The Visitor Pattern lets you outsource operations on objects to other
objects. The main reason to do this is to keep a separation of concerns.
But classes have to define a contract to allow visitors (the
Role::accept method in the example).
The contract is an abstract class but you can have also a clean interface. In that case, each Visitor has to choose itself which method to invoke on the visitor.
You can also find this code on GitHub
.. literalinclude:: RoleVisitorInterface.php :language: php :linenos:
.. literalinclude:: RoleVisitor.php :language: php :linenos:
.. literalinclude:: Role.php :language: php :linenos:
.. literalinclude:: User.php :language: php :linenos:
.. literalinclude:: Group.php :language: php :linenos:
.. literalinclude:: Tests/VisitorTest.php :language: php :linenos: