Merge pull request #82 from tashemi/master

Adapter pattern

Author: fulmicoton

chapters/design_patterns/adapter.md

@@ -0,0 +1,51 @@
+---
+layout: recipe
+title: Adapter patter
+chapter: Design patterns
+---
+## Problem
+
+Suppose we have 3-rd party grid component. We want to apply there our own custom sorting but a small problem. Our custom sorter does not implement required interface by grid component.
+To understand the problem completely best example would be an socket from our usual life. Everybody knows this device. In some countries it has 3 pins and in other contries it has only 2 pins. 
+This is exactly right situation to use [adapter pattern](https://en.wikipedia.org/wiki/Adapter_pattern). 
+
+## Solution
+
+{% highlight coffeescript %}
+# a fragment of 3-rd party grid component
+class AwesomeGrid
+	constructor: (@datasource)->
+		@sort_order = 'ASC' 
+		@sorter = new NullSorter # in this place we use NullObject pattern (another usefull pattern)
+	setCustomSorter: (@customSorter) ->
+		@sorter = customSorter
+	sort: () ->
+		@datasource = @sorter.sort @datasource, @sort_order
+		# don't forget to change sort order
+
+
+class NullSorter
+	sort: (data, order) -> # do nothing; it is just a stub
+	
+class RandomSorter
+	sort: (data)->
+		for i in [data.length-1..1] #let's shuffle the data a bit
+    			j = Math.floor Math.random() * (i + 1)
+    			[data[i], data[j]] = [data[j], data[i]]
+		return data
+
+class RandomSorterAdapter
+	constructor: (@sorter) ->
+	sort: (data, order) ->
+		@sorter.sort data
+
+agrid = new AwesomeGrid ['a','b','c','d','e','f']
+agrid.setCustomSorter new RandomSorterAdapter(new RandomSorter)
+agrid.sort() # sort data with custom sorter through adapter
+
+{% endhighlight %}
+
+## Discussion
+
+Adapter is usefull when you have to organize an interaction between two objects with different interfaces. It can happen when you use 3-rd party libraries  or you work with legacy code. 
+In any case be carefull with adapter: it can be helpfull but it can instigate design errors.