The Visitor Pattern(访问模式)

1.什么是访问模式

以我的理解就是不同模型提供不同的行为,而重点就在于判决不同的模型,这里我们就是通过访问的模式

2.普通风格

---------继承实现抽象的技术来达到"访问模式"而产生不同行为

package shapes {
case class Point(x: Double, y: Double)
abstract class Shape() {
	def draw(): Unit
}
case class Circle(center: Point, radius: Double) extends Shape() {
	def draw() = println("Circle.draw: " + this)
}
case class Rectangle(lowerLeft: Point, height: Double, width: Double) extends Shape() {
	def draw() = println("Rectangle.draw: " + this)
}
case class Triangle(point1: Point, point2: Point, point3: Point)extends Shape() {
	def draw() = println("Triangle.draw: " + this)
	}
}

每个类都具体化了其中的行为,这种是耦合度太低了,不利于源码二次维护

3.另一种提供原本类供外界本类当作"辨别的对象",目的是最大耦合

package shapes {
trait ShapeVisitor {
	def visit(circle: Circle): Unit
	def visit(rect: Rectangle): Unit
	def visit(tri: Triangle): Unit
}
case class Point(x: Double, y: Double)
sealed abstract class Shape() {
	def accept(visitor: ShapeVisitor): Unit
}
case class Circle(center: Point, radius: Double) extends Shape() {
	def accept(visitor: ShapeVisitor) = visitor.visit(this)
}
case class Rectangle(lowerLeft: Point, height: Double, width: Double)
extends Shape() {
	def accept(visitor: ShapeVisitor) = visitor.visit(this)
}
case class Triangle(point1: Point, point2: Point, point3: Point)
extends Shape() {
	def accept(visitor: ShapeVisitor) = visitor.visit(this)
	}
}

 

行为:

package shapes {
class ShapeDrawingVisitor extends ShapeVisitor {
	def visit(circle: Circle): Unit =
	println("Circle.draw: " + circle)
	def visit(rect: Rectangle): Unit =println("Rectangle.draw: " + rect)

	def visit(tri: Triangle): Unit =println("Triangle.draw: " + tri)

	}

}

这种风格就跟上面的很不一样,不同的类产生不同的行为,而该行为分离了本类的主题,实现了耦合,在trait ShapeVisitor中处理三个行为,这就很容易去维护code了,主要对分离的行为进行维护即可,这样不会修改主体

缺点:违背了

OpenClosed Principle

我们都把对象提供给人家(即行为)了-----!不安全

4.比较安全的做法

package shapes2 {
	case class Point(x: Double, y: Double)
	sealed abstract class Shape()
	case class Circle(center: Point, radius: Double) extends Shape()
	case class Rectangle(lowerLeft: Point, height: Double, width: Double) extends Shape()
	case class Triangle(point1: Point, point2: Point, point3: Point) extends Shape()
}

package shapes2 {
class ShapeDrawer(val shape: Shape) {
	def draw = shape match {
		case c: Circle => println("Circle.draw: " + c)
		case r: Rectangle => println("Rectangle.draw: " + r)
		case t: Triangle => println("Triangle.draw: " + t)
	}
}
object ShapeDrawer {
	implicit def shape2ShapeDrawer(shape: Shape) = new ShapeDrawer(shape)
	}
}

我们通过巧妙的方法实现了耦合度

不像上个方法那样把this直接送给行为,这样是主动暴露的行为,而这里是根据不同对象做出不同的行为---是属于被动暴露的的行为