Using the Observer Pattern

At work we are reading “Head First Design Patterns” and discussing the “Observer Pattern” and some people wanted to see a code example.
The first part is the the class that manages the observer classes

   1: public class EventManager<T>

   2: { 

   3:     private Dictionary<string, ISubscriber<T>> _subscribers = new Dictionary<string, ISubscriber<T>>();

   4:     public Dictionary<string,ISubscriber<T>> Subscribers

   5:     {

   6:         get { return _subscribers; }

   7:         set { _subscribers = value; }

   8:     }

   9:  

  10:     public void Notify(T newState)

  11:     {

  12:         foreach (var observer in Subscribers.Values)

  13:         {

  14:             observer.State = newState;

  15:             observer.Update();

  16:         }

  17:     }

  18: }

In this example I created a generic class that has two basic parts a collection of subscribers or observers and a method that loops though and updates the state of the subscribers.  I used a Dictionary<string,ISubscriber<T>> to make it easy to manage my subscribers based on a key, and because ISubscriber<T> is also a generic its a very flexible and strongly typed way to work with the subscribers.

Next let’s take a look at ISubscriber<T> interface

   1: public interface ISubscriber<T>

   2: {

   3:     T State { get; set; }

   4:     void Update();

   5: }

it’s that simple, I use generics to determine the state and the input type for the update method, here is an example implementation

   1: public class TweetLocation : ISubscriber<Location>

   2: {

   3:     public ITwitter TwitterInterfacer = new FakeTwitterClient();

   4:     public Location State { get; set; }

   5:  

   6:     public void Update()

   7:     {

   8:         TwitterInterfacer.TweetMessage(string.Format("currently at ({0},{1})",State.Lat,State.Lon));

   9:     }

  10: }

TweetLocation is observing Location information and calls a Twitter class to Tweet the new location when updated, for something a little more complex

   1: public class SaveLocation : ISubscriber<Location>

   2: {

   3:     public ILocationReposigtory LocationRepository = new FakeLocationRepository();

   4:     public static EventManager<string> ErrorHandler = new EventManager<string>();

   5:  

   6:     public Location State{get;set;}

   7:  

   8:     public SaveLocation()

   9:     {

  10:         ErrorHandler.Subscribers.Add("ErrorMailer",new ErrorMailer());

  11:     }

  12:  

  13:     public void Update()

  14:     {

  15:         bool success = LocationRepository.SaveLocation(State);

  16:         if (!success)

  17:         {

  18:             ErrorHandler.Notify("Failed to save location");

  19:         }

  20:     }

  21: }

SaveLocation is basically the same as TweetLocation except it has it’s own EventManager to handle errors from failed attempts at saving the location.

Finally here is an example of actually using it

   1: class Program

   2: {

   3:     public static EventManager<Location> LocationHandler = new EventManager<Location>();

   4:  

   5:     static void Main(string[] args)

   6:     {

   7:         LocationHandler.Subscribers.Add("Repository",new SaveLocation());

   8:         LocationHandler.Subscribers.Add("Twitter",new TweetLocation());

   9:  

  10:         Location currentLocation = new Location(){Lat = 111,Lon = -121};

  11:  

  12:         LocationHandler.Notify(currentLocation);

  13:  

  14:         Console.ReadKey();

  15:  

  16:         currentLocation.Lon = 1211;

  17:         LocationHandler.Subscribers.Remove("Twitter");

  18:         LocationHandler.Notify(currentLocation);

  19:  

  20:         Console.ReadKey();

  21:         

  22:         

  23:     }

  24: }

This is a simple console app that demonstrates adding subscribers, updating state, and removing a subscriber. 

A real world example of using “Observer Pattern” is a service bus like NServiceBus or MassTransit, where an event driven application needs to send messages to one or more subscribers.

As always here is a link to the project source.

Another walk in the cloud with Amazon Web Services

Talking a look at some of the offering that Amazon Cloud Services has I decided to take a look at Amazon Simple Storage Service or simply Amazon S3. 

Like I wrote in “A walk in the Cloud” the Amazon cloud can be accessed though a web service so you have the option of rolling your own tools or using their API lib.  The has SDK has defiantly improved and the install process is a lot easier now.  Before you had to download the source, build the project, etc, now all you have to do is download AWSSDKForNET and install it.  This adds the Clint to your GAC, and add templates into Visual Studio, if you still want to see what they are doing under the hood you can still download the source. 

Amazon may not provide you with any tools but there are a few free ones that work fairly well.  CloudBerry Explorer is a fairly solid tool for managing your S3 storage, it looks kind of like an ftp client and it lets you manage multiple accounts, create buckets, upload/download content, generate web URLs(http and https), search, etc. Another client is the Firefox plug-in S3Fox, basically the same as CloudBerry but it runs in Firefox.

Pulling data from S3 is relatively simple, this is a code sample for pulling your list of buckets, then building a list of links to the images in each bucket

   1: AmazonS3 s3Client = AWSClientFactory.CreateAmazonS3Client(

   2:     ConfigurationManager.AppSettings["AWSAccessKey"],

   3:     ConfigurationManager.AppSettings["AWSSecretKey"]);

   4:  

   5: ListBucketsResponse response = s3Client.ListBuckets();

   6:    

   7: foreach (S3Bucket bucket in response.Buckets)

   8: {

   9:     HtmlGenericControl header = new HtmlGenericControl("div")

  10:     {

  11:         InnerText = bucket.BucketName

  12:     };

  13:  

  14:     DivlistOfImages.Controls.Add(header);

  15:  

  16:     ListObjectsRequest request = new ListObjectsRequest

  17:      {

  18:          BucketName = bucket.BucketName

  19:      };

  20:     ListObjectsResponse imageList = s3Client.ListObjects(request);

  21:  

  22:     foreach (S3Object s3Object in imageList.S3Objects)

  23:     {

  24:         HtmlAnchor imageLink = new HtmlAnchor

  25:        {

  26:            InnerText = s3Object.Key

  27:        };

  28:         string bucketName = bucket.BucketName;

  29:         string objectKey = s3Object.Key;

  30:         GetPreSignedUrlRequest preSignedUrlRequest = new GetPreSignedUrlRequest

  31:          {

  32:              BucketName = bucketName,

  33:              Key = objectKey,

  34:              Protocol = Protocol.HTTP,

  35:              Expires = DateTime.Now.AddDays(7)

  36:          };

  37:  

  38:  

  39:         imageLink.HRef = s3Client.GetPreSignedURL(preSignedUrlRequest);

  40:         DivlistOfImages.Controls.Add(imageLink);

  41:     }

  42: }

it’s that simple.

With this you can have your content be managed by another department with a tool like CloudBerry and remove the work of updating images and other forms of content. 

As always here is a link to the sample code project