设计模式之09命令模式（笔记）

1 定义：

1.1 定义：Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.(将一个请求封装成一个对象，从而让你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)。

1.2 通用类图：



个人认为以下类图更利于理解：



类图中包含的Client从来不是用作测试的，这里的client是为了将“请求”与“执行者”对应起来，若使用此模式，则可以依次准备好：Receiver、Command、Invoker，最后直接调用Invoker的方法。所以这里，为便于理解，将其类图作以细化。

1.3 通用代码：

注意上述client的静态main方法为人为添加的测试，非原模式的一部分。

public abstract class Receiver {
		// 抽象接收者，定义每个接收者都必须完成的业务
		public abstract void action();
	}

	public class ConcreteReceiver1 extends Receiver {
		// 每个接受者都必须处理一定的业务逻辑
		public void action() {
		}
	}

	public abstract class Command {
		// 每个命令类都必须有一个执行命令的方法
		public abstract void execute();
	}

	public class ConcreteCommand1 extends Command {
		// 也对那个Receiver类进行命令处理
		private Receiver receiver;

		// 构造函数传递接收者
		public ConcreteCommand1(Receiver _receiver) {
			this.receiver = _receiver;
		}

		// 每个具体的命令都必须实现一个命令
		public void execute() {
			// 业务处理
			this.receiver.action();
		}
	}

	public class Invoker {
		private Command command;

		// 受气包，接受命令
		public void setCommand(Command _command) {
			this.command = _command;
		}

		// 执行命令
		public void doSomething() {
			this.command.execute();
		}
	}

	public class Client {
		public static Command prepareCommand() {
			// 定义接收者
			Receiver receiver = new ConcreteReceiver1();
			// 定义一个发送给接收者的命令
			Command command = new ConcreteCommand1(receiver);
			return command;
		}

		public static void main(String[] args) {
			// 首先声明出调用者Invoker
			Invoker invoker = new Invoker();
			// 把命令交给调用者去执行
			invoker.setCommand(prepareCommand());
			invoker.doSomething();
		}
	}

2 优点
2.1 类间解耦：调用者与接收者角色之间没有任何依赖关系，调用者实现功能时只须调用Command抽象类的execute方法就可以，不需要了解到底是哪个接收者执行。

2.2 可扩展性：Command的子类可以非常容易地扩展，而调用者Invoker和高层次的模块Client不产生严重的代码耦合。

3 缺点

如果命令多，Command的子类就多，这个类会膨胀很大，需慎用。

4 应用场景

触发－反馈机制处理的场所。

5 注意事项

6 扩展

6.1 撤销－重做：这种涉及状态改变的情况，可以有以下两种方法解决。

A：结合备忘录模式还原最后状态，该方法适合接收者为状态的变更情况，而不适合事件处理。

B：通过增加一个新的命令，实现事件的回滚。

6.2 对于接收者不多的情形下，我们可以适当地将某一接收者作为默认参数，实在需要改变接收者时再重新设置。这样可以减少高层模块对低层模块的依赖关系，提高系统的稳定性。

7 范例

7.1撤销－重做（仅支持单步撤销的计算数字的例子）

类图如下：



源代码如下：

package _09_Command.example;
	public abstract class Command {
		Receiver receiver;
		int parm;

		public Command(Receiver rec, int parm) {
			this.receiver = rec;
			this.parm = parm;
		}

		public abstract void execute();
	}

	public class AddCommand extends Command {
		public AddCommand(Receiver rec, int parm) {
			super(rec, parm);
		}

		@Override
		public void execute() {
			receiver.action(CMD.ADD, parm);
		}
	}

	public class SubCommand extends Command {
		public SubCommand(Receiver rec, int parm) {
			super(rec, parm);
		}

		@Override
		public void execute() {
			receiver.action(CMD.SUB, parm);
		}
	}

	public class MulCommand extends Command {
		public MulCommand(Receiver rec, int parm) {
			super(rec, parm);
		}

		@Override
		public void execute() {
			receiver.action(CMD.MUL, parm);
		}
	}

	public class DivCommand extends Command {
		public DivCommand(Receiver rec, int parm) {
			super(rec, parm);
		}

		@Override
		public void execute() {
			receiver.action(CMD.DIV, parm);
		}
	}

	public class SetCommand extends Command {
		public SetCommand(Receiver rec, int parm) {
			super(rec, parm);
		}

		@Override
		public void execute() {
			receiver.action(CMD.SET, parm);
		}
	}

	public class UndoCommand extends Command {
		public UndoCommand(Receiver rec, int parm) {
			super(rec, 0);
		}

		@Override
		public void execute() {
			receiver.action(CMD.UNDO, 0);
		}
	}

	enum CMD {
		ADD, SUB, MUL, DIV, SET, UNDO
	};

	public abstract class Receiver {
		protected int result = 0;
		protected int lastParm;
		protected CMD lastAction;

		public abstract void action(CMD cmd, int parm);
	}

	public class ConcreteReceiver extends Receiver {
		@Override
		public void action(CMD cmd, int parm) {
			switch (cmd) {
			case ADD:
				System.out.print(result + " + " + parm + " = ");
				result += parm;
				lastParm = parm;
				lastAction = CMD.ADD;
				System.out.println(result);
				System.out
						.println("----------------------------------------------");
				break;
			case SUB:
				System.out.print(result + " - " + parm + " = ");
				result -= parm;
				lastParm = parm;
				lastAction = CMD.SUB;
				System.out.println(result);
				System.out
						.println("----------------------------------------------");
				break;
			case MUL:
				System.out.print(result + " x " + parm + " = ");
				result *= parm;
				lastParm = parm;
				lastAction = CMD.MUL;
				System.out.println(result);
				System.out
						.println("----------------------------------------------");
				break;
			case DIV:
				System.out.print(result + " / " + parm + " = ");
				result /= parm;
				lastParm = parm;
				lastAction = CMD.DIV;
				System.out.println(result);
				System.out
						.println("----------------------------------------------");
				break;
			case SET:
				result = parm;
				lastParm = 0;
				lastAction = CMD.SET;
				System.out.println("清除并设置值： " + result);
				System.out
						.println("----------------------------------------------");
				break;
			case UNDO:
				if (lastAction != CMD.UNDO && lastAction != CMD.SET) {
					switch (lastAction) {
					case ADD:
						result -= lastParm;
						break;
					case SUB:
						result += lastParm;
						break;
					case MUL:
						result /= lastParm;
						break;
					case DIV:
						result *= lastParm;
						break;
					}
					lastAction = CMD.UNDO;
					System.out.println("撤销上步操作后值： " + result);
					System.out
							.println("----------------------------------------------");
				}
				break;
			}
		}
	}

	public class Invoker {
		Command command;

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

		public void setCommand(Command cmd) {
			command = cmd;
		}
	}

	public class Client {
		public static void main(String[] args) {// // just for test
			Receiver rec = new ConcreteReceiver();
			Command cmds[] = { new SetCommand(rec, 5), new AddCommand(rec, 5),
					new SubCommand(rec, 5), new UndoCommand(rec, 5),
					new MulCommand(rec, 5), new DivCommand(rec, 5) };
			Invoker inv = new Invoker();
			for (int i = 0; i < 6; i++) {
				inv.setCommand(cmds[i]);
				inv.action();
			}
		}
	}

结果如下：

清除并设置值： 5
----------------------------------------------
5 + 5 = 10
----------------------------------------------
10 - 5 = 5
----------------------------------------------
撤销上步操作后值： 10
----------------------------------------------
10 x 5 = 50
----------------------------------------------
50 / 5 = 10
----------------------------------------------