Mixin

Mixin是面向對象程序設計語言中的類，提供了方法的實現。其他類可以訪問mixin類的方法而不必成為其子類。Mixin有時被稱作"included"而不是"inherited"。mixin為使用它的class提供額外的功能，但自身卻不單獨使用（不能單獨生成實例對象，屬於抽象類）。因為有以上限制，Mixin類通常作為功能模塊使用，在需要該功能時「混入」，而且不會使類的關係變得複雜。使用者與Mixin不是「is-a」的關係，而是「-able」關係。

Mixin有利於代碼復用又避免了多繼承的複雜。使用Mixin享有單一繼承的單純性和多重繼承的共有性。接口與mixin相同的地方是都可以多繼承，不同的地方在於mixin是帶實現的。Mixin也可以看作是帶實現的interface。這種設計模式實現了依賴反轉原則。

## 歷史

Mixin最初出現在Symbolics.com的面向對象Flavors系統（由Howard Cannon開發），使用了Lisp Machine Lisp的面向對象方法。名稱起源於麻薩諸塞州薩默維爾的Steve's Ice Cream：這家冰淇淋店提供基本口味的冰淇淋（香草、巧克力等），混合入其他額外成分（堅果、曲奇、乳脂軟糖等）並稱這些為"mix-in",還註冊了商標。

## 實現

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 CALL-NEXT-METHOD. 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.

## 程式語言支持

除了Flavors與CLOS (作為Common Lisp的部分)，其他語言的支持:

- Ada語言 (擴展已存在的tagged record)
- Cobra
- ColdFusion (基於類的includes，基於對象的賦值方法)
- Curl (Curl RTE)
- D語言 (稱作 "template mixins" ; D語言包含 "mixin" 語句.)
- Dart
- Factor語言
- Groovy
- JavaScript Delegation - Functions 作為 Roles (Traits 與 Mixins)
- OCaml
- Perl (通過 Moose_(Perl)的roles)
- Perl 6
- PHP的"Trait"
- Python
- Racket ( mixins documentation )
- Ruby
- Scala
- XOTcl/ TclOO
- Sass
- Vala
- Swift語言
- SystemVerilog

一些語言允許運行時從一個對象拷貝方法到另一個對象。這可以「借」mixin的方法。

C♯與Visual Basic.NET支持接口的擴展方法（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：

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

`button` 類別擁有一段文字。

```lisp
(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)。

```lisp
(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。

```lisp
(defclass border-mixin () ())
```

定義其`object-width +`方法。單純的回傳4。

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

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

```lisp
(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)

```lisp
? (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。

```lisp
? (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。

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

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

### Python

Python中，除了使用protocol以外，也可以用多繼承的形式來實現Mixin。

- 首先它必須表示某一種功能，而不是某個物品。Mixin必須責任單一，如果有多個功能，那就寫多個Mixin類。
- Mixin不依賴於子類的實現
- 子類即便沒有繼承這個Mixin類，也照樣可以工作，只是缺少了某個功能
- 多重繼承時候，Mixin類應該在基本的父類之前（即左側）
- 為了區分普通的多繼承，mixin類的類名一般都會帶上後綴：Mixin、able、ible等。比如Python 2中的類`UserDict.DictMixin`。`DictMixin`類包括部分實現，使用者的類只要實現幾個必須的函數接口，如：`__getitem__()`, `__setitem__()`, `__delitem__()`, `keys()`。

Python的`SocketServer`模塊提供了`UDPServer`類與`TCPServer`類，作為UDP與TCP的socket伺服器。有兩個mixin類：`ForkingMixIn`與 `ThreadingMixIn`。通過如以下代碼的方式使用`ThreadingMixIn`擴展`TCPServer`：

```python
class ThreadingTCPServer(ThreadingMixIn, TCPServer):
  pass
```

`ThreadingMixIn`類為TCP伺服器添加了新功能，使每個新連接都會創建出新線程。而如果是`ForkingMixIn`，則會使每個新連接fork出新的進程。

### Ruby

在ruby中，並不直接使用Mixin這個單詞，而是使用在類的聲明中include一個module的辦法。

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:

```ruby
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);`實現。Mixin可以有自己的父類。

#### 對象-文字與`extend`方法

技術上可以通過綁定函數到對象的鍵，來給對象增加行為。但這導致在狀態與行為之間缺少分離等缺點：

- 它擾亂了模型域屬性與實現域屬性。
- 共同的行為沒有共享。元對象解決此問題通過分離對象的域相關屬性與行為相關的屬性。

一個擴展函數(來自Underscore.js庫，把源對象的所有功能複製到目標對象, 特性, 函數等)用於混合行為：

```javascript
// 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');
```

#### 基於飛行Mixin方法的純函數與委託

上述描述方法得到廣泛使用。但下述方法更接近於JavaScript語言的基礎核心 - Delegation.

兩個基於模式的函數對象不需要`extend`的第三方實現就可完成這種技巧。

```javascript
// 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
```

### 其他語言

In the Curl web-content language, multiple inheritance is used as classes with no instances may implement methods. Common mixins include all skinnable `ControlUI`s 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.

## 接口與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 Methods2 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.

```scala
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`.

```scala
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:

```scala
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.

```swift
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.")
    }
}
```

## 外部連結

- 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.