UMLR — Notation for ordering action input and output pins

Input and output pins of an action in an activity are ordered, and their order corresponds to the order of the parameters of actions such as CallOperationAction or CallBehaviorAction. This is the only way pins correlate with their corresponding parameters. A Pin has no direct ownedAttribute that refers to its corresponding Parameter.

Ordering is sufficient for correlating pins with parameters, but it is difficult to visualize on a diagram. An action may have many input and output pins oriented around the action, and there is no way to visually determine which pin is associated with which parameter. In many cases, the pin name may correspond directly to the parameter name providing a visual connection. However, this is not possible for ActionInputPins or ValuePins where the pin name refers to some structural feature or value specification.

UML2 should consider a presentation option that shows pins ordered starting from the bottom right corner of the action and following clockwise around the action ending at the right bottom side. The first pin is always the target input pin for a CallOperationAction. The last pin corresponds to the return parameter if any.

This makes pin ordering visible in the diagrams, and establishes an unambiguous correlation between pins and parameters in call actions. However, it may result in some layout restrictions and extra object flow crossings in diagrams. This should not be much of an issue in practice because the pins can be moved around the edge of the action, just not reordered. And ActionInputPins allow reference to inputs without requiring explicit object flows.