observer pattern example

Author: anupavanm

DesignPatterns.sln

@@ -37,6 +37,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MediatorPattern", "Mediator
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MementoPattern", "Memento\MementoPattern.csproj", "{93F0318A-696F-4C09-800F-7091BAEC2178}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ObserverPattern", "ObserverPattern\ObserverPattern.csproj", "{88754C6C-4DA4-4C4B-8E53-4DDD9425A387}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -111,6 +113,10 @@ Global
 		{93F0318A-696F-4C09-800F-7091BAEC2178}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{93F0318A-696F-4C09-800F-7091BAEC2178}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{93F0318A-696F-4C09-800F-7091BAEC2178}.Release|Any CPU.Build.0 = Release|Any CPU
+		{88754C6C-4DA4-4C4B-8E53-4DDD9425A387}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{88754C6C-4DA4-4C4B-8E53-4DDD9425A387}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{88754C6C-4DA4-4C4B-8E53-4DDD9425A387}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{88754C6C-4DA4-4C4B-8E53-4DDD9425A387}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

ObserverPattern/ObserverPattern.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

ObserverPattern/Program.cs

@@ -0,0 +1,121 @@
+using System;
+using System.Collections.Generic;
+
+namespace ObserverPattern
+{
+class JobPost
+{
+  public string Title { get; private set; }
+
+  public JobPost(string title)
+  {
+    Title = title;
+  }
+}
+class JobSeeker : IObserver<JobPost>
+{
+  public string Name { get; private set; }
+
+  public JobSeeker(string name)
+  {
+    Name = name;
+  }
+
+  //Method is not being called by JobPostings class currently
+  public void OnCompleted()
+  {
+    //No Implementation
+  }
+
+  //Method is not being called by JobPostings class currently
+  public void OnError(Exception error)
+  {
+    //No Implementation
+  }
+
+  public void OnNext(JobPost value)
+  {
+    Console.WriteLine("Hi {0} ! New job posted: {1}", Name, value.Title);
+  }
+}
+
+class JobPostings : IObservable<JobPost>
+{
+  private List<IObserver<JobPost>> mObservers;
+  private List<JobPost> mJobPostings;
+
+  public JobPostings()
+  {
+    mObservers = new List<IObserver<JobPost>>();
+    mJobPostings = new List<JobPost>();
+  }
+
+  public IDisposable Subscribe(IObserver<JobPost> observer)
+  {
+    // Check whether observer is already registered. If not, add it
+    if (!mObservers.Contains(observer))
+    {
+      mObservers.Add(observer);
+    }
+    return new Unsubscriber<JobPost>(mObservers, observer);
+  }
+
+  private void Notify(JobPost jobPost)
+  {
+    foreach(var observer in mObservers)
+    {
+      observer.OnNext(jobPost);
+    }
+  }
+
+  public void AddJob(JobPost jobPost)
+  {
+    mJobPostings.Add(jobPost);
+    Notify(jobPost);
+  }
+
+}
+
+internal class Unsubscriber<JobPost> : IDisposable
+{
+  private List<IObserver<JobPost>> mObservers;
+  private IObserver<JobPost> mObserver;
+
+  internal Unsubscriber(List<IObserver<JobPost>> observers, IObserver<JobPost> observer)
+  {
+    this.mObservers = observers;
+    this.mObserver = observer;
+  }
+
+  public void Dispose()
+  {
+    if (mObservers.Contains(mObserver))
+      mObservers.Remove(mObserver);
+  }
+}
+
+
+  class Program
+  {
+    static void Main(string[] args)
+    {
+//Create Subscribers
+var johnDoe = new JobSeeker("John Doe");
+var janeDoe = new JobSeeker("Jane Doe");
+
+//Create publisher and attch subscribers
+var jobPostings = new JobPostings();
+jobPostings.Subscribe(johnDoe);
+jobPostings.Subscribe(janeDoe);
+
+//Add a new job and see if subscribers get notified
+jobPostings.AddJob(new JobPost("Software Engineer"));
+
+//Output
+// Hi John Doe! New job posted: Software Engineer
+// Hi Jane Doe! New job posted: Software Engineer
+
+Console.ReadLine();
+    }
+  }
+}