Mixin

Mixin是面向物件程式設計語言中的類，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。因為有以上限制，Mixin類通常作為功能模組使用，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有利於程式碼複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。介面與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。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的面向物件系統並稱這些為"mix-in",還註冊了商標。實現.已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。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.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.程式語言支援.除了Flavors與CLOS (作為Common Lisp的部分)，其他語言的支援:一些語言允許執行時從一個物件複製方法到另一個物件。這可以“借”mixin的方法。C#與Visual Basic.NET支援介面的擴充套件方法。例子.Common Lisp.已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。Common Lisp 在CLOS (Common Lisp Object System)也提供Minin設計方法，其相似於Flavors。codice_12 是一個接受一個引數的通用函式，並使用codice_8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：(:method-combination +))codice_14 類別擁有一段文字。((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codice_8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codice_8並不可省(將會報錯)。(* 10 (length (slot-value object 'text))))定義另一個類別codice_17。 做為示例，只有名字沒有特別成員、沒有父類別。定義其codice_18方法。單純的回傳4。4)最後，codice_9類別同時繼承了codice_17和codice_14.我們現在可以呼叫button的object-width，會得到 80。這個結果只呼叫了codice_14這意類別的codice_12方法。? (object-width (make-instance 'button))80嘗試呼叫codice_9的codice_12，會得到84。這結果是呼叫codice_14的codice_12和codice_17的codice_12結果總和。? (object-width (make-instance 'bordered-button))84我們還可以繼續在增加codice_9的codice_12方法。很簡單的想讓結果在加2。2)? (object-width (make-instance 'bordered-button))86Python.Python中，除了使用protocol以外，也可以用多繼承的形式來實現Mixin。Python的codice_38模組提供了codice_39類與codice_40類，作為UDP與TCP的socket伺服器。有兩個mixin類：codice_41與 codice_42。透過如以下程式碼的方式使用codice_42擴充套件codice_40：class ThreadingTCPServer(ThreadingMixIn, TCPServer):passcodice_42類為TCP伺服器添加了新功能，使每個新連線都會創建出新執行緒。而如果是codice_41，則會使每個新連線fork出新的程序。Ruby.在ruby中，並不直接使用Mixin這個單詞，而是使用在類的宣告中include一個module的辦法。已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。Most of the Ruby world is based around mixins via codice_47. The concept of mixins is implemented in Ruby by the keyword codice_48 to which we pass the name of the module as parameter.Example:class Studentinclude Comparable # The class Student inherits the Comparable module using the 'include' keywordattr_accessor :name, :scoredef initialize(name, score)@name = name@score = scoreend# 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.scoreend# Here's the good bit - I get access to = and other methods of the Comparable Interface for free.ends1 = Student.new("Peter", 100)s2 = Student.new("Jason", 90)s1 &gt; s2 #trues1 &lt;= s2 #falseJavaScript.在JavaScript中，Mixin可以用codice_49實現。Mixin可以有自己的父類。物件-文字與codice_50方法.技術上可以透過繫結函式到物件的鍵，來給物件增加行為。但這導致在狀態與行為之間缺少分離等缺點：一個擴充套件函式(來自Underscore.js庫，把源物件的所有功能複製到目標物件, 特性, 函式等)用於混合行為：// 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 methodssam.rename('Samwise', 'Gamgee');frodo.rename('Frodo', 'Baggins');基於"飛行Mixin方法"的純函式與委託.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.兩個基於模式的函式物件不需要codice_50的第三方實現就可完成這種技巧。// Implementationvar 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(); // 1a.last(); // 3其他語言.已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。In the Curl web-content language, multiple inheritance is used as classes with no instances may implement methods. Common mixins include all skinnable codice_52s inheriting from codice_53, user interface delegate objects that require dropdown menus inheriting from StandardBaseDropdownUI and such explicitly named mixin classes as codice_54, codice_55 and codice_56 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.介面與trait.已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。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 codice_57 or codice_58 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{//more methodsclass Birds extends SingerHere, 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 codice_59 is also used to inherit from a super class, in case of a trait codice_59 is used if no super class is inherited and only for mixin in the first trait. All following traits are mixed in using keyword codice_61.class Personclass Actor extends Person with Singerclass Actor extends Singer with PerformerScala 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{//more methodsval singingPerson = new Person with SingersingingPerson.singSwift.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 errorprint(message)struct NetworkManager : ErrorDisplayable{func onError() {error("Please check your internet Connection.")