Swift extension of a class ONLY when it conforms to a specific protocol

Question

Hi there =) I was just faced with a design problem where I need to (essentially) do the following:

I want to inject a bit of code on viewWillAppear: of any UIViewController subclass that conforms to a protocol MyProtocol. Explained in code:

protocol MyProtocol {     func protocolFunction() {         //do cool stuff...     } }  extension UIViewController where Self: MyProtocol //<-----compilation error {     public override class func initialize()     {         //swizzling stuff switching viewWillAppear(_: Bool) with xxx_viewWillAppear(animated: Bool)     }      //  MARK: - Swizzling      func xxx_viewWillAppear(animated: Bool)     {         self.xxx_viewWillAppear(animated)          //invoke APIs from           self.protocolFunction() // MyProtocol APIs         let viewLoaded = self.isViewLoaded // UIViewController APIs      } } 

The main issue here is that I need to 2 two things in the UIVIewController extension:

1. Invoke both MyProtocol and UIViewController API's
2. Override UIViewController method initialize() in order to be able to swizzle viewWillAppear:

These 2 capabilities seem incompatible (as of Swift 3) because:

1. We can't extend classes with conditions (i.e extension UIViewController where Self: MyProtocol)
2. if we instead extend the protocol, we CAN add conditions extension MyProtocol where Self: UIViewController but we CAN'T override methods from a class in a protocol extension, meaning we can't public override class func initialize() which is needed for swizzling.

So I was wondering if there's somebody out there who can offer a Swifty solution to this problem I'm facing? =)

Thanks in advance!!

Answer 1

You were near the solution. Just need to make it other way around. Extend protocol only if its part of UIViewController.

protocol MyProtocol {   func protocolFunction() {     //do cool stuff...   } }  extension MyProtocol where Self: UIViewController {   public override class func initialize()   {     //swizzling stuff switching viewWillAppear(_: Bool) with xxx_viewWillAppear(animated: Bool)   }    //  MARK: - Swizzling    func xxx_viewWillAppear(animated: Bool)   {     self.xxx_viewWillAppear(animated)      //invoke APIs from       self.protocolFunction() // MyProtocol APIs     let viewLoaded = self.isViewLoaded // UIViewController APIs   } }