Create observer.md

Author: tashemi

chapters/design_patterns/observer.md

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+---
+layout: recipe Observer Pattern
+title: Observer Pattern
+chapter: Design patterns
+---
+## Problem
+
+You have to notify some objects about an event happen
+
+## Solution
+
+Use an [Observer Pattern](http://en.wikipedia.org/wiki/Observer_pattern)
+
+{% highlight coffeescript %}
+class PostOffice
+  constructor: () ->
+    @subscribers = []
+  sendNewItemReleased: (item) ->
+    subscriber.callback(item) for subscriber in @subscribers when subscriber.item is item
+    return
+  subscribe: (to, onNewItemReleased) ->
+    @subscribers.push({'item':to, 'callback':onNewItemReleased})
+class MagazineSubscriber
+  onNewMagazine: (item) ->
+    alert "I've got new "+item
+class NewspaperSubscriber
+  onNewNewspaper: (item) ->
+    alert "I've got new "+item
+
+postOffice = new PostOffice()
+sub1 = new MagazineSubscriber()
+sub2 = new NewspaperSubscriber()
+postOffice.subscribe "Mens Health", sub1.onNewMagazine
+postOffice.subscribe "Times", sub2.onNewNewspaper
+postOffice.sendNewItemReleased "Times"
+postOffice.sendNewItemReleased "Mens Health"
+{% endhighlight %}
+
+## Discussion
+
+Here you have an observer object (PostOffice) and observable objects (MagazineSubscriber, NewspaperSubscriber). 
+To be notified about an event of publishing new periodical observable object should make subscribtion on PostOffice. 
+Every of subscribed objects is stored internaly in the PostOffice array of subscribers. 
+Every subscriber is notified on new concrete periodical is published.