This is something that was discussed some time ago but never resolved.
To summarize:
When working with CollectionValueHolder one can only see whether the collection as a whole changed and not how, e.g.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
col := OrderedCollection new asValueHolder.
col add: 1.
col add: 2.
col add: 3.
col add: 4.
col whenChangedDo: [ :c :changedValue |
1halt.
].
"col = #(1 2 3 4)"
col add: 4. "whenChangedDo: c = col, changedValue = 4"
col remove: 4. "whenChangedDo: c = col, changedValue = 4"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the last two cases the changedValue is the same so I can't tell from the block if the value was added or removed.
My proposition:
* introduce (at least) two more Announcements: ValueAdded and ValueRemoved
* add messages #whenAddedDo:/#whenChangedDo:
* maybe add #when:do: so it can be used as an announcable object (valueHolder when: ValueSomething do: [ :ann | ... ])
The ValueAdded/ValueRemoved would be fired when an item is added, resp. removed.
Thierry also proposed to add an index (I'd make it optional).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
col whenAddedDo: [ :col :addedValue :optionalIndex | "here we know for sure that value was added" ].
col whenRemovedDo: [ :col :addedValue :optionalIndex | "here we know for sure that value was removed" ]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Finally Thierry mentioned a way to handle situations where values are replaced (either single one or multiple ones). As this is edge use-case and the current behavior is pretty much random I wouldn't address the replacement unless a defined use case emerges.
Any thoughts,
Peter
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