public override Task <bool> WaitToReadAsync(CancellationToken cancellationToken)
{
// Outside of the lock, check if there are any items waiting to be read. If there are, we're done.
return(!_parent._items.IsEmpty ?
ChannelUtilities.s_trueTask :
WaitToReadAsyncCore(cancellationToken));
Task <bool> WaitToReadAsyncCore(CancellationToken ct)
{
// Now check for cancellation.
if (ct.IsCancellationRequested)
{
return(Task.FromCanceled <bool>(ct));
}
SingleConsumerUnboundedChannel <T> parent = _parent;
ReaderInteractor <bool> oldWaiter = null, newWaiter;
lock (parent.SyncObj)
{
// Again while holding the lock, check to see if there are any items available.
if (!parent._items.IsEmpty)
{
return(ChannelUtilities.s_trueTask);
}
// There aren't any items; if we're done writing, there never will be more items.
if (parent._doneWriting != null)
{
return(parent._doneWriting != ChannelUtilities.s_doneWritingSentinel ?
Task.FromException <bool>(parent._doneWriting) :
ChannelUtilities.s_falseTask);
}
// Create the new waiter. We're a bit more tolerant of a stray waiting reader
// than we are of a blocked reader, as with usage patterns it's easier to leave one
// behind, so we just cancel any that may have been waiting around.
oldWaiter = parent._waitingReader;
parent._waitingReader = newWaiter = ReaderInteractor <bool> .Create(parent._runContinuationsAsynchronously, ct);
}
oldWaiter?.TrySetCanceled();
return(newWaiter.Task);
}
}
/// <summary>Gets or creates a "waiter" (e.g. WaitForRead/WriteAsync) interactor.</summary>
/// <param name="waiter">The field storing the waiter interactor.</param>
/// <param name="runContinuationsAsynchronously">true to force continuations to run asynchronously; otherwise, false.</param>
/// <param name="cancellationToken">The token to use to cancel the wait.</param>
internal static Task <bool> GetOrCreateWaiter(ref ReaderInteractor <bool> waiter, bool runContinuationsAsynchronously, CancellationToken cancellationToken)
{
// Get the existing waiters interactor.
ReaderInteractor <bool> w = waiter;
// If there isn't one, create one. This explicitly does not include the cancellation token,
// as we reuse it for any number of waiters that overlap.
if (w == null)
{
waiter = w = ReaderInteractor <bool> .Create(runContinuationsAsynchronously);
}
// If the cancellation token can't be canceled, then just return the waiter task.
// If it can, we need to return a task that will complete when the waiter task does but that can also be canceled.
// Easiest way to do that is with a cancelable continuation.
return(cancellationToken.CanBeCanceled ?
w.Task.ContinueWith(t => t.Result, cancellationToken, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default) :
w.Task);
}