Working with modeling tools that provide different views on the same model (like UML with diagrams) can be an interesting experience. The relationship between model and views introduces some challenges for usability, which is addressed differently by tools. These aspects can easily confuse those that do their first steps in modeling and annoy those that work with several tools. I suggest that you might want to have some checklist for your tool for quick reference to navigate these quirks. In a number of blog posts, I will propose and explain this checklist.

For illustration, I am working with Papyrus/MDT. Papyrus at the time of writing is available in version 0.7.0. Usability might change, but Papyrus is only used to illustrate the checklist, not to comment on the tool itself.

Checklist: In your tool, what commands are used for local deletion (in diagram) and from model.

Assume we have a small UML model with three classes, RainSensor, RainSensor Variant and SunRoof in two diagrams A and B.

Diagram A

Diagram B

It is important to remember, that diagrams are only views on the model. The actual model can be seen in the tree view. Obviously we should only find one RainSensor in the model.

Model Viewer

Diagrams are views on the model.  This has a view implications on usability. If we have a look on the context menu of Papyrus, we see two commands for deletion:

Deletion Commands

These two kinds of deletion commands can be found with all kinds of different naming conventions in graphical modeling tools. 2) will remove the symbol from the diagram without modifying the model. 1) will actually remove the model element. If you try 2), the model will not be changed. However, this is how the screenshots change after you choose 1): Since the model element has been deleted, its symbol is removed from Diagram B.

Don’t confuse the two, you might accidentally remove elements. It would be great if your tool had functionality to show you the impact of a deletion (the impact of any change would even be better).

Model elements can also be deleted by using the pop-up from the tree view:

Checklist: Check the semantics of Undo

If you try the above in any of your tools, try the undo function as well and see what your tool does. If you try the steps above and use undo after deleting from within the diagram, you will get the following results:

and, unexpectedly:

This tool does not undo any side effect a change has on other diagrams! Be sure to check the semantics of undo, to make sure you do not loose any data.

Checklist: Changes on closed diagrams

If you close the diagram (i.e. it is not open for editing), changes can have a different semantics. Again, deleting the model element RainSensor from within Diagram A, but having closed Diagram B results in the expected deletion of the model element. This is how B looks after opening it again:

Suddenly, there is an element “model” in there? What happened? Tools that do not actively update closed diagrams when the model changes, have to check the consistency of diagram and model upon opening. This specific tool seems to follow the strategy to replace references to non existing model elements with references to others. Check what your tool does!

Checklist: Collaboration

Several people working on one model at the same time opens another can of worms. Without going into detail: Check how deletions by one person are propagated throughout the system.

Checklist: Find unreferenced elements

You can easily imagine, that you might delete an element from the diagram, actually forgetting about it. But it is still there in the model. Does your tool have functionality to find unreferenced elements? You wouldn’t want to browse through all the diagrams just to find the unused items.

The next post on the topic will be on element renaming.