Add stock example on Observer pattern

src/ObserverPattern/StockUpdateEvents/AaplObserver.cs

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+using System;
+
+namespace ObserverPattern.StockUpdateEvents
+{
+    /// <summary>
+    /// Observer
+    /// </summary>
+    public class AaplObserver
+    {
+        double? oldValue;
+        public AaplObserver(StockSubject stockObservable)
+        {
+            stockObservable.StockUpdated += (obj, e) => PrintNewValue(e.Stock);
+        }
+
+        private void PrintNewValue(Stock stock)
+        {
+            if (stock.Name == "AAPL")
+            {
+                if (oldValue.HasValue)
+                    Console.WriteLine("Apple price updated from {0} to {1}", oldValue, stock.Value);
+                else
+                    Console.WriteLine("Apple has a new price, new value is: {0}", stock.Value);
+
+                oldValue = stock.Value;
+            }
+        }
+    }
+}

src/ObserverPattern/StockUpdateEvents/LondonStockObserver.cs

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+using System;
+
+namespace ObserverPattern.StockUpdateEvents
+{
+    public class LondonStockObserver
+    {
+        double? oldValue;
+        public LondonStockObserver(StockSubject stockObservable)
+        {
+            stockObservable.StockUpdated += (obj, e) => PrintStockValue(e.Stock);
+        }
+
+        private void PrintStockValue(Stock stock)
+        {
+            if (stock.Name == "FTSE")
+            {
+                if (oldValue.HasValue)
+                    Console.WriteLine("The Financial Times Stock Exchange 100 Index price updated from {0} to {1}", oldValue, stock.Value);
+                else
+                    Console.WriteLine("The Financial Times Stock Exchange 100 Index has a new price, new value is: {0}", stock.Value);
+                oldValue = stock.Value;
+            }
+        }
+    }
+}

src/ObserverPattern/StockUpdateEvents/Stock.cs

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+namespace ObserverPattern.StockUpdateEvents
+{
+    public class Stock
+    {
+        public string Name { get; set; }
+        public int Quantity { get; set; }
+
+        public double Value { get; set; }
+    }
+}

src/ObserverPattern/StockUpdateEvents/StockSubject.cs

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+using System;
+
+namespace ObserverPattern.StockUpdateEvents
+{
+    /// <summary>
+    /// Subject
+    /// </summary>
+    public class StockSubject
+    {
+
+        public void UpdateStockValue(Stock s)
+        {
+            StockUpdated?.Invoke(this, new StockUpdateEventArgs(s));
+        }
+
+        public event EventHandler<StockUpdateEventArgs> StockUpdated;
+    }
+}

src/ObserverPattern/StockUpdateEvents/StockUpdateEventArgs.cs

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+namespace ObserverPattern.StockUpdateEvents
+{
+    public class StockUpdateEventArgs
+    {
+        public Stock Stock { get; set; }
+
+        public StockUpdateEventArgs(Stock stock)
+        {
+            Stock = stock;
+        }
+    }
+}

src/ObserverPattern/StockUpdateEvents/StockUpdateEventsExample.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reactive;
+using System.Reactive.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern.StockUpdateEvents
+{
+    public class StockUpdateEventsExample
+    {
+        public void RunSimple()
+        {
+            StockSubject stockSubject = new StockSubject();
+
+            LondonStockObserver londonObserver = new LondonStockObserver(stockSubject);
+
+            AaplObserver aaplObserver = new AaplObserver(stockSubject);
+
+            PublishUpdateStocks(stockSubject);
+        }        
+
+        public void RunReactive()
+        {
+            StockSubject stockSubject = new StockSubject();
+
+            var londonObserver = Observable.FromEventPattern<StockUpdateEventArgs>(
+                ev => stockSubject.StockUpdated += ev,
+                ev => stockSubject.StockUpdated -= ev).Where(s => s.EventArgs.Stock.Name == "FTSE");
+
+            var aaplObserver = Observable.FromEventPattern<StockUpdateEventArgs>(
+                ev => stockSubject.StockUpdated += ev,
+                ev => stockSubject.StockUpdated -= ev).Where(s => s.EventArgs.Stock.Name == "AAPL");
+
+            using (londonObserver.Subscribe(PrintLondonStockPriceUpdate))
+            using (aaplObserver.Subscribe(PrintAaplStockPriceUpdate))
+            {
+                PublishUpdateStocks(stockSubject);
+            }
+        }
+
+        private void PrintAaplStockPriceUpdate(EventPattern<StockUpdateEventArgs> eventP)
+        {
+            Console.WriteLine("apple updated, new value: {0}", eventP.EventArgs.Stock.Value);
+        }
+
+        private void PrintLondonStockPriceUpdate(EventPattern<StockUpdateEventArgs> eventP)
+        {
+            Console.WriteLine("london stock updated, new value: {0}", eventP.EventArgs.Stock.Value);
+        }
+
+        private static void PublishUpdateStocks(StockSubject stockSubject)
+        {
+            stockSubject.UpdateStockValue(new Stock { Name = "AAPL", Value = 3 });
+            stockSubject.UpdateStockValue(new Stock { Name = "AAPL", Value = 4 });
+            stockSubject.UpdateStockValue(new Stock { Name = "AAPL", Value = 5 });
+            stockSubject.UpdateStockValue(new Stock { Name = "AAPL", Value = 6 });
+
+            stockSubject.UpdateStockValue(new Stock { Name = "FTSE", Value = 6 });
+            stockSubject.UpdateStockValue(new Stock { Name = "FTSE", Value = 12 });
+            stockSubject.UpdateStockValue(new Stock { Name = "FTSE", Value = 2 });
+            stockSubject.UpdateStockValue(new Stock { Name = "FTSE", Value = 3 });
+        }
+    }
+}