UMLR — Need more flexible notation for activity partitions

Activity diagrams currently support two notation styles for denoting membership in a partition: horizontal or vertical, and possibly nested "swimlanes" and a parenthesized list of partition names displayed above an activity node. These notation styles are adequate in many cases, but can result in diagrams that are either difficult to layout or don't sufficiently display relationships between nodes that are in the same partition. In addition, control nodes often shouldn't belong to any partition at all, but may need to be displayed in a partition with adjoining nodes for diagram layout purposes. In this case, apparent inclusion in the partition on the diagram should not necessarily imply membership in the partition.

One way to address these issues would be to include additional notation styles for partitions. For example, a partitions could be displayed as in concentric, overlapping circles, ovals, or rounded rectangles with activity nodes in the partitions displayed inside the partition shape. The same partition could be displayed many times on the same diagram allowing separation of members in the partition for diagram layout purposes.

Control nodes often connect activity nodes across partitions and can have a strong influence on diagram layout. Having control nodes treated like other activity nodes in partitions can result in overly restrictive layout constraints, or accidental modification of the activity model for diagram layout purposes. For control nodes, perhaps the default should be that they are not placed in partitions unless explicitly stated by some tool action, and they could have a more restricted notation, such as requiring the parenthesized list only. This would allow control nodes to be freely interspersed between elements in a partition without necessarily belonging to that partition and keep diagram layout concerns from accidentally modifying the underlying activity model.