[swift-evolution] Specified Protocol Conformances in Subclasses

[Adrian Zubarev]

I kinda feel this is somehow related: http://discourse.natecook.com/t/pitch-return-a-subclass-for-a-protocol-method-without-the-need-for-an-associatedtype/2355

It would be really handy to be able to override any type A with another type B where the relationship is B : A.



-- 
Adrian Zubarev
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Am 10. März 2017 um 13:08:37, Rod Brown via swift-evolution (swift-evolution at swift.org) schrieb:

Hi everyone. I found something odd that seems baked into how Cocoa Touch does protocols, but Swift cannot model it:


@interface UIScrollView: UIView

@property (weak, nonatomic) id <UIScrollViewDelegate> delegate;

@end

@interface UITableView: UIScrollView

@property (weak, nonatomic) id <UITableViewDelegate> delegate;

@end

@protocol UITableViewDelegate: UIScrollViewDelegate
...
@end



Subclasses can further specify the conformance of a property’s protocol-conforming object to state a further type on that property. I tried to do something extremely similar today in Swift, and it failed, saying the protocols were different:


class SourceBar: UIScrollView {

override var delegate: SourceBarDelegate? {
get { return super.delegate as? SourceBarDelegate }
set { super.delegate = newValue }
}

}  


@objc protocol SourceBarDelegate: UIScrollViewDelegate {
func foo()
}


Considering the fact that the protocol SourceBarDelegate conforms to UIScrollViewDelegate, I can’t see why this override should fail. Can anyone enlighten me as to why this is a limitation in the language?

Thanks,

Rod
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