Re: Strange XCode Warning
Re: Strange XCode Warning
- Subject: Re: Strange XCode Warning
- From: Kyle Sluder <email@hidden>
- Date: Tue, 15 Sep 2015 14:45:46 -0500
On Tue, Sep 15, 2015, at 01:23 PM, Jens Alfke wrote:
>
> > On Sep 15, 2015, at 10:52 AM, Kyle Sluder <email@hidden> wrote:
> >
> > Class<ProtocolX> really is a Class (instance of metaclass).
>
> That seems to be true, based on a bit of testing I just did. But I don’t
> think a type like that makes sense, since it says that the metaclass
> implements the protocol, which isn’t true; the metaclass would have to
> implement the “meta-protocol”, and I don’t know if such an entity exists
> explicitly in Obj-C. (I can’t rely on analogy to Smalltalk-80 because
> Smalltalk didn’t have protocols.)
>
> For example, if a Class implemented NSCoding that would imply the
> metaclass had methods like encodeWithCoder:, which isn’t true.
Recall that class methods are simply instance methods on the metaclass.
So Class<SomeProto> makes a statement about what messages you can send
to the class: the ones that are prefixed with a + in the protocol
definition.
So while Class<NSCoding> isn't a useful type because NSCoding doesn't
define any class methods, Class<NSSecureCoding> is useful because it
tells you that the class object responds to
@selector(supportsSecureCoding).
>
> > Check out UIAppearance.h; that's how containment-based appearance works.
>
> Hm, I don’t see anything related to metaclasses there, just protocols
> with class methods.
See the type for the argument to +appearanceWhenContainedIn:. It's a
Class<UIAppearanceContainer>. Not an explicit mention of metaclasses,
but at least proof that we use the Class<SomeProto> syntax to refer to
classes, not instances.
--Kyle Sluder
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