[swift-evolution] Explicit Protocol Conformance for Child Classes

[Kevin Ballard]

Seems to me this doesn't actually make sense on protocols at all. The
scenario of a base superclass like UITableViewController that conforms
to a protocol but only makes sense if its children override methods
doesn't actually have anything to do with the protocol at all, as it's
certainly not true any class that conforms to
UITableViewDelegate/DataSource wants subclasses to override methods,
that's only true of UITableViewController.

Given that, the proposed @abstract is actually something that would make
sense to put on the implementation of a method in a class, not on a
protocol. There's simply no such thing as an abstract protocol. And when
put on a method, it would probably be better to just call it
@requires_override, because that's all it means.

All that said, it doesn't actually make sense to put the attribute on
UITableViewController's methods either. And the reason for that is that
if you use a storyboard with static cells, you can have a
UITableViewController that functions perfectly fine without overriding
any of the UITableViewDelegate/DataSource methods. So requiring that a
subclass implement any of those methods would actually be wrong.

So overall, there are use-cases for a @requires_override attribute, but
UITableViewController actually isn't one of them (in fact, I don't think
anything in UIKit/Foundation qualifies).

Regarding `required`, that's not appropriate because `required` means
that every single descendant class must provide an implementation. And
it's only allowed on init methods because the whole point of `required`
is to remove the special-cased behavior of init for subclassing (e.g.
that it's not inherited by subclasses). There's no other place besides
init() where it makes sense to require every single descendant class to
provide an implementation because every other method will get inherited
(and even if your base class's implementation should always be
overridden, if your child overrides it, your grandchild certainly
doesn't have to!).

-Kevin Ballard

On Sun, Dec 13, 2015, at 02:16 PM, Jacob Bandes-Storch via swift-evolution wrote:
> How about reusing the keyword "required" for this? It's already used
> (and has a similar meaning) for initializers.
>
> I think NSSecureCoding is a good example of a use case.
>
> On Sat, Dec 12, 2015 at 9:14 AM, Arthur Ariel Sabintsev via swift-
> evolution <swift-evolution at swift.org> wrote:
>> TL;DR: Force explicit protocol conformance of non-optional methods
>> for child classes on an opt-in basis.
>>
>> Currently, a child class that inherits their parent class' protocol
>> conformations are not explicitly required to override their parent
>> classes implementation of a protocol. For example, a custom class,
>> MyTableViewController, does not explicitly have to override the non-
>> optional methods of the protocols (i.e., UITableViewDataSource,
>> UITableViewDelegate) to which UITableViewController conforms.
>>
>> For many protocols, the superclass implementation of a protocol
>> method may be enough. Therefore, this feature should off by default,
>> and only enabled if the creator of the protocol requires it (for
>> whatever reason). I suggest using a new `@abstract` attribute  to
>> designate if a protocol requires explicit conformance for children
>> of a conforming parent class. This choice of naming for the
>> attribute is along the lines of what other languages call an
>> Abstract Class, which is what I'm trying to have added to Swift in
>> this fairly long-winded post.
>>
>> @abstract protocol P: class {     // All methods defined in here
>> would need to be explicitly overridden by the conforming class's
>> children. }
>>
>> If you want to have optional methods in your protocol, then using the
>> @abstract attribute gets kind of messy, as you also have the @objc
>> attribute.
>>
>> @abstract @objc protocol P: class {     // All non-optional methods
>> defined in here would need to be explicitly overridden by the
>> conforming class's children. }
>>
>> Thoughts?
>>
>> Best,
>>
>> Arthur / Sabintsev.com[1]
>>
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>
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Links:

  1. http://sabintsev.com/
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