sequence diagram: allow message to span beyond the involved participants

[kplatt1]

In a sequence diagram, if you have a long message between 2 participants, PlantUML tries to fit the message in the horizontal space between the 2 participants, and this can cause a big and unnecessary gap that wastes horizontal space, for example:

Instead, it would be much better to allow the message to span beyond the 2 involved participants and save horizontal space. There's plenty of horizontal space to put the message text above the arrow and going all the way across as far as it needs to, like so:

Not only does it save space, but it is common practice when drawing UML sequence diagrams to extend the message text outside of the region between the 2 participants.

PlantUML doesn't support this that I'm aware of. If this feature were supported, you could set up a diagram without having to worry about either (a) artificially adding a newline character \n somewhere in the message text, or (b) setting an artificial wrap limit on the text width via skinparam maxMessageSize 100 as in Issue #35. One of the big conveniences of PlantUML, especially for sequence diagrams, is that it drastically reduces the need for people to worry about layout and format. But without this feature, people have to hand-tune the layout.

I think the proposed behavior should be the default, but if someone wants to force the old behavior, they can specify a parameter to let them do so.

[kplatt1]

I'm honestly shocked that the proposed behavior is not supported. In just about every sequence diagram that I've created with PlantUML, I've run into this issue, and had to put a \n somewhere in the message text to make it less wide and waste less space.

[arnaudroques]

Ok, let’s give this a try!

With the latest snapshot:

• https://github.com/plantuml/plantuml/releases/tag/snapshot

You need to enable the teoz layout engine and activate sequenceMessageSpan.

Here is an example:

@startuml
!pragma teoz true
!pragma sequenceMessageSpan true

participant ParticipantNumber1
participant ParticipantNumber2
participant ParticipantNumber3

ParticipantNumber1 -> ParticipantNumber2 : this is a very long message that is very long
ParticipantNumber2 -> ParticipantNumber3 : foo2
@enduml

This is still very experimental and has only been lightly tested.
There will certainly be side effects, so we would really appreciate your feedback!

[kplatt1]

Thanks for that fix!
It works EXCEPT for self-messages. Self-messages still cause a big gap to form to the right of the self-message.
Is that fixable?

[arnaudroques]

Thanks for that fix! It works EXCEPT for self-messages. Self-messages still cause a big gap to form to the right of the self-message. Is that fixable?

Thanks for the report! Self-messages should be handled in the latest snapshot.

@startuml
!pragma teoz true
!pragma sequenceMessageSpan true

participant ParticipantNumber1
participant ParticipantNumber2
participant ParticipantNumber3

ParticipantNumber1 -> ParticipantNumber1 : this is a very long message that is very long 
ParticipantNumber2 -> ParticipantNumber3 : foo2

@enduml

There may still be rough edges with notes and a few corner cases, so if you hit anything odd, please share a minimal example and we will take a look.

[kplatt1]

The self-message fix seems to work as well, thanks again!
I'll let you know if I run into anything weird...