SourceQueue

Offers an element to a stream and returns a Future that:

• completes with Enqueued if the element is consumed by a stream
• completes with Dropped when the stream dropped the offered element
• completes with QueueClosed when the stream is completed whilst the Future is active
• completes with Failure(f) in case of failure to enqueue element from upstream
• fails when stream is already completed

Additionally when using the backpressure overflowStrategy:

• If the buffer is full the Future won't be completed until there is space in the buffer
• Calling offer before the Future is completed, in this case it will return a failed Future

Returns a Future that will be completed if this operator completes, or will be failed when the operator faces an internal failure.

Note that this only means the elements have been passed downstream, not that downstream has successfully processed them.