Overriding delegate property of UIScrollView in Swift (like UICollectionView does)

Question

UIScrollView has a delegate property which conforms to UIScrollViewDelegate

protocol UIScrollViewDelegate : NSObjectProtocol {
    //...
}
class UIScrollView : UIView, NSCoding {
    unowned(unsafe) var delegate: UIScrollViewDelegate?
    //...
}

UICollectionView overrides this property with a different type UICollectionViewDelegate

protocol UICollectionViewDelegate : UIScrollViewDelegate, NSObjectProtocol {
   //...
}

class UICollectionView : UIScrollView {
     unowned(unsafe) var delegate: UICollectionViewDelegate?
   //...
}

When I try to override UIScrollViews delegate with my protocol like so:

protocol MyScrollViewDelegate : UIScrollViewDelegate, NSObjectProtocol {
    //...
}

class MyScrollView: UIScrollView {
    unowned(unsafe) var delegate: MyScrollViewDelegate?

}

the compiler gives me two warnings:

• Property 'delegate' with type 'MyScrollViewDelegate?' cannot override a property with type 'UIScrollViewDelegate?'
• 'unowned' cannot be applied to non-class type 'MyScrollViewDelegate?'

How can I subclass UIScrollView and override type of delegate property (i.e. use a custom delegate protocol) ?

Answer 1

I think overriding an inherited property is something that's possible in Objective-C but not (at least currently) in Swift. The way I've handled this is to declare a separate delegate as a computed property of the correct type that gets and sets the actual delegate:

@objc protocol MyScrollViewDelegate : UIScrollViewDelegate, NSObjectProtocol {
    func myHeight() -> CGFloat
    // ...
}

class MyScrollView: UIScrollView {
    var myDelegate: MyScrollViewDelegate? {
        get { return self.delegate as? MyScrollViewDelegate }
        set { self.delegate = newValue }
    }
}

This way anything that calls the scroll view delegate normally still works, and you can call your particular delegate methods on self.myDelegate, like this:

if let height = self.myDelegate?.myHeight() {
    // ...
}