visitor.md

Visitor Design Pattern

The Visitor Pattern allows you to separate an algorithm from the object structure on which it operates. It allows adding further operations to objects without modifying them.

How to apply

// Visitor

interface GenreVisitor
{
    public function visitFiction(Fiction $fiction): void;
    public function visitFantasy(Fantasy $fantasy): void;
}

class BookGenreVisitor implements GenreVisitor
{
    public function visitFiction(Fiction $fiction): void
    {
        $fiction->printGenre();
    }

    public function visitFantasy(Fantasy $fantasy): void
    {
        $fantasy->renderGenre();
    }
}

// Component

interface Genre
{
    public function accept(GenreVisitor $visitor): void;
}

class Fiction implements Genre
{
    public function printGenre(): void
    {
        echo "Visiting Fiction genre";
    }

    public function accept(GenreVisitor $visitor): void
    {
        $visitor->visitFiction($this);
    }
}

class Fantasy implements Genre
{
    public function renderGenre(): void
    {
        echo "Visiting Fantasy genre";
    }

    public function accept(GenreVisitor $visitor): void
    {
        $visitor->visitFantasy($this);
    }
}

Usage:

$fiction = new Fiction();
$fantasy = new Fantasy();

$visitor = new BookGenreVisitor();

$fiction->accept($visitor);
$fantasy->accept($visitor);

Output:

Visiting Fiction genre
Visiting Fantasy genre