Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Web/Util/SubscriptionQueue.cs

@@ -0,0 +1,85 @@
+//------------------------------------------------------------------------------
+// <copyright file="SubscriptionQueue.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+//------------------------------------------------------------------------------
+
+namespace System.Web.Util {
+    using System;
+    using System.Collections.Generic;
+    using System.Web;
+
+    // Similar to a Queue<T>, but allows unsubscribing from the underlying queue.
+    //
+    // !! WARNING !!
+    // Mutable struct for performance reasons; optimized for case where Enqueue is never called.
+    // Be careful with usage, e.g. no readonly declarations of this type.
+    //
+    // Type is not thread safe.
+
+    internal struct SubscriptionQueue<T> {
+
+        private LinkedList<T> _list;
+
+        public bool IsEmpty {
+            get { return (_list == null || _list.Count == 0); }
+        }
+
+        public ISubscriptionToken Enqueue(T value) {
+            if (_list == null) {
+                // lazily instantiate the list
+                _list = new LinkedList<T>();
+            }
+
+            LinkedListNode<T> node = _list.AddLast(value);
+            return new SubscriptionToken(node);
+        }
+
+        public void FireAndComplete(Action<T> action) {
+            try {
+                T value;
+                // Use a while loop instead of a foreach since the list might be changing
+                while (TryDequeue(out value)) {
+                    action(value);
+                }
+            }
+            finally {
+                _list = null;
+            }
+        }
+
+        private bool TryDequeue(out T result) {
+            if (_list != null && _list.First != null) {
+                LinkedListNode<T> theNode = _list.First;
+                _list.RemoveFirst(); // also marks the SubscriptionToken as inactive
+                result = theNode.Value;
+                theNode.Value = default(T); // unroot the value in case it's large
+                return true;
+            }
+            else {
+                result = default(T); // unroot the value in case it's large
+                return false;
+            }
+        }
+
+        private sealed class SubscriptionToken : ISubscriptionToken {
+            private readonly LinkedListNode<T> _node;
+
+            public SubscriptionToken(LinkedListNode<T> node) {
+                _node = node;
+            }
+
+            public bool IsActive {
+                get { return (_node.List != null); }
+            }
+
+            public void Unsubscribe() {
+                if (IsActive) {
+                    _node.List.Remove(_node);
+                    _node.Value = default(T); // unroot the value in case it's large
+                }
+            }
+        }
+
+    }
+}