Hi Joe
Thanks for your input on this.
> This is likely due to the runtime not handling cycles properly in type
> metadata initialization—a subclass T needs its base class metadata for
> BaseObject<T>, which in turn needs the metadata for T to instantiate the
> generic type. This is something we plan to fix next year since it requires
> runtime ABI changes.
Hmmm. I'm finding this really frustrating, waiting for features that I have
been using for years, in another popular language (C#), to see the light of day.
Maybe application programmers are happy with Swift so far but, when it comes to
my speciality of designing frameworks, I am constantly finding myself blocked
by lack of metadata APIs :-(
I am talking about a couple of frameworks that underpin one of the largest
business management systems in Europe, for which the client is asking a
MacOS/iOS version.
> Would a protocol-based approach suffice? `Self` in a protocol extension would
> give you access to the concrete type in a similar way:
>
> protocol Base: AnyObject {
> var properties: [PartialKeyPath<Self> : AnyProperty] { get }
> }
>
> extension Base {
> func value<T>(for keyPath: KeyPath<Self, T>) -> T?
> {
> guard let property = properties[keyPath] else
> {
> return nil
> }
>
> return property.getValue()
> }
>
> /*etc.*/
> }
>
> class Test: Base {
> let properties = [\Test.name: Property<String>()]
>
> var name: String {
> get { return value(for: \.name) }
> set { set(value: newValue, for: \.name) }
> }
> }
At first glance, I thought, Yes! But then I realised that this then requires a
final class to cater for the use of Self in the protocol; something that would
be a blocker for the class hierarchies that most users of the framework would
want to create on top of the Base functionality.
I suppose I could look at using protocol-oriented composition but trying to
avoid associated types in protocols in frameworks that use generics extensively
is akin to hitting yourself repeatedly on the head with a hammer ;-)
And I really don't want to have to keep on repeating type-erasure boilerplate
code to achieve that.
I think the realisation that I may well lose the chance of a pretty massive
contract and that my client will lose an important business opportunity is
finally dawning :-(
Joanna
--
Joanna Carter
Carter Consulting
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