.. include:: /Includes.rst.txt .. _cgl-traits: ====== Traits ====== Characteristica =============== * A trait MAY access properties or methods of the class it is embedded in. * A trait MUST be combined with an interface. Classes using a trait must implement at least this interface. * A trait interface MUST have a default implementation trait. Rationale ========= There is one specific feature that traits provide other abstraction solutions like services or static extraction do not: A trait is embedded within the class that consumes it and as such can directly access methods and properties of this class. A trait typically holds state in a property of the class. If this feature is not needed, traits should not be used. Thus, the trait itself may even have a dependency to the class it is embedded in, even if this is rather discouraged. A simple way to look at this is to see the interface as the main feature with the trait providing a single or maybe two default implementations of the interface for a specific class. One usage of traits is the removal of boilerplate code. While object creation and dependency injection is still a not resolved issue in the core, this area is probably a good example where a couple of traits would be really useful to autowire default functionality like logging into classes with very little developer effort and in a simple and understandable way. It should however be kept in mind that traits must always be used with care and should stay as a relatively seldom used solution. This is one reason why the current :php:`getLanguageService()` and similar boilerplate methods are kept within classes directly for now and is not extracted to traits: Both container system and global scope objects are currently not finally decided and we don’t want to have relatively hard to deprecate and remove traits at this point. Good Examples ============= * :php:`\Symfony\Component\DependencyInjection\ContainerAwareInterface` with :php:`\Symfony\Component\DependencyInjection\ContainerAwareTrait` as default implementation * The :php:`ContainerAwareInterface` is tested to within the dependency injection system of symfony and the trait is a simple default implementation that easily adds the interface functionality to a given class. * Good naming. * Clear scope. * :php:`LoggerAwareInterface` with a default trait. Bad Examples ============ * Old :php:`\TYPO3\CMS\FluidStyledContent\ViewHelpers\Menu\MenuViewHelperTrait` (available in previous TYPO3 versions) * Contains only protected methods, can not be combined with interface. * Contains :php:`getTypoScriptFrontendController()`, hides this dependency in the consuming class. * No interface. * It would have probably been better to add the trait code to a full class and just use it in the according view helpers (composition) or implement it as abstract. For these reasons the trait has been dissolved into an `AbstractMenuViewHelper`. Further Reading =============== See https://www.rosstuck.com/how-i-use-traits.