Swift override protocol methods in sub classes

Question

I've a base class that implements an extension that conforms to a protocol as below:

protocol OptionsDelegate {
    func handleSortAndFilter(opt: Options)
}

extension BaseViewController: OptionsDelegate {
    func handleSortAndFilter(opt: Options) {
        print("Base class implementation")
    }
}

I've a subclass "InspirationsViewController" that inherits from BaseViewController. And I'm overriding protocol method in the extension as below:

extension InspirationsViewController {
    override func handleSortAndFilter(opt: Options) {
        print("Inside inspirations")
    }
}

I'm getting error when I override "handleSortAndFilter" function in subclass extension: "Declerations in extensions cannot override yet"

But I'm not seeing similar problem when I implemented UITableView datasource and delegate methods.

How to avoid this error?

`InspirationsViewController` is not a subclass, its an extension. I think you defined it wrong. It should be a class `InspirationsViewController: BaseViewController` — Hossam Ghareeb, Dec 27 '16 at 11:31
@HossamGhareeb he already said `InspirationsViewController` is subclass of `BaseViewController` , this is extension for it. This problem is probably haven't got implemented in Swift, you should do override from main class, extension is for adding more function — Tj3n, Dec 27 '16 at 11:34
In extension we can not override methods of the super class. — Aman Gupta, Dec 27 '16 at 11:39
Please post the class definitions also. The code compiles fine here adding `class BaseViewController: UIViewController{}` and `class InspirationsViewController: BaseViewController {}`. — shallowThought, Dec 27 '16 at 12:19
@shallowThought, Class declaration that you mentioned is correct. — Satyam, Dec 27 '16 at 12:36

Igor Bidiniuc · Accepted Answer · 2020-04-21T18:02:21.867

27

Use protocol extension with where clause. It works. But I would not recommend you to have such things in your codebase.

class BaseViewController: UIViewController {

}

extension OptionsDelegate where Self: BaseViewController {
  func handleSortAndFilter(opt: Options) {
    print("Base class implementation")
  }
}

extension BaseViewController: OptionsDelegate {

}

class InsipartionsViewController: BaseViewController {

}

extension OptionsDelegate where Self: InsipartionsViewController {
  func handleSortAndFilter(opt: Options) {
    print("Inspirations class implementation")
  }
}

edited Apr 21 '20 at 18:02

answered Dec 27 '16 at 11:40

Igor Bidiniuc

1,540
1
16
27

Thanks a lot for the answer. Can you please provide some explanation to understand the implementation? – Satyam Dec 27 '16 at 12:18
1

Its compiling perfect. But its never calling override function implemented in "InsipartionsViewController". – Satyam Dec 27 '16 at 12:35
https://talk.objc.io/episodes/S01E29-protocols-class-hierarchies checkout this video about protocols and class hierarchies in swift. – Igor Bidiniuc Dec 28 '16 at 11:52
Hmmm. I tried this, but I'm not able to call the protocol method. E.g., let x = InsipartionsViewController(); x.handleSortAndFilter(opt:...) doesn't compile for me. – Chris Prince Jul 11 '17 at 03:41
Ah. Got it now, I had to make UIViewController abide by the protocol. – Chris Prince Jul 11 '17 at 03:44
This is great! Unfortunately the methods cannot access internal properties (private) to the class. Ex: it can't access private vars in InspirationsViewController – TheJeff Dec 27 '20 at 00:29

score 5 · Answer 2 · answered Dec 27 '16 at 11:40

As far as I know you can't override methods in extensions. Extensions can only do the following: “Extensions in Swift can:

Add computed instance properties and computed type properties
Define instance methods and type methods
Provide new initializers
Define subscripts
Define and use new nested types
Make an existing type conform to a protocol”

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 3.0.1).”

Swift override protocol methods in sub classes

2 Answers2

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