> On Jul 10, 2016, at 1:37 AM, Károly Lőrentey via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> On 2016-07-09 04:39:01 +0000, Jordan Rose via swift-evolution said:
>
>> I wanted to share a concrete use case that Daniel Dunbar relayed to me. He
>> was working on a closed class hierarchy like the ones discussed here, where
>> all of the subclasses are within a single module, but they are all public.
>> The class also has a required initializer for dynamic construction, so that
>> they could write something like this:
>> internal struct ModelContext { /*…*/ }
>> public class ModelBase {
>> internal required init(context: ModelContext) { /*…*/ }
>> // …
>> }
>> public class SimpleModel: ModelBase {
>> internal required init(context: ModelContext) { /*…*/ }
>> }
>> public class MoreComplicatedModel: ModelBase { /*…*/ }
>> // (within some other type)
>> public func instantiateModelObject<Model: ModelBase>(_ type: Model) -> Model
>> {
>> return type.init(context: self.context)
>> }
>> That is, a public entry point calls a required initializer with an internal
>> argument type. This is the only way to instantiate Model objects, and the
>> internal context type doesn’t leak out into the public API.
>> Of course, Swift doesn’t allow this. If someone outside of the module
>> subclasses ModelBase, there’s no way for them to provide the
>> dynamically-dispatched 'init(context:)’, because they don’t have access to
>> the internal ModelContext. The author of the library has to make the
>> required initializers public, and either set the ModelContext separately or
>> make it public as well. Even though no one outside the module should be
>> using these APIs.
>
> Can you remind us why does Swift need required initializers to have the same
> access as the containing class?
Because without required initializers you can't do something like:
let cl: MyClass.Type = ...
let instance = cl.init(value: 0)
where MyClass has a required init(value: Int).
The issue here is that in order to be able to do this, subclasses that
implement another initializer must implement this one as well - so it must be
visible outside of the module if it is public.
>
> Having only package-internal constructors in the public base class is the
> usual pattern for exporting a sealed class hierarchy in some other languages,
> like Java.
>
> --
> Károly
> @lorentey
>
>
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