Unable to conform to a protocol using another conforming protocol

Question

I came across the following code and am not sure why it doesn’t compile:

protocol CellDelegate: class {}

protocol DelegatingCellViewModel {
    var delegate: CellDelegate? { get }
}

protocol ProductCellViewModelDelegate: CellDelegate {}

// Error: Type 'ProductCellViewModel' does not conform to protocol 'DelegatingCellViewModel'
class ProductCellViewModel: DelegatingCellViewModel {
    weak var delegate: ProductCellViewModelDelegate?
}

Full error message:

error: Playground.playground:9:7: error: type 'ProductCellViewModel' does not conform to protocol 'DelegatingCellViewModel'
class ProductCellViewModel: DelegatingCellViewModel {
      ^

Playground.playground:10:14: note: candidate has non-matching type 'ProductCellViewModelDelegate?'
    weak var delegate: ProductCellViewModelDelegate?
             ^

Playground.playground:4:9: note: protocol requires property 'delegate' with type 'CellDelegate?'; do you want to add a stub?
    var delegate: CellDelegate? { get }
        ^

Is this a language limitation or am I missing something? How should this code be written so it compiles and keeps the intent?

can you please post the exact error message? – shivi_shub May 24 '19 at 12:51 — shivi_shub, May 24 '19 at 12:51
Maybe weak is the reason? – Andrey Chernukha May 24 '19 at 12:55 — Andrey Chernukha, May 24 '19 at 12:55
Actually the error messages are pretty descriptive. – vadian May 24 '19 at 12:56 — vadian, May 24 '19 at 12:56
@AndreyChernukha nope, it make no difference – Nikita Kukushkin May 24 '19 at 12:58 — Nikita Kukushkin, May 24 '19 at 12:58
@vadian any idea how to work around the error? – Nikita Kukushkin May 24 '19 at 13:00 — Nikita Kukushkin, May 24 '19 at 13:00

shivi_shub · Answer 1 · 2019-05-24T13:08:15.040

0

You need to change the type of delegate or add the protocol stubs.

class ProductCellViewModel: DelegatingCellViewModel {
    weak var delegate: CellDelegate?
}

OR you must change the name of that particular delegate like this. Because delegate name already used by conforming protocol -

class ProductCellViewModel: DelegatingCellViewModel {
    var delegate: CellDelegate?
    weak var productDelegate: ProductCellViewModelDelegate?
}

edited May 24 '19 at 13:08

answered May 24 '19 at 13:03

shivi_shub

1,018
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15

Unfortunately, this approach looses the more concrete delegate type `ProductCellViewModelDelegate`. Is there a way to write the code so I can keep it? – Nikita Kukushkin May 24 '19 at 13:07
@NikitaKukushkin check the edited answer. – shivi_shub May 24 '19 at 13:08

Unable to conform to a protocol using another conforming protocol

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