Mixin是什么概念?
在浏览tornado的代码时，auth中的类都以Mixin命名，这个词好奇怪啊，查了一下资料，有人解释Mixin为mix in，混入的意思，类似于多重继承。auth模块实现OpenID和OAuth，为什么要用Mixin方式？Mixin的应用场景？与“接口”概念有什么区别？
回复内容：
Mixin 实质上是利用语言特性（比如 Ruby 的 include 语法、Python 的多重继承）来更简洁地实现组合模式。以如下 Java 伪码为例，实现一个可复用的“打标签”组件（Taggable），并且应用到帖子（Post）模型上：
<span class="kn">import</span> <span class="nn">java.util.List</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">java.util.ArrayList</span><span class="o">;</span>
<span class="kd">interface</span> <span class="nc">Entity</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">getId</span><span class="o">();</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">getKind</span><span class="o">();</span>
<span class="o">}</span>
<span class="kd">interface</span> <span class="nc">Taggable</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">addTag</span><span class="o">(</span><span class="kt">int</span> <span class="n">tagId</span><span class="o">);</span>
<span class="kd">public</span> <span class="n">List</span><span class="o"><</span><span class="n">Integer</span><span class="o">></span> <span class="nf">getTags</span><span class="o">();</span>
<span class="o">}</span>
<span class="kd">class</span> <span class="nc">TaggableImpl</span> <span class="kd">implements</span> <span class="n">Taggable</span> <span class="o">{</span>
<span class="kd">private</span> <span class="n">Entity</span> <span class="n">target</span><span class="o">;</span>
<span class="kd">public</span> <span class="nf">TaggableImpl</span><span class="o">(</span><span class="n">Entity</span> <span class="n">target</span><span class="o">)</span> <span class="o">{</span>
<span class="k">this</span><span class="o">.</span><span class="na">target</span> <span class="o">=</span> <span class="n">target</span><span class="o">;</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">addTag</span><span class="o">(</span><span class="kt">int</span> <span class="n">tagId</span><span class="o">)</span> <span class="o">{</span>
<span class="kt">int</span> <span class="n">id</span> <span class="o">=</span> <span class="n">target</span><span class="o">.</span><span class="na">getId</span><span class="o">();</span>
<span class="kt">int</span> <span class="n">kind</span> <span class="o">=</span> <span class="n">target</span><span class="o">.</span><span class="na">getKind</span><span class="o">();</span>
<span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"insert into ... values "</span>
<span class="o">+</span> <span class="n">id</span> <span class="o">+</span> <span class="s">", "</span>
<span class="o">+</span> <span class="n">kind</span> <span class="o">+</span> <span class="s">", "</span>
<span class="o">+</span> <span class="n">tagId</span> <span class="o">+</span> <span class="s">")"</span><span class="o">);</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="n">ArrayList</span><span class="o"><</span><span class="n">Integer</span><span class="o">></span> <span class="nf">getTags</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// query from database</span>
<span class="k">return</span> <span class="k">new</span> <span class="n">ArrayList</span><span class="o"><</span><span class="n">Integer</span><span class="o">>();</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="kd">class</span> <span class="nc">Post</span> <span class="kd">implements</span> <span class="n">Entity</span><span class="o">,</span> <span class="n">Taggable</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kd">final</span> <span class="kd">static</span> <span class="kt">int</span> <span class="n">KIND</span> <span class="o">=</span> <span class="mi">1001</span><span class="o">;</span>
<span class="kd">private</span> <span class="n">Taggable</span> <span class="n">taggable</span><span class="o">;</span>
<span class="kd">private</span> <span class="kt">int</span> <span class="n">id</span><span class="o">;</span>
<span class="kd">private</span> <span class="n">String</span> <span class="n">title</span><span class="o">;</span>
<span class="kd">public</span> <span class="nf">Post</span><span class="o">(</span><span class="kt">int</span> <span class="n">id</span><span class="o">,</span> <span class="n">String</span> <span class="n">title</span><span class="o">)</span> <span class="o">{</span>
<span class="k">this</span><span class="o">.</span><span class="na">id</span> <span class="o">=</span> <span class="n">id</span><span class="o">;</span>
<span class="k">this</span><span class="o">.</span><span class="na">title</span> <span class="o">=</span> <span class="n">title</span><span class="o">;</span>
<span class="k">this</span><span class="o">.</span><span class="na">taggable</span> <span class="o">=</span> <span class="k">new</span> <span class="n">TaggableImpl</span><span class="o">(</span><span class="k">this</span><span class="o">);</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">getId</span><span class="o">()</span> <span class="o">{</span>
<span class="k">return</span> <span class="n">id</span><span class="o">;</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">getKind</span><span class="o">()</span> <span class="o">{</span>
<span class="k">return</span> <span class="n">KIND</span><span class="o">;</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">addTag</span><span class="o">(</span><span class="kt">int</span> <span class="n">tagId</span><span class="o">)</span> <span class="o">{</span>
<span class="n">taggable</span><span class="o">.</span><span class="na">addTag</span><span class="o">(</span><span class="n">tagId</span><span class="o">);</span> <span class="c1">// delegate</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="n">ArrayList</span><span class="o"><</span><span class="n">Integer</span><span class="o">></span> <span class="nf">getTags</span><span class="o">()</span> <span class="o">{</span>
<span class="k">return</span> <span class="n">taggable</span><span class="o">.</span><span class="na">getTags</span><span class="o">();</span> <span class="c1">// delegate</span>
<span class="o">}</span>
<span class="o">}</span>
和多重继承类似（其实可以把 Mixin 看作多重继承的一种在特定场景下的应用），但通常混入 Mixin 的类和 Mixin 类本身不是 is-a 的关系，混入 Mixin 类是为了添加某些（可选的）功能。自由地混入 Mixin 类就可以灵活地为被混入的类添加不同的功能。
传统的「接口」概念中并不包含实现，而 Mixin 包含实现。实际上 Mixin 的作用和 Java 中的众多以「able」结尾的接口很相似。不同的是 Mixin 提供了（默认）实现，而 Java 中实现了 -able 接口的类需要类自身来实现这些混入的功能（Serializable 接口是个例外）。 趁着午休来答一个。
如楼上很多答主一样，谈到Mixin就不得不谈到多重继承，因为Mixin的出现就是为了解决多重继承的问题，那么多重继承有什么问题呢？
在《松本行弘的程序世界》一书中，作者列举了以下三点：
- 结构复杂化：如果是单一继承，一个类的父类是什么，父类的父类是什么，都很明确，因为只有单一的继承关系，然而如果是多重继承的话，一个类有多个父类，这些父类又有自己的父类，那么类之间的关系就很复杂了。
- 优先顺序模糊：假如我有A，C类同时继承了基类，B类继承了A类，然后D类又同时继承了B和C类，所以D类继承父类的方法的顺序应该是D、B、A、C还是D、B、C、A，或者是其他的顺序，很不明确。
- 功能冲突：因为多重继承有多个父类，所以当不同的父类中有相同的方法是就会产生冲突。如果B类和C类同时又有相同的方法时，D继承的是哪个方法就不明确了，因为存在两种可能性。
所以为能够利用多继承的优点又解决多继承的问题，提出了规格继承和实现继承这两样东西。
简单来讲，规格继承指的是一堆方法名的集合，而实现继承除了方法名还允许有方法的实现。
Java 选择了规格继承，在 Java 中叫 interface（不过Java8中已经有默认方法了），而 Ruby 选择了实现继承，也可以叫Mixin，在 Ruby 中叫 module。
从某种程度上来说，继承强调 I am，Mixin 强调 I can。当你 implement 了这个接口或者 include 这个 module 的时候，然后就你行你上。
所以这又可以扯到 duck typing 去了，不细说。要想了解具体的可以看一下《松本行弘的程序世界》这本书。 这叫迷信方法，你想知道好处，打开py源码，搜搜 mixin，试着不用迷信实现一个本来用了迷信的模块，就能切身感受一下了。 为了解决多重继承的问题，Java引入了接口 （interface）技术，Lisp、Ruby引入了 Mix-in 技术。
以 Ruby 为例，Mix-in 有效地降低多重继承复杂性（谁是你爹，哪个爹的优先级高，你的把妹方法是继承自哪个爹的等）。 Ruby中 Mix-in 的单位是 模块 （module）。
Mix-in 技术按一下规则来限制多重继承：
- 继承用但一继承；
- 第二个及两个以上的父类必须是 Mix-in 的抽象类。
Mix-in 类是具有以下特征的抽象类：
- 不能单独生成实例；
- 不能继承普通类。
按照以上的原则，类在层次上具有单一继承一样的树结构，同时又可以实现功能的共享（方法是：把共享的功能放在 Mix-in 类中，再把 Mix-in 类插入到树结构里）。
Java 用 接口 解决规格继承（类都有哪些方法）的问题，Mix-in 则解决了实现继承（类中都用了什么数据结构和什么算法）的问题。
逼逼了这么多，对于 Mix-in 的理解是，Mix-in 只不过是实现多重继承的一个技巧而已。 被约束的多重继承。 mixin不是多继承，mixin是duck type的糖，让你可以不用去把一坨坨Interface继承一遍然后才能彼此调用接口。 mixin 并没有特别权威的标准，非要让我下个定论的话：mixin 其实就是在语言不提供标准多重继承的情况下，变相实现多重继承的一个语法糖。 不同版本的 mixin 实现不太一样，但出发点都是在允许继承接口和继承实现的基础上，简化继承关系，避免多重继承的坑。
我认为 C# 的扩展方法其实就是一种 mixin，但又不是通过代码拷贝等动态特性实现的，而是在编译器层面很严格的帮你进行了封装。对其它静态语言来说，如果没有多重继承，要自己实现 mixin 是很困难的。动态语言有些就支持 mixin，不支持的话自己造一个也很容易。
我最近也利用 javascript 实现了一个版本的 mixin 机制，为了支持可视化编辑，添加了类似组件系统的能力，将整合进 0.5 的 Fireball-x，到时会进一步更新答案。
手机码字求赞，欢迎更多讨论。 就是编译的时候把一段代码复制到另一个地方的意思。 Mixin是一种特殊的多重继承，也就是多重继承的子集。
使用Mixin的好处是，同时享有单一继承的单纯性和多重继承的共有性。
作为Mixin类，需要满足以下条件：
- 不能单独生成实例对象，属于抽象类。
- 不能继承Mixin以外的类。
Java的接口，只提供了“规格”的多重继承。Mixin类则同时提供了“规格”和“实现”的多重继承，使用上相比接口会更加简单。
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