= Mixins = == Introduction == Provide a non inheritance based capability for providing shared/common characteristics and behaviour. Basically, selected traits or characteristics and behaviours are described in a special Type called a 'mixin' (analogous to an interface),[[BR]] these can then be specified on a class and are injected into that class to provide those traits on that class. Mixin behaviours can be specified using object: * vars * properties * methods They do not support constructors (For more see the [http://cobra-language.com/forums/viewtopic.php?f=4&t=306 Forum discussion thread]). == Compared to Extensions == * Extensions enable adding methods to existing classes whereas mixins only augment types that you declare. * Mixins can add state (object variables declared with `var`) whereas extensions cannot. * Mixins can add properties whereas extensions cannot. * Mixin members will be found at run-time via reflection whereas extension members will not. * Mixins create new types. From a VirtualMachine perspective, they are seen as interfaces (since the popular VMs don't support mixins). * Mixins cannot be used from a [wiki:BinaryLibrary binary library]. The source must be included for the compiler. In summary, mixins are more about implementing shared capabilities among the (non common ancestor) types you are declaring. [[BR]] Extensions are about adding convenience methods to existing types (regardless of who declared them). == Restrictions == As we gain more experience with using mixins, and as the implementation of mixins matures, some of the following restrictions are likely to be removed: * Mixins cannot declare initializers. * Mixins cannot inherit from other types. * Mixins cannot implement interfaces. * Mixins cannot add other mixins. * Mixins cannot declare generic parameters (see [http://cobra-language.com/forums/viewtopic.php?f=4&t=532 forum's thread]) nor being generic themselves (see [http://cobra-language.com/forums/viewtopic.php?f=4&t=533 forum's thread]). * Mixins cannot add invariants. * Two or more mixins added to a single type cannot have collision in their member names. * Mixins cannot make use of methods/members already present in the the target type. To support this, a mixin declaration would need to be able to declare the signatures of such members. == Example == {{{ #!cobra # The behaviour desired - a name property mixin Named pro name from var = '' get capName return .name.capitalized # the (otherwise empty) class with mixin added class A adds Named pass class B adds Named pass class P def main is shared a = A() assert a inherits Named assert a.name == '' a.name = 'arosebyanyothername' assert a.name == 'arosebyanyothername' assert a.capName == 'Arosebyanyothername' }}} == See also == * [http://cobra-language.com/trac/cobra/browser/cobra/trunk/Tests/150-mixins/ Test cases] in the Subversion repository * [http://cobra-language.com/forums/viewtopic.php?f=4&t=306 Announcement thread] on discussion forums * [http://cobra-language.com/forums/search.php?keywords=mixin "mixin" search] on discussion forums * [http://bloggingabout.net/blogs/erwyn/archive/2005/10/20/9925.aspx A blog post] by Erwyn Van Der Meer covers limitations experienced without mixins, and the various suboptimal workarounds.