命令模式

UML
Java
Scheme

命令模式把一个请求或者操作封装到一个对象中。命令模式允许系统使用不同的请求把客户端参数化，对请求排队或者记录请求日志，可以提供命令的撤销和恢复功能

UML

Java

定义Command接口：

public interface Command {
    void exec();
}

实现命令执行者Receiver：

public class Receiver {
    public void action() {
        System.out.println("command received!");
    }
}

实现真正的命令，转发给执行者执行

public class MyCommand implements Command {

    private final Receiver receiver;

    public MyCommand(final Receiver receiver) {
        this.receiver = receiver;
    }

    @Override
    public void exec() {
        receiver.action();
    }
}

实现命令发起者Invoker：

public class Invoker {

    private final Command command;

    public Invoker(final Command command) {
        this.command = command;
    }

    public void action() {
        command.exec();
    }
}

测试代码，命令者发起命令，命令转给执行者，执行者执行命令：

public class CommandTest {
    public static void main(String[] args) {
        Receiver receiver = new Receiver();
        Command cmd = new MyCommand(receiver);
        Invoker invoker = new Invoker(cmd);
        invoker.action();
    }
}

测试结果：
```
command received!
```

Scheme

传入command函数作为参数：

(define (invoke command op)
  (command op))

测试：

(invoke
 (lambda (op)
   (begin
     (display "command received")
     (newline)
     "command finished")) '())

;; => command received
;;    command finished

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命令模式

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