GoF Behavioral - Command Pattern

• Intent

to encapsulate a request as an object, thereby letting you parameterize clients with different requests,queue or log requests, and support rollback types of operations.

Command pattern is also known as Action or Transaction

• Benefits

1. 區隔開呼叫程序方法的物件實體以及實際提供物件方法的物件實體，也就是說將動作的要求者以及動作的執行者之間鬆綁


2. 簡化了新增新命令物件的作法，因為已經存在的命令物件不需要被變動到，也就是說 我把每個特定的動作都封裝成物件來處理，每個特殊的動作都有其對應的物件，所以不會對既有的物件產生影響。

• 適用情境

1. 對每一個client動作發生時，都需要將其視為物件作為傳遞訊息使用


2. 傳遞訊息時，從發出訊息到等待柱列、執行是在不同的時間點發生


3. 需要有rollback機制以及logging機制

• 常用API

1. MessageBeans invoke business logic based on content of messages dispatched to them.


2. Servlets/JSPs are invoked corresponding to the type of HTTP request that is recevied by the web container.

• Story

想像一下，假設有個遙控器可以控制電腦開關，所以我們設計了個on off method，但有一天想要做一個萬用遙控器時，希望可以處理10種不同電器，那是不是真的要在遙控器上放10個按鈕?

• Class Diagram

[caption id="attachment_235" align="alignnone" width="448" caption="CommandPattern"][/caption]

• Sample Code

package commandpattern;

interface Command {
	public void execute();
}

class Light {
	public void on() {
		System.out.println("light is on! ");
	}

	public void off() {
		System.out.println("light is off!");
	}
}

class LightOnCommand implements Command {
	Light light;

	@Override
	public void execute() {
		// TODO Auto-generated method stub
		light.on();
	}

	public LightOnCommand(Light light) {
		this.light = light;
	}

}

class SimpleRemoteControl {
	Command slot;

	public SimpleRemoteControl(){}
	public SimpleRemoteControl(Command com) {
		this.slot = com;
	}

	public void setCommand(Command com) {
		this.slot = com;
	}

	public void buttonWasPressed() {
		slot.execute();
	}
}

public class CommandPattern {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		SimpleRemoteControl remote = new SimpleRemoteControl();
		Light light = new Light();
		LightOnCommand lightOn = new LightOnCommand(light);
		remote.setCommand(lightOn);
		remote.buttonWasPressed();
	}

}