Add observer pattern

Update memento pattern examples class to have run as static method
Update behavioral patterns program.cs to maintain the order

src/ObserverPattern/ObserverPattern.xproj

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+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <VisualStudioVersion Condition="'$(VisualStudioVersion)' == ''">14.0</VisualStudioVersion>
+    <VSToolsPath Condition="'$(VSToolsPath)' == ''">$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)</VSToolsPath>
+  </PropertyGroup>
+
+  <Import Project="$(VSToolsPath)\DotNet\Microsoft.DotNet.Props" Condition="'$(VSToolsPath)' != ''" />
+  <PropertyGroup Label="Globals">
+    <ProjectGuid>d48db558-0228-4ace-88a8-a202e5c57849</ProjectGuid>
+    <RootNamespace>ObserverPattern</RootNamespace>
+    <BaseIntermediateOutputPath Condition="'$(BaseIntermediateOutputPath)'=='' ">.\obj</BaseIntermediateOutputPath>
+    <OutputPath Condition="'$(OutputPath)'=='' ">.\bin\</OutputPath>
+    <TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>
+  </PropertyGroup>
+
+  <PropertyGroup>
+    <SchemaVersion>2.0</SchemaVersion>
+  </PropertyGroup>
+  <Import Project="$(VSToolsPath)\DotNet\Microsoft.DotNet.targets" Condition="'$(VSToolsPath)' != ''" />
+</Project>

src/ObserverPattern/ObserverPatternExamples.cs

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+using ObserverPattern.Twits;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern
+{
+    
+    public class ObserverPatternExamples
+    {
+        public static void Run()
+        {
+            ObservableTwitsExample obsTwits = new ObservableTwitsExample();
+            obsTwits.Run();
+        }
+    }
+}

src/ObserverPattern/Properties/AssemblyInfo.cs

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+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+// General Information about an assembly is controlled through the following
+// set of attributes. Change these attribute values to modify the information
+// associated with an assembly.
+[assembly: AssemblyConfiguration("")]
+[assembly: AssemblyCompany("Hewlett-Packard Company")]
+[assembly: AssemblyProduct("ObserverPattern")]
+[assembly: AssemblyTrademark("")]
+
+// Setting ComVisible to false makes the types in this assembly not visible
+// to COM components.  If you need to access a type in this assembly from
+// COM, set the ComVisible attribute to true on that type.
+[assembly: ComVisible(false)]
+
+// The following GUID is for the ID of the typelib if this project is exposed to COM
+[assembly: Guid("d48db558-0228-4ace-88a8-a202e5c57849")]

src/ObserverPattern/Twits/ObservableTwitsExample.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern.Twits
+{
+    public class ObservableTwitsExample
+    {
+        public void Run()
+        {
+            TwitObservable observable = new TwitObservable();
+
+            TwitUser t100 = new TwitUser("t100");
+            TwitUser r2d2 = new TwitUser("r2d2");
+            
+            var t100Subscription = observable.Subscribe(t100);
+            var r2d2Subscription = observable.Subscribe(r2d2);
+
+            observable.AddTwit("El chupacapra");
+
+            t100Subscription.Dispose();
+
+            observable.AddTwit("Vamos vamos mi amor");
+
+            observable.ItsGoingHomeTime();
+
+            
+        }
+    }
+}

src/ObserverPattern/Twits/TwitObservable.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern.Twits
+{
+    /// <summary>
+    /// Concrete observable
+    /// </summary>
+    public class TwitObservable : IObservable<string>
+    {
+        List<IObserver<string>> observers;
+        public TwitObservable()
+        {
+            observers = new List<IObserver<string>>();
+        }
+        public IDisposable Subscribe(IObserver<string> observer)
+        {
+            if (!observers.Contains(observer))
+                observers.Add(observer);
+
+            return new Unsubscriber(observers, observer);
+        }
+
+        public void AddTwit(string twit)
+        {
+            foreach (var observer in observers)
+            {
+                observer.OnNext(twit);
+            }
+        }   
+        
+        public void ItsGoingHomeTime()
+        {
+            foreach (var observer in observers)
+            {
+                observer.OnCompleted();
+            }
+        }     
+
+        private class Unsubscriber : IDisposable
+        {
+            private List<IObserver<string>> _observers;
+            private IObserver<string> _observer;
+
+            public Unsubscriber(List<IObserver<string>> observers, IObserver<string> observer)
+            {
+                _observers = observers;
+                _observer = observer;
+            }
+
+            public void Dispose()
+            {
+                if (!(_observer == null)) _observers.Remove(_observer);
+            }
+        }
+    }
+
+
+}

src/ObserverPattern/Twits/TwitUser.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern.Twits
+{
+    /// <summary>
+    /// Concrete observer
+    /// </summary>
+    public class TwitUser : IObserver<string>
+    {
+        string name;
+        public TwitUser(string name)
+        {
+            this.name = name;
+        }
+        public void OnCompleted()
+        {
+            Console.WriteLine("{0} finished watching twitter", name);
+        }
+
+        public void OnError(Exception error)
+        {
+            Console.WriteLine("Error while watching twitter: {0}", error);
+        }
+
+        public void OnNext(string value)
+        {
+            Console.WriteLine("{0} just observed that something {1} was tweeted", name, value);
+        }
+    }
+}

src/ObserverPattern/project.json

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+{
+  "version": "1.0.0-*",
+
+  "dependencies": {
+    "NETStandard.Library": "1.5.0-rc2-24027"
+  },
+
+  "frameworks": {
+    "netstandard1.5": {
+      "imports": "dnxcore50"
+    }
+  }
+}