# Tutorial 8 - Generic mixins

In this tutorial we will look at how to use generic Fragments. So far all Fragments, i.e. the Concerns, SideEffects, and Mixins, have directly implemented the domain interface. But sometimes it is useful to be able to provide a generic implementation of an interface. An example of this is the HelloWorldState interface. Since it only handles properties, and the old version used the JavaBean rules for naming getters and setters we could create a mixin that handles invocations of such methods automatically for us by storing the properties in a map and use the methods to look them up.

Implementing a generic Fragment is done by creating a class that implements the interface java.lang.proxy.InvocationHandler. This has a single "invoke" method which is passed the object that was invoked (the TransientComposite in this case), the method, and the arguments. The Fragment is then allowed to implement the method any way it wants.

Since interfaces with only Properties is such a common case Zest™ already has a generic Mixin that implements the Properties management described above, but for the builtin Property type instead of the getter/setter variant. The class is aptly named PropertyMixin.

While we could use it, for now we will implement it ourselves to get a feel for how generic Mixins work.

Steps for this tutorial:

1. Remove the HelloWorldStateMixin
2. Add a GenericPropertyMixin, and have it implement InvocationHandler
3. Inject "@State StateHolder state" in the mixin. The StateHolder interface will give you access to the Properties for the TransientComposite which Zest™ manages for you
4. On call to invoke(), delegate to the StateHolder interface to get the Property for the invoked method
5. Add an @AppliesTo annotation to the Mixin and implement the AppliesToFilter with a rule that matches only methods that return Property values.
6. Add the mixin to the TransientComposite.