Command Pattern

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命令模式

定义命令模式

命令模式将”请求”封装成命令对象，以便使用不同的请求、队列或者日志来参数化其他对象，作为命令执行实际的主体，也就是命令实际的接收者。命令模式还可支持可撤销的操作。

一个命令对象通过在特定接收者上绑定一组动作来封装一个请求。要做到这一点，命令对象将动作和接收者包装进对象。这个对象只暴露一个execute()方法，当此方法被调用时，接收者就会进行这些动作。从外面开来，调用对象并不知道究竟哪个接收者进行了哪些动作，只知道如果调用exectue()方法，请求的目的就能达到。

通过对基本命令模式进行扩展，我们可以轻易实现Meta command pattern。meta command pattern可以创建命令的宏，以便依次执行多个命令。

Design_pattern_command_1

Design_pattern_command_2

Design_pattern_command_3

Design_pattern_command_1

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public interface Command {
	public void execute();
}

public class LightOnCommand implements Command {
	Light light;
  
	public LightOnCommand(Light light) {
		this.light = light;
	}
 
	public void execute() {
		light.on();
	}
}

public class LightOffCommand implements Command {
	Light light;
 
	public LightOffCommand(Light light) {
		this.light = light;
	}
 
	public void execute() {
		light.off();
	}
}
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public class SimpleRemoteControl {
	Command slot;
 
	public SimpleRemoteControl() {}
 
	public void setCommand(Command command) {
		slot = command;
	}
 
	public void buttonWasPressed() {
		slot.execute();
	}
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public class RemoteControlTest {
	public static void main(String[] args) {
		SimpleRemoteControl remote = new SimpleRemoteControl();
		Light light = new Light();
		GarageDoor garageDoor = new GarageDoor();
		LightOnCommand lightOn = new LightOnCommand(light);
		GarageDoorOpenCommand garageOpen = 
		    new GarageDoorOpenCommand(garageDoor);
 
		remote.setCommand(lightOn);
		remote.buttonWasPressed();
		remote.setCommand(garageOpen);
		remote.buttonWasPressed();
    }
}

提供撤销操作

通过提供在Command提供undo方法，我们可以实现撤销操作。如果有多个命令对象，则需要记录前面一个操作的是哪个命令对象。

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public interface Command {
	public void execute();
	public void undo();
}
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public class LightOffCommand implements Command {
	Light light;
 
	public LightOffCommand(Light light) {
		this.light = light;
	}
 
	public void execute() {
		light.off();
	}
 
	public void undo() {
		light.on();
	}
}

public class LightOnCommand implements Command {
	Light light;
 
	public LightOnCommand(Light light) {
		this.light = light;
	}
 
	public void execute() {
		light.on();
	}
 
	public void undo() {
		light.off();
	}
}
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public class RemoteControlWithUndo {
	Command[] onCommands;
	Command[] offCommands;
	Command undoCommand;
 
	public RemoteControlWithUndo() {
		onCommands = new Command[7];
		offCommands = new Command[7];
 
		Command noCommand = new NoCommand();
		for(int i=0;i<7;i++) {
			onCommands[i] = noCommand;
			offCommands[i] = noCommand;
		}
		undoCommand = noCommand;
	}
  
	public void setCommand(int slot, Command onCommand, Command offCommand) {
		onCommands[slot] = onCommand;
		offCommands[slot] = offCommand;
	}
 
	public void onButtonWasPushed(int slot) {
		onCommands[slot].execute();
		undoCommand = onCommands[slot];
	}
 
	public void offButtonWasPushed(int slot) {
		offCommands[slot].execute();
		undoCommand = offCommands[slot];
	}
 
	public void undoButtonWasPushed() {
		undoCommand.undo();
	}
  
	public String toString() {
		StringBuffer stringBuff = new StringBuffer();
		stringBuff.append("\n------ Remote Control -------\n");
		for (int i = 0; i < onCommands.length; i++) {
			stringBuff.append("[slot " + i + "] " + onCommands[i].getClass().getName()
				+ "    " + offCommands[i].getClass().getName() + "\n");
		}
		stringBuff.append("[undo] " + undoCommand.getClass().getName() + "\n");
		return stringBuff.toString();
	}
}

使用状态实现撤销

如果需要实现更复杂的撤销操作，我们需要加入状态，并记录前面的状态。

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public class CeilingFan {
	public static final int HIGH = 3;
	public static final int MEDIUM = 2;
	public static final int LOW = 1;
	public static final int OFF = 0;
	String location;
	int speed;
 
	public CeilingFan(String location) {
		this.location = location;
		speed = OFF;
	}
  
	public void high() {
		speed = HIGH;
		System.out.println(location + " ceiling fan is on high");
	} 
 
	public void medium() {
		speed = MEDIUM;
		System.out.println(location + " ceiling fan is on medium");
	}
 
	public void low() {
		speed = LOW;
		System.out.println(location + " ceiling fan is on low");
	}
  
	public void off() {
		speed = OFF;
		System.out.println(location + " ceiling fan is off");
	}
  
	public int getSpeed() {
		return speed;
	}
}
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public class CeilingFanHighCommand implements Command {
	CeilingFan ceilingFan;
	int prevSpeed;
  
	public CeilingFanHighCommand(CeilingFan ceilingFan) {
		this.ceilingFan = ceilingFan;
	}
 
	public void execute() {
		prevSpeed = ceilingFan.getSpeed();
		ceilingFan.high();
	}
 
	public void undo() {
		if (prevSpeed == CeilingFan.HIGH) {
			ceilingFan.high();
		} else if (prevSpeed == CeilingFan.MEDIUM) {
			ceilingFan.medium();
		} else if (prevSpeed == CeilingFan.LOW) {
			ceilingFan.low();
		} else if (prevSpeed == CeilingFan.OFF) {
			ceilingFan.off();
		}
	}
}

使用宏命令

通过宏命令，我们可以一次执行多个命令。宏命令本身也是封装为一个命令对象。

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public class MacroCommand implements Command {
	Command[] commands;
 
	public MacroCommand(Command[] commands) {
		this.commands = commands;
	}
 
	public void execute() {
		for (int i = 0; i < commands.length; i++) {
			commands[i].execute();
		}
	}
 
	public void undo() {
		for (int i = 0; i < commands.length; i++) {
			commands[i].undo();
		}
	}
}
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public class RemoteControl {
	Command[] onCommands;
	Command[] offCommands;
	Command undoCommand;
 
	public RemoteControl() {
		onCommands = new Command[7];
		offCommands = new Command[7];
 
		Command noCommand = new NoCommand();
		for(int i=0;i<7;i++) {
			onCommands[i] = noCommand;
			offCommands[i] = noCommand;
		}
		undoCommand = noCommand;
	}
  
	public void setCommand(int slot, Command onCommand, Command offCommand) {
		onCommands[slot] = onCommand;
		offCommands[slot] = offCommand;
	}
 
	public void onButtonWasPushed(int slot) {
		onCommands[slot].execute();
		undoCommand = onCommands[slot];
	}
 
	public void offButtonWasPushed(int slot) {
		offCommands[slot].execute();
		undoCommand = offCommands[slot];
	}

	public void undoButtonWasPushed() {
		undoCommand.undo();
	}
 
	public String toString() {
		StringBuffer stringBuff = new StringBuffer();
		stringBuff.append("\n------ Remote Control -------\n");
		for (int i = 0; i < onCommands.length; i++) {
			stringBuff.append("[slot " + i + "] " + onCommands[i].getClass().getName()
				+ "    " + offCommands[i].getClass().getName() + "\n");
		}
		stringBuff.append("[undo] " + undoCommand.getClass().getName() + "\n");
		return stringBuff.toString();
	}
}
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public class RemoteLoader {

	public static void main(String[] args) {

		RemoteControl remoteControl = new RemoteControl();

		Light light = new Light("Living Room");
		TV tv = new TV("Living Room");
		Stereo stereo = new Stereo("Living Room");
		Hottub hottub = new Hottub();
 
		LightOnCommand lightOn = new LightOnCommand(light);
		StereoOnCommand stereoOn = new StereoOnCommand(stereo);
		TVOnCommand tvOn = new TVOnCommand(tv);
		HottubOnCommand hottubOn = new HottubOnCommand(hottub);
		LightOffCommand lightOff = new LightOffCommand(light);
		StereoOffCommand stereoOff = new StereoOffCommand(stereo);
		TVOffCommand tvOff = new TVOffCommand(tv);
		HottubOffCommand hottubOff = new HottubOffCommand(hottub);

		Command[] partyOn = { lightOn, stereoOn, tvOn, hottubOn};
		Command[] partyOff = { lightOff, stereoOff, tvOff, hottubOff};
  
		MacroCommand partyOnMacro = new MacroCommand(partyOn);
		MacroCommand partyOffMacro = new MacroCommand(partyOff);
 
		remoteControl.setCommand(0, partyOnMacro, partyOffMacro);
  
		System.out.println(remoteControl);
		System.out.println("--- Pushing Macro On---");
		remoteControl.onButtonWasPushed(0);
		System.out.println("--- Pushing Macro Off---");
		remoteControl.offButtonWasPushed(0);
	}
}

命令模式还可以用于日志请求。某些应用需要我们将所有的动作都记录在日志中，并能在系统死机之后，重新调用这些动作恢复到之前到状态。

也就是执行命令的时候，将历史记录存储在磁盘中。一旦系统死机，就可以将命令对象重新加载，并依次调用这些对象的execture()方法。