@MainActor Doesn't Bridge from Objective-C

Apr 27, 2026 |

This is something I wanted to verify myself after seeing AI suggestions claim otherwise.

Marking a Swift delegate method @MainActor does not cause an automatic hop when called from Objective-C. The annotation is invisible to the ObjC runtime.

// ObjC calls the delegate from a background thread
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [self.delegate testerDidReceiveCallbackWithMessage:@"hello"];
});

extension MyManager: @MainActor ObjCBridgeTesterDelegate {
    @MainActor
    func testerDidReceiveCallback(withMessage message: String) {
        print(Thread.isMainThread) // false — still on the background thread
    }
}

@MainActor is a Swift concurrency construct enforced at compile time. The ObjC runtime dispatches calls using objc_msgSend and has no concept of actors — it just invokes the method on whatever thread it’s already running on.

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