TaskCompletionQueue.cs

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//
// TaskCompletionQueue.cs
//
// Authors:
//    Jérémie Laval <jeremie dot laval at xamarin dot com>
//
// Copyright 2011 Xamarin Inc (http://www.xamarin.com).
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
//

#if NET_4_0 || UNITY_5_3_OR_NEWER

using System;
using System.Threading;
using System.Collections.Generic;
using System.Collections.Concurrent;

namespace System.Threading.Tasks
{
    internal struct TaskCompletionQueue<TCompletion> where TCompletion : class
    {
        TCompletion single;
        ConcurrentOrderedList<TCompletion> completed;

        public void Add (TCompletion continuation)
        {
            if (single == null && AotInterlocked.CompareExchange (ref single, continuation, null) == null)
                return;
            if (completed == null)
                AotInterlocked.CompareExchange (ref completed, new ConcurrentOrderedList<TCompletion> (), null);
            completed.TryAdd (continuation);
        }

        public bool Remove (TCompletion continuation)
        {
            TCompletion temp = single;
            if (temp != null && temp == continuation && AotInterlocked.CompareExchange (ref single, null, continuation) == continuation)
                return true;
            if (completed != null)
                return completed.TryRemove (continuation);
            return false;
        }

        public bool HasElements {
            get {
                return single != null || (completed != null && completed.Count != 0);
            }
        }

        public bool TryGetNextCompletion (out TCompletion continuation)
        {
            continuation = null;

            if (single != null && (continuation = AotInterlocked.Exchange (ref single, null)) != null)
                return true;

            return completed != null && completed.TryPop (out continuation);
        }
    }
}

#endif