README.md

Visitor

Elements

• Visitor: interface used to declare the visit operations;
• ConcreteVisitor: the one that implement visitor interface;
• Visitable: declares the accept operation, it enables an object to be "visited";
• ConcreteVisitable: each class that implements the Visitable interface and receive viditor;
• ObjectStructure: containing all the objects that can be visited;

Description

The visitor pattern is a way to separate algorithm from an object structure. It is one way to implement the open/closed principle of SOLID. It allows to add functionalities without object modification. A visitor take the object instance as input and implements the algorithm.

Implementation

In the following implementation we will create a visitor that will visit all cards inside a project. Each card represents a Task or a Bug. Each card contains a title and some points. Here, card is an interface that for simplicity implement just a part of a complete card.

Let's start from the card interface. Each card will contains, at least, GetTitle() and GetPoints() method. In a real world example each card could contains more and more specific methods. This is just a trivial example.

type Card interface {
	GetTitle() string
	GetPoints() int
}

Now let's implement both task and bug object within interface Card.

type Task struct {
	Title string
	Time  int
}

func (b *Task) GetTitle() string {
	return b.Title
}

func (b *Task) GetPoints() int {
	return b.Time
}

type Bug struct {
	Title string
	Time  int
}

func (b *Bug) GetTitle() string {
	return b.Title
}

func (b *Bug) GetPoints() int {
	return b.Time
}

Until now we have not yet visitor pattern elements. We just have bugs and tasks. In the visitor pattern there always be Visitable and Visitor items. Our visitor will visit a card. All visitable item must accept a visitor.

type Visitable interface {
	Accept(v Visitor)
}

type Visitor interface {
	Visit(t Card)
}

Now we want that all cards are visitable.

func (b *Task) Accept(v Visitor) {
	v.Visit(b)
}

func (b *Bug) Accept(v Visitor) {
	v.Visit(b)
}

And here we have the visitor: a service that sum each cards points. As we can see the logic is not in Bug object nor in Task.

type EstimationVisitor struct {
	Sum int
}

func (e *EstimationVisitor) Visit(t Card) {
	e.Sum += t.GetPoints()
}

Finally, the main function, where it is visible the Visitor in action.

func main() {
	nextRelease := []Visitable{
		&Task{"Do stuff", 1},
		&Task{"Implement Foo Bar", 5},
		&Bug{"Error 500 on resource /foo/bar", 3},
	}

	storyPoint := new(EstimationVisitor)

	for _, i := range nextRelease {
		i.Accept(storyPoint)
	}

	// "Next release is calulated in 9 story points"
	fmt.Println(
		"Next release is calulated in",
		storyPoint.Sum,
		"story points",
	)
}