[swift-evolution] Custom didSet methods

[cocoadev at charlessoft.com]

did/willSetAtIndex could be redundant, since that information could be 
equally well expressed with a range having a length of 1.

The other thing it's missing is a way to tell what kind of change it 
was. If something were inserted or removed such that the number of items 
in the collection were changed, you'd need to know about it to avoid 
having to redraw whatever view correspond to every single element in the 
collection coming after the index you've changed.

What KVO did was to offer an NSKeyValueSetMutationKind enum containing 
constants for insertion and removal as well as OR and AND operations 
with other sets. If you don't want to do that, another possible 
interface could be to provide two ranges, one representing the range of 
the affected region before the change, and another representing it 
afterward. If you had an empty range for "before" and a non-empty range 
for "after", that would represent an insertion. The reverse would be 
removal, and anything else would be a replacement of some kind.

Charles

On 2016-01-12 22:04, David Sweeris wrote:
> What about did/willAdd for all collection types, did/willSetAtIndex
> for collection types with indexes, and
> did/willSetAtIndexRange for collection types that support subscripting
> with ranges?
> 
> - Dave
> 
>> On Jan 12, 2016, at 13:26, Charles Srstka via swift-evolution 
>> <swift-evolution at swift.org> wrote:
>> 
>> Currently, didSet already gets called when a set property is mutated. 
>> The only difference I see in this proposal is that it lets you know 
>> more precisely what changed, so that you can more efficiently update 
>> the UI to reflect the change; i.e. if each item is represented by a 
>> view object, and you insert an object, the redraw only needs to occur 
>> for the view corresponding to the newly inserted item instead of 
>> having to recreate all the views.
>> 
>> Seems reasonable to me. KVO currently provides this facility in 
>> Objective-C.
>> 
>> Charles
>> 
>>> On 2016-01-12 07:33, 肇鑫 via swift-evolution wrote:
>>> I don't know if this is a good idea. If you insert an item to an
>>> array, what should happen? The items after the inserted item should 
>>> be
>>> considered as removed or not? They indexes are changed but the values
>>> not.
>>> zhaoxin
>>> On Tue, Jan 12, 2016 at 7:24 PM, James Campbell via swift-evolution
>>> <swift-evolution at swift.org> wrote:
>>>> When declaring a collection, it would be nice to have a way of
>>>> knowing when an item is set/added and removed.
>>>> For example you could trigger a layer to be added to the layer
>>>> hiearchy.
>>>> var shapeLayers: [CAShapeLayer] {
>>>> didAdd(item) {
>>>> addSubLayer(item)
>>>> }
>>>> willRemove(item){
>>>> item.removeFromSuperlayer()
>>>> }
>>>> }
>>>> --
>>>>  Wizard
>>>> james at supmenow.com
>>>> +44 7523 279 698 [1]
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution [2]
>>> --
>>> Owen Zhao
>>> Links:
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