Mixin

title: 实现.
content: In Simula, classes are defined in a block in which attributes, methods and class initialization are all defined together; thus all the methods that can be invoked on a class are defined together, and the definition of the class is complete.  In Flavors, a Mixin is a class from which another class can inherit slot definitions and methods. The Mixin usually does not have direct instances. Since a Flavor can inherit from more than one other Flavor, it can inherit from one or more Mixins. Note that the original Flavors did not use generic functions.  In New Flavors (a successor of Flavors) and CLOS, methods are organized in "generic functions". These generic functions are functions that are defined in multiple cases (methods) by class dispatch and method combinations.  CLOS and Flavors allow mixin methods to add behavior to existing methods: codice_1 and codice_2 daemons, whoppers and wrappers in Flavors. CLOS added codice_3 methods and the ability to call shadowed methods via. So, for example, a stream-lock-mixin can add locking around existing methods of a stream class. In Flavors one would write a wrapper or a whopper and in CLOS one would use an codice_3 method. Both CLOS and Flavors allow the computed reuse via method combinations. codice_1, codice_2 and codice_3 methods are a feature of the standard method combination. Other method combinations are provided.  An example is the codice_8 method combination, where the resulting values of each of the applicable methods of a generic function are arithmetically added to compute the return value. This is used, for example, with the border-mixin for graphical objects. A graphical object may have a generic width function. The border-mixin would add a border around an object and has a method computing its width. A new class codice_9 (that is both a graphical object and uses the codice_10 mixin) would compute its width by calling all applicable width methods—via the codice_8 method combination. All return values are added and create the combined width of the object.
title: 接口与trait.
content: Java 8 introduces a new feature in the form of default methods for interfaces. Basically it allows a method to be defined in an interface with application in the scenario when a new method is to be added to an interface after the interface class programming setup is done. To add a new function to the interface means to implement the method at every class which uses the interface. Default methods help in this case where they can be introduced to an interface any time and have an implemented structure which is then used by the associated classes. Hence default methods adds a possibility of applying the concept in a mixin sort of a way.  Interfaces combined with aspect-oriented programming can also produce full-fledged mixins in languages that support such features, such as C# or Java. Additionally, through the use of the marker interface pattern, generic programming, and extension methods, C# 3.0 has the ability to mimic mixins. With C# 3.0 came the introduction of Extension Methods[2] and they can be applied, not only to classes but, also, to interfaces. Extension Methods provide additional functionality on an existing class without modifying the class. It then becomes possible to create a static helper class for specific functionality that defines the extension methods. Because the classes implement the interface (even if the actual interface doesn’t contain any methods or properties to implement) it will pick up all the extension methods also.  Here, Bird has mixed in all methods of the trait into its own definition as if class Bird had defined method sing() on its own.

Summary: Mixin是面向对象程序设计语言中的类,提供了方法的实现。其他类可以访问mixin类的方法而不必成为其子类。Mixin有时被称作"included"而不是"inherited"。mixin为使用它的class提供额外的功能,但自身却不单独使用(不能单独生成实例对象,属于抽象类)。因为有以上限制,Mixin类通常作为功能模块使用,在需要该功能时“混入”,而且不会使类的关系变得复杂。使用者与Mixin不是“is-a”的关系,而是「-able」关系

Mixin: In object-oriented programming languages, a mixin (or mix-in) is a class that contains methods for use by other classes without having to be the parent class of those other classes. How those other classes gain access to the mixin's methods depends on the language. Mixins are sometimes described as being "included" rather than "inherited".