No `update` event emitted for some slot creations and removals

[asolove]

In a MultiArgMorph showing an arbitrary list of upvars or strings:

• When you use the < and > buttons to add and remove slots, an update event is fired so the surrounding context can react.
• When you use the right-click menus to delete and insert slots, no update event is fired.

Reproduction steps

• Define a language with a block that has an input of type %mult%upvar or %mult%s
• Add an update event handler to the top-level IDE_Morph
• Drag in an instance of the block and create multiple values:
• Delete the last one by using the < button
• Notice that the update event handler fires
• Delete the next-to-last one by opening the context menu and clicking "delete variable"
• Notice that the UI updates as before, but not update event is fired.

Research notes

• A similar issue also happens for adding a new item to the multi-arg block, where if you use the > arrow, an update is emitted, but if you right-click and select "insert a variable", it is added in the UI but not update event is emitted.
• The issue seems to happen whether the embedded item is an upvar (as it was in my initial testing) or a plain string slot, as I've since tested with.

How to fix

I tried debugging through this to understand why the event gets thrown in one case and not another, but can't quite find it. I think it might just be a side-effect of the first case deleting and re-creating the FrameMorph wrapping the arrows.

There are some code comment notes about a deliberate difference between remove and delete, so I wanted to ask before making arbitrary changes.

Tracing through what happens yielded:

• When you click the < button at the end of the list to remove the last item...

  • This is handled by MultiArgMorph#removeInput
  • Which then triggers a removeChild on both a TemplateSlotMorph and a FrameMorph
  • It also triggers an update event so we can re-run code generation to take account of the removed item.

• When you right-click and delete the last item in the list

  • This is handled by MultiArgMorph#deleteSlot
  • Which only calls removeChild for the TemplateSlotMorph and not the FrameMorph
  • It does not trigger an update event, so the surrounding context can't update because of the removed item.

[jmoenig]

Thanks, Adam, good catch! This is going to be a quick and easy fix, I think. I'm all busy and traveling this week, but hope to get around to fixing it either over the weekend or early next week.