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
