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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.
\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.
LoggerAwareInterfacewith a default trait
Contains only protected methods, can not be combined with interface
getTypoScriptFrontendController(), hides this dependency in the consuming class
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)