完成源包装 第四版

    public class AsyncCompletion
    {
        static AsyncCompletion()
        {

            CompletedSource = new TaskCompletionSource<Unit>();
            CompletedSource.SetResult(Unit.Default);

        }
        int completed = 0;
        bool completionFetched ;

        /// <summary>
        /// 静态完成源
        /// </summary>

        static readonly TaskCompletionSource<Unit> CompletedSource;

        /// <summary>
        /// 跟着实例走的默认未完成源
        /// </summary>
        Lazy<TaskCompletionSource<Unit>> _incompletByDefaultSource =
            new Lazy<TaskCompletionSource<Unit>>(() => new TaskCompletionSource<Unit>(), true);


        public Task Completion
        {
            get
            {
                completionFetched = true;     

                //如果完成了就只返回静态完成源，否则返回引用中的默认未完成源
                return (completed == 0) ? _incompletByDefaultSource.Value.Task : CompletedSource.Task;
            }

        }

        public void Complete()
        {
            //标记完成

            var isFirstComplete = Interlocked.Exchange(ref completed, 1) == 0;  
            
            //如果有人拿过默认未完成源 且当前是第一次完成 那么就把未完成源设置为完成]
            if (completionFetched && isFirstComplete)
            {
                var cts = _incompletByDefaultSource.Value;
                cts.SetResult(Unit.Default);
            }

        }
    }

    public class AsyncCompletion
    {
        static AsyncCompletion()
        {

            CompletedSource = new TaskCompletionSource<Unit>();
            CompletedSource.SetResult(Unit.Default);

        }

        public AsyncCompletion()
        {
            _incompletByDefaultSource = new Lazy<TaskCompletionSource<Unit>>(
                       () => (completed == 0) ? new TaskCompletionSource<Unit>() : CompletedSource,
                       true);

        }
        int completed = 0;


        /// <summary>
        /// 静态完成源
        /// </summary>

        static readonly TaskCompletionSource<Unit> CompletedSource;

        /// <summary>
        /// 跟着实例走的默认未完成源
        /// </summary>
        Lazy<TaskCompletionSource<Unit>> _incompletByDefaultSource;


        public Task Completion
        {
            get
            {
                //如果完成了就只返回静态完成源，否则返回引用中的默认未完成源
                return (completed == 0) ? _incompletByDefaultSource.Value.Task : CompletedSource.Task;
            }

        }

        public void Complete()
        {
            //标记完成

            var mark = Interlocked.Exchange(ref completed, 1);
            //如果有人拿过默认未完成源 且状态为未完 那么就把未完成源设置为完成（根据替换的值判断是否执行过）
            if (mark == 0) //没有执行过
            {
                if (_incompletByDefaultSource.IsValueCreated)
                {
                    var cts = _incompletByDefaultSource.Value;
                    if (!cts.Task.IsCompleted)
                    {
                        cts.SetResult(Unit.Default);
                    }
                }
            }

        }
    }