通过协议扩展将选择器与协议方法的默认实现一起使用

Question

I am attempting to make a Protocol that ViewControllers can implement to adjust their view to accommodate keyboard show/hide.

protocol KeyboardAdaptable {
    func keyboardWillShow(notification: NSNotification)
    func keyboardWillHide(notification: NSNotification)
    func addKeyboardNotificationObservers()
}

extension KeyboardAdaptable where Self: UIViewController, Self: NSObject {
    func keyboardWillShow(notification: NSNotification) {
        if let keyboardSize = (notification.userInfo?[UIKeyboardFrameBeginUserInfoKey] as? NSValue)?.cgRectValue {
            if self.view.frame.origin.y == 0{
                self.view.frame.origin.y -= keyboardSize.height
            }
        }
    }

    func keyboardWillHide(notification: NSNotification) {
        if let keyboardSize = (notification.userInfo?[UIKeyboardFrameBeginUserInfoKey] as? NSValue)?.cgRectValue {
            if self.view.frame.origin.y != 0{
                self.view.frame.origin.y += keyboardSize.height
            }
        }
    }

    func addKeyboardNotificationObservers() {
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillShow), name: NSNotification.Name.UIKeyboardWillShow, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillHide), name: NSNotification.Name.UIKeyboardWillHide, object: nil)
    }
}

The error: "Argument of #selector refers to instance methods 'keyboardWillShow' that is not exposed to Objective-C."

I know that selectors are a feature of Objective-C and that referenced functions must be compatible. I tried to solve this by marking the protocol itself as well as the methods with the @objc annotation, but then the compiler insisted I mark the default implementations in the protocol extension with @objc as well. When I did that, it yelled at me to remove the @objc annotation because "@objc can only be used with members of classes, @objc protocols, and concrete extensions of classes" (ie not in protocol extensions?)

Does anyone know of a way to achieve this goal? I know at first blush it might seem like there is no way around it, but I also know that UIViewController is a child of NSObject, and typically instance methods on UIViewControllers are allowed to be targets for selectors. I thought by placing constraints on my protocol extension requiring it be a subclass of UIViewController, that I could target the default implementations contained therein with selectors.

Thoughts?

Answer 1

Can't be done.

Everyone first thinks this would be a cool idea, (myself included) but it simply can't.

Swift protocol extensions are invisible to Objective-C. Can't do anything about that. Even with #selector doesn't work because:

Your protocol functions are only defined in the protocol extension.

People keep trying to inject Objective-C-callable functionality (selector, delegate method, whatever) into a class via a protocol extension. Sorry.

Answer 2

I fully agree with the previous answer - this can't be done. However I decided to fool around a bit with your code and create the most acceptable solution I could find. Maybe it will come in handy:

class KeyboardAdapter {

   private weak var view: UIView!

   init(view: UIView) {
      self.view = view
      addKeyboardNotificationObservers()
   }

   @objc
   func keyboardWillShow(notification: NSNotification) {
      if let keyboardSize = (notification.userInfo?[UIKeyboardFrameBeginUserInfoKey] as? NSValue)?.cgRectValue {
         if self.view.frame.origin.y == 0 {
            self.view.frame.origin.y -= keyboardSize.height
         }
      }
   }

   @objc
   func keyboardWillHide(notification: NSNotification) {
      if let keyboardSize = (notification.userInfo?[UIKeyboardFrameBeginUserInfoKey] as? NSValue)?.cgRectValue {
         if self.view.frame.origin.y != 0 {
            self.view.frame.origin.y += keyboardSize.height
         }
      }
   }

   private func addKeyboardNotificationObservers() {
      NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillShow), name: NSNotification.Name.UIKeyboardWillShow, object: nil)
      NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillHide), name: NSNotification.Name.UIKeyboardWillHide, object: nil)
   }
}

protocol KeyboardAdaptable: class {
   var keyboardAdapter: KeyboardAdapter! { get set }
   func configureAdapter()
}

extension KeyboardAdaptable where Self: UIViewController {
   func configureAdapter() {
      keyboardAdapter = KeyboardAdapter(view: self.view)
   }
}