Merge pull request #78 from jamesonquinn/master

Mixins chapter

Author: peterhellberg

Diff for: chapters/classes_and_objects/mixins.md

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+---
+layout: recipe
+title: Mixins for classes
+chapter: Classes and Objects
+---
+## Problem
+
+You have a few utility methods that you want to include in a number of different classes.
+
+## Solution
+
+Use a mixOf factory function that generates a mixed superclass for you.
+
+{% highlight coffeescript %}
+mixOf = (base, mixins...) ->
+  class Mixed extends base
+  for mixin in mixins by -1 #earlier mixins override later ones
+    for name, method of mixin::
+      Mixed::[name] = method
+  Mixed
+
+...
+
+class DeepThought
+  answer: ->
+    42
+    
+class PhilosopherMixin
+  pontificate: ->
+    console.log "hmm..."
+    @wise = yes
+
+class DeeperThought extends mixOf DeepThought, PhilosopherMixin
+  answer: ->
+    @pontificate()
+    super()
+    
+earth = new DeeperThought
+earth.answer()
+# hmm...
+# => 42
+{% endhighlight %}
+
+## Discussion
+
+This is intended for lightweight mixins. Thus you inherit methods of the
+base and its ancestors, and those of the mixins, but not those of the ancestors of 
+the mixins. Also, after declaring a mixed class, further changes in the mixins are not 
+reflected.