void undo() {
        c.msg = c.msg.substring(0,c.msg.length()-strToInsert.length());
    }
}

4、CopyCommand 复制命令

package com.mashibing.command;
public class CopyCommand extends Command{
    Content c;
    public CopyCommand(Content c) {
        this.c = c;
    }
    @Override
 public void doit() {
        c.msg = c.msg+c.msg;
    }
    @Override
 public void undo() {
        c.msg = c.msg.substring(0,c.msg.length()/2);
    }
}

5、DeleteCommand删除命令

package com.mashibing.command;
public class DeleteCommand extends Command{
    Content c;
    String deleted;
    public DeleteCommand(Content c) {
        this.c = c;
    }
    @Override
 public void doit() {
        deleted = c.msg.substring(0,5);
        c.msg = c.msg.substring(5,c.msg.length());
    }
    @Override
 public void undo() {
        c.msg = deleted + c.msg;
    }
}

6、Main 测试

package com.mashibing.command;
import java.util.ArrayList;
import java.util.List;
/**
 * command模式一般跟责任链模式结合，实现一连串的undo
 * 也可以用双向链表的方式来做
 */
public class Main {
    public static void main(String[] args) {
        Content c = new Content();
        //单个执行命令
 Command insertCommand = new InsertCommand(c);
        insertCommand.doit();
        insertCommand.undo();
        Command copyCommand = new CopyCommand(c);
        copyCommand.doit();
        copyCommand.undo();
        Command deleteCommand = new DeleteCommand(c);
        deleteCommand.doit();
        deleteCommand.undo();
        System.out.println(c.msg);
        //串在一起一连串执行
 List<Command> commands = new ArrayList<>();
        commands.add(new InsertCommand(c));
        commands.add(new CopyCommand(c));
        commands.add(new DeleteCommand(c));
        for(Command command : commands) {
            command.doit();
        }
        System.out.println(c.msg);
        //一连串撤回
 for (int i = commands.size()-1; i >=0; i--){
            commands.get(i).undo();
        }
        System.out.println(c.msg);
    }
}

测试结果：