GoF Behavior - Visitor Pattern

• Intent

To present an operation to be performed on elements of an object structure. The visitor pattern allows for the addition of a new operation without changing the classes of the elements on which it is to operate.

Visitor 模式是要講述關於在類別方法中的設計觀念，他允許了新增一個新的method實作內容，但卻不用變更目前實際在使用中的物件類別。

可以舉個例子來看，假設我們有個組裝車，裏頭放置了有各式各項的組成聚合物(DVD player , GPRS, air condition, ...)，有一天我需要知道車內各項聚合物的耗電量時，就得一個一個聚合的元素慢慢找( ex: getEnergyUsage() , 當然前提是這些聚合物都時做同樣的介面)

• Benefits

1. 允許對合成結構加入新的操作，而無須改變結構本身。


2. 想要加入新的操作是輕而易舉的事情。


3. 參觀者所進行的操作，程式碼是被集中在一起的。

• 適用情境

1. Head First Design Pattern說到，當想要為一個 Aggregation class 增加新的功能，且封裝並不是那麼重要時，可以直接這麼做。

• Class Diagram

[caption id="attachment_282" align="alignnone" width="600" caption="Visitor Pattern 2"][/caption]

• Sample Code

package visitorpattern;

interface CarElementVisitor {
	void visit(Wheel wheel);

	void visit(Engine engine);

	void visit(Body body);

	void visit(Car car);
}

interface CarElement {
	void accept(CarElementVisitor visitor); // CarElements have to provide
											// accept().
}

class Wheel implements CarElement {
	private String name;

	public Wheel(String name) {
		this.name = name;
	}

	public String getName() {
		return this.name;
	}

	public void accept(CarElementVisitor visitor) {
		visitor.visit(this);
	}
}

class Engine implements CarElement {
	public void accept(CarElementVisitor visitor) {
		visitor.visit(this);
	}
}

class Body implements CarElement {
	public void accept(CarElementVisitor visitor) {
		visitor.visit(this);
	}
}

class Car implements CarElement {
	CarElement[] elements;

	public CarElement[] getElements() {
		return elements.clone(); // Return a copy of the array of references.
	}

	public Car() {
		this.elements = new CarElement[] { new Wheel("front left"), new Wheel("front right"), new Wheel("back left"),
				new Wheel("back right"), new Body(), new Engine() };
	}

	public void accept(CarElementVisitor visitor) {
		for (CarElement element : this.getElements()) {
			element.accept(visitor);
		}
		visitor.visit(this);
	}
}

class CarElementPrintVisitor implements CarElementVisitor {
	public void visit(Wheel wheel) {
		System.out.println("Visiting " + wheel.getName() + " wheel");
	}

	public void visit(Engine engine) {
		System.out.println("Visiting engine");
	}

	public void visit(Body body) {
		System.out.println("Visiting body");
	}

	public void visit(Car car) {
		System.out.println("Visiting car");
	}
}

class VisitorDemo implements CarElementVisitor {
	public void visit(Wheel wheel) {
		System.out.println("Kicking my " + wheel.getName() + " wheel");
	}

	public void visit(Engine engine) {
		System.out.println("Starting my engine");
	}

	public void visit(Body body) {
		System.out.println("Moving my body");
	}

	public void visit(Car car) {
		System.out.println("Starting my car");
	}
}

public class TestVisitorDemo {
	public static void main(String[] args) {
		Car car = new Car();
		car.accept(new CarElementPrintVisitor());
		car.accept(new VisitorDemo());
	}
}