public override bool ExecuteAsync(AsyncOperation operation, bool preferSync)
{
EnsureNonBlocking();
try
{
operation.CurrentAsyncContext = this;
if (operation.IsReadNotWrite)
{
return(_readQueue.ExecuteAsync(operation, preferSync));
}
else
{
return(_writeQueue.ExecuteAsync(operation, preferSync));
}
}
catch
{
operation.Next = null;
CancellationRequestResult result = operation.RequestCancellationAsync(OperationCompletionFlags.CompletedCanceledSync);
Debug.Assert(result == CancellationRequestResult.Cancelled);
if (result == CancellationRequestResult.Cancelled)
{
operation.Complete();
}
throw;
}
}
// Requests cancellation of a queued operation.
// If the operation is not on the queue (because it already completed), NotFound is returned.
// If the operation is not executing, it is removed from the queue and Cancelled is returned.
// If the operation is executing, it stays on the queue, the operation gets marked as cancellation requested,
// and Requested is returned.
protected CancellationRequestResult RequestCancellationAsync(AsyncOperation operation, OperationCompletionFlags flags)
{
CancellationRequestResult result = CancellationRequestResult.NotFound;
if (_tail == operation) // We're the last operation
{
result = operation.RequestCancellationAsync(flags);
if (result == CancellationRequestResult.Cancelled)
{
if (operation.Next == operation) // We're the only operation.
{
_tail = null;
}
else
{
// Update tail
do
{
_tail = _tail.Next;
} while (_tail !.Next != operation);
// Update head
_tail.Next = operation.Next;
}
operation.Next = null;
}
}
else if (_tail != null) // The list is multiple operations and we're not the last
{
ref AsyncOperation nextOperation = ref _tail.Next !;
do
{
if (nextOperation == operation)
{
result = operation.RequestCancellationAsync(flags);
if (result == CancellationRequestResult.Cancelled)
{
nextOperation = operation.Next !;
operation.Next = null;
}
break;
}
nextOperation = ref nextOperation.Next !;
} while (nextOperation != _tail);
}
