objc async selectors

07 Dec 2024

I very recently learned it the hard way that objc selectors can not be async.

In simple words, this will crash at runtime:

let button = UIButton(frame: buttonFrame)
view.addSubview(button)
button.setTitle("This is a button", for: .normal)
button.addTarget(
  self,
  action: #selector(handleTap), 
  for: .touchUpInside
)
  
@objc func handleTap() async {
  Task {
    debugPrint("handle Tap")
  }
}

No warnings, no errors, no messages on console, nothing. Just crash!

As it turns out all objc selector marked as async will crash. For example:

 perform(#selector(handleTap))

So what is going on here. Let’s first print the selector generated by the compiler for async methods.

debugPrint(NSStringFromSelector(#selector(handleTap)))
// "handleTapWithCompletionHandler:"

As per swift objc interops guidelines, Swift maps all objc methods with suffix "WithCompletionHandler" as `async. So these two are equivalent:

- (void)stopRecordingWithCompletionHandler: void(^ _Nullable)(RPPreviewViewController * _Nullable, NSError * _Nullable)handler;

func stopRecording() async throws -> RPPreviewViewController

So our func handleTap() becomes - (void) handleTapWithCompletionHandler:(CompletionHandler completionHandler) in objc.

The next question naturally is what is the type of CompletionHandler?

I tried to search online if others have also faced this problem. And yes there are quite a number of discussions on this topic. This one even has nice a workaround to the problem:

let completion: @convention(block) () -> Void = {
  debugPrint("completion handler")
}

perform(#selector(handleTap), with: completion)

// prints:
// completion handler
// handle Tap

@convention is a attribute provided by swift for calling conventions in Swift. And @convention(block) is the attribute to represent Objective-C blocks as Swift closures.

Now, with all this acquired knowledge can we fix our crash? We still can’t use the #selector(handleTap) for target action pattern because that pattern is very well documented to support only following method signatures:

func doSomething()
func doSomething(sender: Any)
func doSomething(sender: Any, forEvent event: UIEvent)

Here the sender and event is autofilled by objc runtime while invoking the provided selector. And as you can see there is no place to provide arbitrary arguments. But for this entire mechanism to work we want the sender to be of type @convention(block) () -> Void.

So we can create our own custom UIControl subclass and hijack the target-action mechanism to update the sender. In other words, we need to override the sendAction() method and then call the performSelector.

class MyButton: UIButton {
  override func sendAction(_ action: Selector, to target: Any?, for event: UIEvent?) {
    let sender: @convention(block) () -> Void = {
      debugPrint("completion handler")
    }
    (target as? NSObject)?.perform(action, with: sender)
  }
}

class ViewController: UIViewController {
  
  override func viewDidLoad() {
    super.viewDidLoad()
    let buttonFrame = CGRect(
      x: 20,
      y: (view.bounds.height * 0.5) - 30,
      width: view.bounds.width - 40,
      height: 60
    )
    let button = MyButton(frame: buttonFrame)
    view.addSubview(button)
    button.setTitleColor(.black, for: .normal)
    button.setTitle("This is a button", for: .normal)
    button.addTarget(
      self,
      action: #selector(handleTap),
      for: .touchUpInside
    )
  }
  
  @objc func handleTap() async {
    Task {
      debugPrint("handle Tap")
    }
  }
}

And that is how you can have a button action as async.

Whacky Labs

objc async selectors

References