Annotate Objective-C Block Type with NS_SWIFT_UI_ACTOR

Using Swift
1

Hi folks,

I'm trying to update some Objective-C code to be friendlier with respect to Swift Concurrency. In this case, I have a protocol definition like this:

@import Foundation;

typedef void (^FooAction)(void);

@protocol Foo
@property (nonatomic, nullable) FooAction didDoFoo;
- (void)doFoo;
@end

I have a Swift class in another module that needs to be able to conform to that protocol. In this case, I need to attribute FooAction with @MainActor to express the guarantees of the callers from Objective-C.

class Bar: NSObject, Foo {
    var didDoFoo: FooAction?

    func `do`() {
        Task {
            await didDoFoo?()
        }
    }
}

No matter where I try attributing NS_SWIFT_UI_ACTOR to the FooAction typedef, if I create an instance of Bar and try setting didDoFoo, I get errors that "converting function value...loses global actor 'MainActor'". Is this interaction between Objective-C and Swift supported, and if so, what is the proper way to attribute an Objective-C typedef for the main actor?

2

Currently, that's only recognized on parameters, but putting it on the typedef would be extremely reasonable, and it's probably the only reasonable way we could express that the property getter returns a @MainActor function. I've filed [SR-15885] NS_SWIFT_UI_ACTOR isn't recognized on block typedefs · Issue #58150 · apple/swift · GitHub to track this.

I don't think there's a pure-Swift workaround that doesn't look totally awful, but you can put helper functions in your bridging header that work within the limits of what the import logic can handle:

static inline void setDidDoFoo(_Nonnull id<Foo> foo,
                               _Nullable FooAction action NS_SWIFT_UI_ACTOR) {
  foo.didDoFoo = action;
}

NS_SWIFT_UI_ACTOR
static inline void callDidDoFoo(_Nonnull id<Foo> foo) {
  FooAction action = foo.didDoFoo;
  if (action) action();
}
1 Like
3

Thank you John. I'm actually relieved to hear it's just an oversight. I'll make a note of this in my team's documentation.