> On Mar 24, 2017, at 10:09 AM, Douglas Gregor <[email protected]> wrote:
>
>> I'm actually not worried about methods so much as properties. KVC is
>> completely untyped on the Objective-C side, and there are several different
>> mechanisms there which use KVC with poorly validated external strings, like
>> bindings, sort descriptors, and predicates. Tons of migration errors are
>> going to escape into production if we do this,
>
> We can avoid these by migrating conservatively (have the migrator add @objc
> everywhere it’s inferred in Swift 3).
We can do that, but personally, I really hate these kinds of conservative
migrations. It might be unavoidable, though.
>> Have you considered a deprecation cycle (for instance, having Swift 4 thunks
>> log a warning that they're going away in Swift 5)?
>
> I think Swift 3 -> Swift 4 is the deprecation cycle, no?
But there was no indication during Swift 3 that this feature was going away. As
I understand it, a deprecation cycle introduces advanced warning of a change so
you have time to prepare for it; that's not available here.
My concern is that, because the tools are not really aware of KVC, we can't
count on the compiler to lead developers to missing `@objc` properties. Folks
are only going to find those mistakes through testing, and they're inevitably
going to miss a few spots. So some poor schmuck is going to migrate their code
to Swift 4 without realizing this is an issue at all, accidentally miss a few
spots in their testing, ship it, and have to deal with weird crashes out of
nowhere. They're going to say, "My code worked just fine before. Swift 4 broke
it!" And they won't be wrong.
I'd be more comfortable with a version-long deprecation cycle that gave
developers plenty of time to notice these bugs. Failing that, I'd at least like
to see them get backtraces containing a symbol name like
`YouCantInvokeASwiftMemberThroughTheObjectiveCRuntimeUnlessItsMarkedWithAtObjc`
so the nature of the problem and its solution will be more obvious.
(Preferably, this function would log the instance and selector, so if people
got both the logs and the backtrace, the diagnosis would be as simple as we can
make it.)
(Actually, I wonder if we could install a `-doesNotRecognizeSelector:` override
in Swift classes which looked for a matching member in the Swift runtime
metadata and, if it found one, called the `YouCantInvoke…` function? That would
be lower overhead than generating stubs at compile time, and the slowness of
searching the runtime metadata wouldn't matter much since it was going to crash
anyway. I'm not sure if it might remove useful information from the backtrace,
though. Maybe in Swift 5, when these bugs will be more rare. Or maybe in
`SwiftObject`.)
> Plus, inheritance from an Objective-C class is often incidental: you do it
> because you need an NSObjectProtocol conformance, or something else expects
> NSObject. I haven’t heard of developers inheriting from NSObject solely to
> get @objc inference for their members.
You do it because you need a particular object to interact with Objective-C. In
that circumstance, I don't think the compiler is wrong to assume that you want
to expose as many members as possible to Objective-C.
>> you already have to specify `dynamic` to avoid optimizations;
>
> Conceptually, ‘dynamic’ is orthogonal to ‘@objc’. In today’s implementation,
> we can only implement ‘dynamic’ via the Objective-C runtime, hence this
> proposal’s requirement to write both.
I understand that, but again, I think it's defensible for the compiler to
assume that, if you want dynamic behavior in a class where you've already
enabled Objective-C interop, you probably want that dynamic behavior to be
compatible with Objective-C.
I guess we just take different standpoints on Objective-C interop. My belief is
that, if you state an intention to have a type interoperate with Objective-C,
Swift should try to expose as many of its members to Objective-C as possible. I
think you believe that Swift should expose as *little* as possible to
Objective-C.
Because of that difference, I actually think I'd be *more* likely to support
removing inference by requiring an explicit `@nonobjc` on members of
Objective-C-compatible classes which aren't compatible with Objective-C. That
is, writing:
class Foo: NSObject {
var bar: Int?
}
Is an error; you have to write:
class Foo: NSObject {
@nonobjc var bar: Int?
}
I don't really like that answer very much, but I like it more than I would like
requiring `@objc` if `bar` were a plain `Int`.
>> Would you like the request for bridging notarized and filed in triplicate?
(By the way, in reading this later, I realized this part might have sounded a
little too angry. I was trying to be funny. Nobody complained, but I'm sorry if
I missed the mark here.)
--
Brent Royal-Gordon
Architechies
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