[Dev] collections and notifications

[Alec Flett]

John Anderson wrote:
>> On the collections API, you've switched a create at construction time 
>> model. Does this mean it will be impossible to restructure the 
>> collection tree, short of destroying a subtree and constructing an 
>> entirely new one?
>>
>> Yes, that will be impossible without destroying and re-constructing. 
>> The way the code was written before wouldn't have worked anyway, the 
>> hard part here is that when a collection tree is modified in place, a 
>> large, potentially huge, number of notifications have to be sent 
>> around (at least to maintain upstream indexes). None of that was in 
>> place in the old code, except for the add/removeSource method in 
>> collections.py, which is still there and still works as before. Once 
>> we have a use case for such in-place collection structure 
>> rearranging, beyond what is implemented and functional today, we can 
>> reconsider this.
> I think we could get away with not sending any notifications when 
> rearranging the collection tree and recompute everything after the 
> rearrangement. Independent of whether or not the old code actually 
> worked, I suspect not allowing rearrangement will end eventually end 
> up biting us, but I don't think we should work on it until we end up 
> with a use case.

+1 - trying to optimize for rearranging/restructuring the collection 
tree seems...well, non-optimal. We currently have no use case or 
requirement for this. The way-off-in-the-future rule builder is about 
the closest thing to a use case than I can think of, and I don't think 
it would suffer from destroying/recreating a collection tree.

Alec