Hello,
I'm rather new in the PlantUML forum, but after exploring the Sequence Diagram syntax, I've come to conclusion that there is no such option to send messages while current slanted arrow is in progress.
I'm working with kind of message queue framework(*) and sometimes commands are not "dequeued" immediately because of current assignment which is still in progress. In such case, the command will be dequeued only when current one is done.
But this doesn't mean new messages cannot be sent meanwhile. Take a look at the following image:
I understand my issue solving might mess with the diagram timing, but this may be solved if slanted arrows degree is based on time steps rather than plain pixels.
Either way, I'm sure it will be pretty useful.
Cheers,
Ran
(*) A message queue framework is best explained as a system made of multiple independent state machines, each is designated for specific set of actions. The communication between state machines is made by queueing commands between one and other and a new queue command is processed only after completing the last one. Any state machine can send queues (Enqueueing) to any other, but for any "communication channel" there is only one target state machine - dequeuer.
I guess the implementation may vary between software languages, but believe the core idea is the same. I'm using LabView programming language to implement it.