Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Web/UI/PageAsyncTaskManager.cs

@@ -0,0 +1,54 @@
+//------------------------------------------------------------------------------
+// <copyright file="PageAsyncTaskManager.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+//------------------------------------------------------------------------------
+
+namespace System.Web.UI {
+    using System;
+    using System.Collections.Generic;
+    using System.Threading;
+    using System.Threading.Tasks;
+    using System.Web;
+
+    // Handles execution of asynchronous tasks that are registered with a Page
+
+    internal sealed class PageAsyncTaskManager {
+
+        private bool _executeTasksAsyncHasCompleted = false;
+        private readonly Queue<IPageAsyncTask> _registeredTasks = new Queue<IPageAsyncTask>();
+
+        public void EnqueueTask(IPageAsyncTask task) {
+            if (_executeTasksAsyncHasCompleted) {
+                // don't allow multiple calls to the execution routine
+                throw new InvalidOperationException(SR.GetString(SR.PageAsyncManager_CannotEnqueue));
+            }
+
+            _registeredTasks.Enqueue(task);
+        }
+
+        public async Task ExecuteTasksAsync(object sender, EventArgs e, CancellationToken cancellationToken, AspNetSynchronizationContextBase syncContext, IRequestCompletedNotifier requestCompletedNotifier) {
+            try {
+                while (_registeredTasks.Count > 0) {
+                    // if canceled, propagate exception to caller and stop executing tasks
+                    cancellationToken.ThrowIfCancellationRequested();
+
+                    // if request finished, stop executing tasks
+                    if (requestCompletedNotifier.IsRequestCompleted) {
+                        return;
+                    }
+
+                    // execute this task
+                    IPageAsyncTask task = _registeredTasks.Dequeue();
+                    using (syncContext.AllowVoidAsyncOperationsBlock()) {
+                        await task.ExecuteAsync(sender, e, cancellationToken);
+                    }
+                }
+            }
+            finally {
+                _executeTasksAsyncHasCompleted = true;
+            }
+        }
+
+    }
+}