purpose
The visitor pattern allows you to outsource object operations to other objects.
The main reason for this is the separation of concerns (data structure and data manipulation). But the class being visited must make a contract to accept the visitor. (See the Role::accept method in the example for details)
The contract can be an abstract class or an interface. In this case, each visitor must choose which method of the visitor to call.
UML graphics
Code
- RoleVisitorInterface.php
<?php
namespace DesignPatterns\Behavioral\Visitor;
/**
* 注意：访问者不能自行选择调用哪个方法，
* 它是由 Visitee 决定的。
*/
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);
}
}
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]);
}
}
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