Added task service (#374)

* Added task service

test/Microsoft.SqlTools.ServiceLayer.UnitTests/TaskServices/DatabaseOperationStub.cs

@@ -0,0 +1,61 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.SqlTools.ServiceLayer.TaskServices;
+
+namespace Microsoft.SqlTools.ServiceLayer.UnitTests.TaskServices
+{
+    public class DatabaseOperationStub
+    {
+        private CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
+        public void Stop()
+        {
+            IsStopped = true;
+        }
+
+        public void FailTheOperation()
+        {
+            Failed = true;
+        }
+
+        public TaskResult TaskResult { get; set; }
+
+        public bool IsStopped { get; set; }
+
+        public bool Failed { get; set; }
+
+        public async Task<TaskResult> FunctionToRun(SqlTask sqlTask)
+        {
+            sqlTask.TaskCanceled += OnTaskCanceled;
+            return await Task.Factory.StartNew(() =>
+            {
+                while (!IsStopped)
+                {
+                    //Just keep running
+                    if (cancellationTokenSource.Token.IsCancellationRequested)
+                    {
+                        throw new OperationCanceledException();
+                    }
+                    if (Failed)
+                    {
+                        throw new InvalidOperationException();
+                    }
+                    sqlTask.AddMessage("still running", SqlTaskStatus.InProgress, true);
+                }
+                sqlTask.AddMessage("done!", SqlTaskStatus.Succeeded);
+
+                return TaskResult;
+            });
+        }
+
+        private void OnTaskCanceled(object sender, TaskEventArgs<SqlTaskStatus> e)
+        {
+            cancellationTokenSource.Cancel();
+        }
+    }
+}