3.12. Visitor

3.12.1. Purpose

The Visitor Pattern lets you outsource operations on objects to otherobjects. The main reason to do this is to keep a separation of concerns.But classes have to define a contract to allow visitors (theRole::accept method in the example).

The contract is an abstract class but you can have also a cleaninterface. In that case, each Visitor has to choose itself which methodto invoke on the visitor.

3.12.2. UML Diagram

3.12.3. Code

You can also find this code on GitHub

RoleVisitorInterface.php

<?php
 
namespace DesignPatterns\Behavioral\Visitor;
 
/**
 * Note: the visitor must not choose itself which method to
 * invoke, it is the Visitee that make this decision
 */
interface RoleVisitorInterface
{
    public function visitUser(User $role);
 
    public function visitGroup(Group $role);
}

RoleVisitor.php

<?php
 
namespace DesignPatterns\Behavioral\Visitor;
 
class RoleVisitor implements RoleVisitorInterface
{
    /**
     * @var Role[]
     */
    private $visited = [];
 
    public function visitGroup(Group $role)
    {
        $this->visited[] = $role;
    }
 
    public function visitUser(User $role)
    {
        $this->visited[] = $role;
    }
 
    /**
     * @return Role[]
     */
    public function getVisited(): array
    {
        return $this->visited;
    }
}

Role.php

<?php
 
namespace DesignPatterns\Behavioral\Visitor;
 
interface Role
{
    public function accept(RoleVisitorInterface $visitor);
}

User.php

<?php
 
namespace DesignPatterns\Behavioral\Visitor;
 
class User implements Role
{
    /**
     * @var string
     */
    private $name;
 
    public function __construct(string $name)
    {
        $this->name = $name;
    }
 
    public function getName(): string
    {
        return sprintf('User %s', $this->name);
    }
 
    public function accept(RoleVisitorInterface $visitor)
    {
        $visitor->visitUser($this);
    }
}

Group.php

<?php
 
namespace DesignPatterns\Behavioral\Visitor;
 
class Group implements Role
{
    /**
     * @var string
     */
    private $name;
 
    public function __construct(string $name)
    {
        $this->name = $name;
    }
 
    public function getName(): string
    {
        return sprintf('Group: %s', $this->name);
    }
 
    public function accept(RoleVisitorInterface $visitor)
    {
        $visitor->visitGroup($this);
    }
}

3.12.4. Test

Tests/VisitorTest.php

<?php
 
namespace DesignPatterns\Tests\Visitor\Tests;
 
use DesignPatterns\Behavioral\Visitor;
use PHPUnit\Framework\TestCase;
 
class VisitorTest extends TestCase
{
    /**
     * @var Visitor\RoleVisitor
     */
    private $visitor;
 
    protected function setUp()
    {
        $this->visitor = new Visitor\RoleVisitor();
    }
 
    public function provideRoles()
    {
        return [
            [new Visitor\User('Dominik')],
            [new Visitor\Group('Administrators')],
        ];
    }
 
    /**
     * @dataProvider provideRoles
     *
     * @param Visitor\Role $role
     */
    public function testVisitSomeRole(Visitor\Role $role)
    {
        $role->accept($this->visitor);
        $this->assertSame($role, $this->visitor->getVisited()[0]);
    }
}