Event map for Transfer (and its SendTransfer / ReceiveTransfer subclasses), mapping each TransferEvent to its listener argument tuple.
Successful completion, carrying the transfer id.
Failure or reasoned cancellation, carrying the FileTransferError.
Progress update carrying the latest TransferProgress snapshot.
State transition: the new state followed by the previous one.
Event map for Transfer (and its SendTransfer / ReceiveTransfer subclasses), mapping each TransferEvent to its listener argument tuple.