add async foreach example (#715)

Author: tg123

examples/watch/Program.cs

@@ -1,18 +1,36 @@
 using System;
 using System.Threading;
+using System.Threading.Tasks;
 using k8s;
 using k8s.Models;
+using Microsoft.Rest;
 
 namespace watch
 {
     internal class Program
     {
-        private static void Main(string[] args)
+        private async static Task Main(string[] args)
         {
             var config = KubernetesClientConfiguration.BuildConfigFromConfigFile();
 
             IKubernetes client = new Kubernetes(config);
 
+            var podlistResp = client.ListNamespacedPodWithHttpMessagesAsync("default", watch: true);
+            // C# 8 required https://docs.microsoft.com/en-us/archive/msdn-magazine/2019/november/csharp-iterating-with-async-enumerables-in-csharp-8
+            await foreach (var (type, item) in podlistResp.WatchAsync<V1Pod, V1PodList>())
+            {
+                Console.WriteLine("==on watch event==");
+                Console.WriteLine(type);
+                Console.WriteLine(item.Metadata.Name);
+                Console.WriteLine("==on watch event==");
+            }
+
+            // uncomment if you prefer callback api
+            // WatchUsingCallback(client);
+        }
+
+        private static void WatchUsingCallback(IKubernetes client)
+        {
             var podlistResp = client.ListNamespacedPodWithHttpMessagesAsync("default", watch: true);
             using (podlistResp.Watch<V1Pod, V1PodList>((type, item) =>
             {

src/KubernetesClient/WatcherExt.cs

@@ -66,6 +66,15 @@ public static Watcher<T> Watch<T, L>(
             return Watch(Task.FromResult(response), onEvent, onError, onClosed);
         }
 
+        /// <summary>
+        /// create an IAsyncEnumerable from a call to api server with watch=true
+        /// see https://docs.microsoft.com/en-us/archive/msdn-magazine/2019/november/csharp-iterating-with-async-enumerables-in-csharp-8
+        /// </summary>
+        /// <typeparam name="T">type of the event object</typeparam>
+        /// <typeparam name="L">type of the HttpOperationResponse object</typeparam>
+        /// <param name="responseTask">the api response</param>
+        /// <param name="onError">a callbak when any exception was caught during watching</param>
+        /// <returns>IAsyncEnumerable of watch events</returns>
         public static IAsyncEnumerable<(WatchEventType, T)> WatchAsync<T, L>(
             this Task<HttpOperationResponse<L>> responseTask,
             Action<Exception> onError = null)