如何獲得 ActionBlock 的訪問輸入隊列？

Question

我正在傳遞給某個類的 Actionblock 實例。 如果我打電話

cancellationSource.Cancel();

然后處理將停止。 但是有些實例可以留在 ActionBlock 的輸入隊列中。 我需要訪問組織中剩余的實例以釋放一些資源。

我怎樣才能實現這個目標？

Answer 1

如果您迫切需要一個帶有公開輸入緩沖區的ActionBlock ，您可以嘗試下面的自定義實現。 它支持ActionBlock所有內置功能，還包括自定義IEnumerable<T> InputQueue屬性。 當ActionBlockEx在錯誤或取消狀態下完成時，不會清空輸入緩沖區。

public class ActionBlockEx<T> : ITargetBlock<T>
{
    private readonly ITargetBlock<object> _actionBlock;
    private readonly Queue<T> _queue;

    public ActionBlockEx(Func<T, Task> action,
        ExecutionDataflowBlockOptions dataflowBlockOptions = null)
    {
        if (action == null) throw new ArgumentNullException(nameof(action));
        _actionBlock = new ActionBlock<object>(_ =>
        {
            T item; lock (_queue) item = _queue.Dequeue();
            return action(item);
        }, dataflowBlockOptions ?? new ExecutionDataflowBlockOptions());
        _queue = new Queue<T>();
    }

    public ActionBlockEx(Action<T> action,
        ExecutionDataflowBlockOptions dataflowBlockOptions = null) : this(
            item => { action(item); return Task.CompletedTask; }, dataflowBlockOptions)
    {
        if (action == null) throw new ArgumentNullException(nameof(action));
    }

    public int InputCount { get { lock (_queue) return _queue.Count; } }
    public IEnumerable<T> InputQueue { get { lock (_queue) return _queue.ToList(); } }

    public Task Completion => _actionBlock.Completion;
    public void Complete() => _actionBlock.Complete();
    void IDataflowBlock.Fault(Exception ex) => _actionBlock.Fault(ex);

    DataflowMessageStatus ITargetBlock<T>.OfferMessage(DataflowMessageHeader header,
        T item, ISourceBlock<T> source, bool consumeToAccept)
    {
        var sourceProxy = source != null ? new SourceProxy(source, this) : null;
        lock (_queue)
        {
            var offerResult = _actionBlock.OfferMessage(header, null, sourceProxy,
                consumeToAccept);
            if (offerResult == DataflowMessageStatus.Accepted
                && (sourceProxy == null || !sourceProxy.ConsumeMessageInvoked))
            {
                _queue.Enqueue(item);
            }
            return offerResult;
        }
    }

    private class SourceProxy : ISourceBlock<object>
    {
        private readonly ISourceBlock<T> _realSource;
        private readonly ActionBlockEx<T> _realTarget;

        public bool ConsumeMessageInvoked { get; private set; }

        public SourceProxy(ISourceBlock<T> realSource, ActionBlockEx<T> realTarget)
        {
            _realSource = realSource;
            _realTarget = realTarget;
        }

        object ISourceBlock<object>.ConsumeMessage(DataflowMessageHeader header,
            ITargetBlock<object> target, out bool messageConsumed)
        {
            this.ConsumeMessageInvoked = true;
            lock (_realTarget._queue)
            {
                var item = _realSource.ConsumeMessage(header, _realTarget,
                    out messageConsumed);
                if (messageConsumed) _realTarget._queue.Enqueue(item);
            }
            return null;
        }

        bool ISourceBlock<object>.ReserveMessage(DataflowMessageHeader header,
            ITargetBlock<object> target)
        {
            return _realSource.ReserveMessage(header, _realTarget);
        }

        void ISourceBlock<object>.ReleaseReservation(DataflowMessageHeader header,
            ITargetBlock<object> target)
        {
            _realSource.ReleaseReservation(header, _realTarget);
        }

        Task IDataflowBlock.Completion => throw new NotSupportedException();
        void IDataflowBlock.Complete() => throw new NotSupportedException();
        void IDataflowBlock.Fault(Exception ex) => throw new NotSupportedException();
        IDisposable ISourceBlock<object>.LinkTo(ITargetBlock<object> target,
            DataflowLinkOptions linkOptions) => throw new NotSupportedException();
    }

}

此實現基於一個內部ActionBlock<object> ，它提供了虛擬null消息。 它與鏈接的ISourceBlock通信被攔截，以便獲取實際消息並將其存儲在內部Queue<T> 。 這種間接性增加了一些開銷（在收到的每條消息上都會進行對象分配），因此請謹慎使用此類！