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有利于代码复用又避免了多继承的复杂。使用Mixin享有单一继承的单纯性和多重继承的共有性。接口与mixin相同的地方是都可以多继承，不同的地方在于mixin是带实现的。Mixin也可以看作是带实现的。这种设计模式实现了依赖反转原则。

历史 	
Mixin最初出现在Symbolics.com的面向对象系统（由Howard Cannon开发），使用了的面向对象方法。名称起源于马萨诸塞州萨默维尔的： 这家冰淇淋店提供基本口味的冰淇淋（香草、巧克力等），混合入其他额外成分（坚果、曲奇、乳脂軟糖等）并称这些为"mix-in",还注册了商标。

实现

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: :before and :after daemons, whoppers and wrappers in Flavors. CLOS added :around 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 :around method. Both CLOS and Flavors allow the computed reuse via method combinations. :before, :after and :around methods are a feature of the standard method combination. Other method combinations are provided.

An example is the + 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 bordered-button (that is both a graphical object and uses the border mixin) would compute its width by calling all applicable width methods—via the + method combination. All return values are added and create the combined width of the object.

In an OOPSLA 90 paper, Gilad Bracha and William Cook reinterpret different inheritance mechanisms found in Smalltalk, Beta and CLOS as special forms of a mixin inheritance.

编程语言支持
除了Flavors与CLOS (作为Common Lisp的部分)，其他语言的支持:

Ada语言 (扩展已存在的tagged record)
Cobra
ColdFusion (基于类的includes，基于对象的赋值方法)
Curl (Curl RTE)
D语言 (称作"template mixins" ; D语言包含"mixin" 语句.)
Dart
Factor语言
Groovy
JavaScript  Delegation - Functions 作为 Roles (Traits 与 Mixins)
OCaml
Perl (通过 的roles)
Perl 6
PHP的"Trait"
Python
Racket (mixins documentation )
Ruby
Scala
/TclOO  
Sass
Vala
Swift
SystemVerilog

一些语言允许运行时从一个对象拷贝方法到另一个对象。这可以“借”mixin的方法。

C#与Visual Basic.NET支持接口的扩展方法（extension method）。

例子

Common Lisp

Common Lisp 在CLOS (Common Lisp Object System)也提供Minin設計方法，其相似於Flavors。

object-width 是一個接受一個參數的通用函式，並使用+方式結合。 這種結合方式，會讓所有應用這個方法的互叫，自動以加法結合其結果。

看個例子：

(defgeneric object-width (object)
  (:method-combination +))

button 類別擁有一段文字。

(defclass button ()
  ((text :initform "click me")))

接著實現button物件的方法object-with，會計算文字長度後乘10作為回傳值。結合+方法，會使此方法每次都被呼叫，並以加法結合(繼續看下面例子)。

另外，使用:method-combination定義個通用函式，在之後定義的方法，必須都有相同簽名。意思是對於同樣名稱方法object-width，其+並不可省(將會報錯)。

(defmethod object-width + ((object button))
   (* 10 (length (slot-value object 'text))))

定義另一個類別border-mixin。 做為示例，只有名字沒有特別成員、沒有父類別。

(defclass border-mixin () ())

定義其object-width +方法。單純的回傳4。

(defmethod object-width + ((object border-mixin))
  4)

最後，bordered-button類別同時繼承了border-mixin和button.

(defclass bordered-button (border-mixin button) ())

我們現在可以呼叫button的object-width，會得到 80。這個結果只呼叫了button這意類別的object-width方法。
(預設字串"click me"長度為8，8*10=80)

? (object-width (make-instance 'button))
80

嘗試呼叫bordered-button的object-width，會得到84。這結果是呼叫button的object-width和border-mixin的object-width結果總和。 

? (object-width (make-instance 'bordered-button))
84

我們還可以繼續在增加bordered-button的object-width方法。很簡單的想讓結果在加2。

(defmethod object-width + ((object bordered-button))
  2)

? (object-width (make-instance 'bordered-button))
86

Python
Python中，除了使用protocol以外，也可以用多继承的形式来实现Mixin。
 首先它必须表示某一种功能，而不是某个物品。Mixin必须责任单一，如果有多个功能，那就写多个Mixin类。
 Mixin不依赖于子类的实现
 子类即便没有继承这个Mixin类，也照样可以工作，只是缺少了某个功能
 多重继承时候，Mixin类应该在基本的父类之前（即左侧）
 为了区分普通的多继承，mixin类的类名一般都会带上后缀：Mixin、able、ible等。比如Python 2中的类UserDict.DictMixin。DictMixin类包括部分实现，使用者的类只要实现几个必须的函数接口，如：__getitem__(), __setitem__(), __delitem__(), keys()。
	
Python的SocketServer模块提供了UDPServer类与TCPServer类，作为UDP与TCP的socket服务器。有两个mixin类：ForkingMixIn与 ThreadingMixIn。通过如以下代码的方式使用ThreadingMixIn扩展TCPServer：

class ThreadingTCPServer(ThreadingMixIn, TCPServer):
  pass

ThreadingMixIn类为TCP服务器添加了新功能，使每个新连接都会创建出新线程。而如果是ForkingMixIn，则会使每个新连接fork出新的进程。

Ruby
在ruby中，并不直接使用Mixin这个单词，而是使用在类的声明中include一个module的办法。

Most of the Ruby world is based around mixins via Modules. The concept of mixins is implemented in Ruby by the keyword include to which we pass the name of the module as parameter.

Example:

class Student
  include Comparable # The class Student inherits the Comparable module using the 'include' keyword
  attr_accessor :name, :score

  def initialize(name, score)
    @name = name
    @score = score
  end

  # Including the Comparable module requires the implementing class to define the <=> comparison operator
  # Here's the comparison operator. We compare 2 student instances based on their scores.

  def <=>(other)
    @score <=> other.score
  end

  # Here's the good bit - I get access to <, <=, >,>= and other methods of the Comparable Interface for free.
end

s1 = Student.new("Peter", 100)
s2 = Student.new("Jason", 90)

s1 > s2 #true
s1 <= s2 #false

JavaScript
在JavaScript中，Mixin可以用Object.assign(MyClass.prototype, MixinClass);实现。Mixin可以有自己的父类。

对象-文字与extend方法
技术上可以通过绑定函数到对象的键，来给对象增加行为。但这导致在状态与行为之间缺少分离等缺点：
 它扰乱了模型域属性与实现域属性；
 共同的行为没有共享。元对象解决此问题通过分离对象的域相关属性与行为相关的属性。

一个扩展函数(来自Underscore.js库，把源对象的所有功能复制到目标对象, 特性, 函数等)用于混合行为：

// This example may be contrived.
// It's an attempt to clean up the previous, broken example.
var Halfling = function (fName, lName) {
    this.firstName = fName;
    this.lastName = lName;
}

var NameMixin = {
    fullName: function () {
        return this.firstName + ' ' + this.lastName;
    },
    rename: function(first, last) {
        this.firstName = first;
        this.lastName = last;
        return this;
    }
};

var sam = new Halfling('Sam', 'Lowry');
var frodo = new Halfling('Freeda', 'Baggs');

// Mixin the other methods
_.extend(Halfling.prototype, NameMixin);

// Now the Halfling objects have access to the NameMixin methods
sam.rename('Samwise', 'Gamgee');
frodo.rename('Frodo', 'Baggins');

基于飞行Mixin方法的纯函数与委托
上述描述方法得到广泛使用。但下述方法更接近于JavaScript语言的基础核心 - Delegation.

两个基于模式的函数对象不需要extend的第三方实现就可完成这种技巧。 

// Implementation
var EnumerableFirstLast = (function () { // function based module pattern.
    var first = function () {
        return this[0];
    },
    last = function () {
        return this[this.length - 1];
    };
    return function () {      // function based Flight-Mixin mechanics ...
        this.first  = first;  // ... referring to ...
        this.last   = last;   // ... shared code.
    };
}());

// Application - explicit delegation:
// applying [first] and [last] enumerable behavior onto [Array]'s [prototype].
EnumerableFirstLast.call(Array.prototype);

// Now you can do:
a = [1, 2, 3];
a.first(); // 1
a.last();  // 3

其他语言

In the Curl web-content language, multiple inheritance is used as classes with no instances may implement methods. Common mixins include all skinnable ControlUIs inheriting from SkinnableControlUI, user interface delegate objects that require dropdown menus inheriting from StandardBaseDropdownUI and such explicitly named mixin classes as FontGraphicMixin, FontVisualMixin and NumericAxisMixin-of class. Version 7.0 added library access so that mixins do not need to be in the same package or be public abstract. Curl constructors are factories that facilitates using multiple-inheritance without explicit declaration of either interfaces or mixins.

接口与trait

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.

ECMAScript (in most cases implemented as JavaScript) does not need to mimic object composition by stepwise copying fields from one object to another. It natively supports Trait and Mixin based object composition via function objects that implement additional behavior and then are delegated via call or apply to objects that are in need of such new functionality.

Scala
Scala has a rich type system and Traits are a part of Scala’s type system which help implement mixin behaviour. As their name reveals, Traits are usually used to represent a distinct feature or aspect that is normally orthogonal to the responsibility of a concrete type or at least of a certain instance.
For example, the ability to sing is modeled as such an orthogonal feature: it could be applied to Birds, Persons, etc.

trait Singer{
  def sing { println(" singing … ") }
  //more methods
}

class Birds extends Singer

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.

As extends is also used to inherit from a super class, in case of a trait extends is used if no super class is inherited and only for mixin in the first trait. All following traits are mixed in using keyword with.

class Person
class Actor extends Person with Singer
class Actor extends Singer with Performer

Scala allows mixing in a trait (creating an anonymous type) when creating a new instance of a class. In the case of a Person class instance, not all instances can sing. This feature comes use then:

class Person{
  def tell {  println (" Human ") }
  //more methods
}

val singingPerson = new Person with Singer
singingPerson.sing

Swift
Mixin can be achieved in Swift by using a language feature called Default implementation in Protocol Extension. 

protocol ErrorDisplayable {
    func error(message:String)
}

extension ErrorDisplayable {
    func error(message:String) {
        // Do what it needs to show an error
        //...
        print(message)
    }
}

struct NetworkManager : ErrorDisplayable{
    func onError() {
        error("Please check your internet Connection.")
    }
}

参见
 抽象类型
 修饰模式
 基于原则设计
 Trait，一种类似结构但不要求线型复合

参考文献

外部链接
 MixIn  at Portland Pattern Repository
 Mixins in ActionScript
 The Common Lisp Object System: An Overview  by Richard P. Gabriel and Linda DeMichiel provides a good introduction to the motivation for defining classes by means of generic functions.

 
面向对象的程序设计