Delete + undo does not work

[amolenaar]

Describe the bug

Deleting a model element and then performing an undo action causes the diagram item to be shown, but it's not backed by a model element.

To Reproduce Steps to reproduce the behavior:

1. Create a Class item
2. Select it and delete it
3. Undo (ctrl-z)
4. A box is drawn, but no content. E.i. the model element is missing

Expected behavior

It to work properly.

Version Version of Gaphor: 2.2.1

[amolenaar]

There are two problem scenario's.

Overtake events

• Diagram item starts unlinking
• It removes all associations, including subject
• Removing subject triggers a handler in sanitize service
• There are no more items linked to the underlaying element, so element is unlinked
• This registers element being deleted
• Now removal of the subject association is registered
• Lastly removal of the diagram item is registered

When undoing:

• first the item is created
• secondly the association to the elements is restored crash (element has not been recreated yet)

Bi-directional associations

• One end removes an element, this triggers the opposite end
• Due to the trigger at the opposite end, the original element is removed (e.g. by the sanitizer service)
• Now the initial end also sends a remove event

When this is undone, first it's trying to create a relation with an element that no longer exists.

[amolenaar]

The solution proved simpler. The undo events need to be ordered: create elements, then set properties and lastly unlink (remove) elements. This is basically the same order we use for loading models and copy/paste functionality.

The options above has the big disadvantage that events could only be sent after all effects were applied. This made it hard to undo the effects. Now undoing can still be done close to where the event happened. The undo mechanism only had to be a little smarter.