I’m using solution similar to yours for a while.
While using it I notice one problem. If you copy DelegatedCall struct you can call closure outside of owner.

Eg:

class Controller {
 
     let downloader = ImageDownloader()
     var image: UIImage?
 
     init() {
         downloader.didDownload.delegate(to: self) { (self, image) in
             self.image = image
         }
     }
 
     func updateImage() {
         downloader.downloadImage(for: /* some image url */)
     }
     func trickDownloader() {
         var callback = downloader.didDownload
         collback.callback?(NSImange()) // This invokes closure from init method
     }
}

Did you spot this problem and if so, do you have any solution to fix that?