Merge pull request mgechev#15 from greglbd/master

updated observer pattern for modern angular style guide and added examples

Author: mgechev

README.md

@@ -36,6 +36,7 @@
   * [Others](#others)
     * [Module Pattern](#module-pattern)
     * [Data Mapper](#data-mapper)
+    * [Observer Pattern as an External Service](#observer-pattern-as-an-external-service)
 * [References](#references)
 
 <!--endtoc-->
@@ -824,6 +825,8 @@ Each scope can subscribe to any event with multiple callbacks (i.e. it can assoc
 
 In the JavaScript community this pattern is better known as publish/subscribe.
 
+For a best practice example see [Observer Pattern as an External Service](#observer-pattern-as-an-external-service)
+
 #### Chain of Responsibilities
 
 >The chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain. A mechanism also exists for adding new processing objects to the end of this chain.
@@ -1088,6 +1091,94 @@ And the following partial:
 </div>
 ```
 
+### Observer Pattern as an External Service
+
+##### About
+Below is an example taken from https://github.com/greglbd/angular-observer-pattern. This is an angular factory which reflects the Observer Pattern.  It works well with the ControllerAs method of working as it can be much more efficient that $scope.$watch and more specific to a unique scope or object than $emit and $broadcast when used correctly. 
+
+**Use Case:** You would use this pattern to communicate between 2 controllers that use the same model but are not connected in anyway
+
+##### Methods
+
+function adds a listener to an event with a callback which is stored against the event with it's corresponding id.
+```
+_observerService.attach = function(callback, event, id)
+```
+
+
+function removes all occurences of one id from all events in the observer object
+```
+_observerService.detachById = function(id)
+```
+
+
+function removes all occurences of the event from the observer Object
+```
+_observerService.detachByEvent = function(event)
+``` 
+
+
+removes all callbacks for an id in a specific event from the observer object
+```
+_observerService.detachByEventAndId = function(event, id)
+```
+
+
+Notifies all observers of a specific event, can pass a params variable of any type
+```
+_observerService.notify = function(event, parameters)
+```
+
+
+##### Controller Example
+Below example shows how to attach, notify and detach an event.
+```javascript
+angular.module('app.controllers')
+  .controller('ObserverExample',ObserverExample);
+ObserverExample.$inject= ['ObserverService','$timeout'];
+function ObserverExample(ObserverService, $timeout) {
+    var vm = this;
+    var id = 'vm1';
+    ObserverService.attach(callbackFunction, 'let_me_know', id)
+    
+    function callbackFunction(params){
+        console.log('now i know');
+        ObserverService.detachByEvent('let_me_know')
+    }
+    
+    $timeout(function(){
+        ObserverService.notify('let_me_know');
+    },5000);
+}
+```
+Alternative way to remove event
+
+```javascript
+angular.module('app.controllers')
+  .controller('ObserverExample',ObserverExample);
+ObserverExample.$inject= ['ObserverService','$timeout', '$scope'];
+function ObserverExample(ObserverService, $timeout, $scope) {
+    var vm = this;
+    var id = 'vm1';
+    ObserverService.attach(callbackFunction, 'let_me_know', id)
+    
+    function callbackFunction(params){
+        console.log('now i know');
+    }
+    
+    $timeout(function(){
+        ObserverService.notify('let_me_know');
+    },5000);
+    
+    // Cleanup listeners when this controller is destroyed
+    $scope.$on('$destroy', function handler() {
+        ObserverService.detachByEvent('let_me_know')
+    });
+}
+```
+**Public Repository to get you started**
+https://github.com/greglbd/angular-observer-pattern
+
 ## References
 
 1. [Wikipedia](https://en.wikipedia.org/wiki). The source of all brief descriptions of the design patterns is wikipedia.