- 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.
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
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 ¶
\Symfony\Component\DependencyInjection\ContainerAwareTraitas default implementation
ContainerAwareInterfaceis 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.
LoggerAwareInterfacewith a default trait.
Bad Examples ¶
- Contains only protected methods, can not be combined with interface.
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).