[swift-users] Optional conformance warnings with Protocols

[Zhao Xin]

I pasted you code into Xcode 8 beta3 playground, everything seemed fine
with below code.

import Cocoa

import CoreLocation


class ManifestItem: NSObject {

    let value:    String

    let title:    String

    let subtitle: String?

    dynamic var coordinate: CLLocationCoordinate2D



    init(value:String, title:String, subtitle:String? = nil, coordinate:
CLLocationCoordinate2D) {

        self.value = value

        self.title = title

        self.subtitle = subtitle

        self.coordinate = coordinate

    }

}


@objc public protocol MKAnnotation : NSObjectProtocol {



    // Center latitude and longitude of the annotation view.

    // The implementation of this property must be KVO compliant.

    var coordinate: CLLocationCoordinate2D { get }



    // Title and subtitle for use by selection UI.

    @objc optional var title: String? { get }

    @objc optional var subtitle: String? { get }

}

I didn't encounter the error you mentioned. But I had to remove public and
add @objc as Xcode asked.

Zhaoxin

On Sun, Jul 24, 2016 at 12:09 PM, Rod Brown via swift-users <
swift-users at swift.org> wrote:

> Hi Swift Users,
>
> I just ran across an issue where I had the following code:
>
> class ManifestItem: NSObject {
>     let value:    String
>     let title:    String
>     let subtitle: String?
>     dynamic var coordinate: CLLocationCoordinate2D
> }
>
>
> As part of utilising this object, I needed it to conform to MKAnnotation,
> which declares:
>
>
> public protocol MKAnnotation : NSObjectProtocol {
>
>
>     // Center latitude and longitude of the annotation view.
>     // The implementation of this property must be KVO compliant.
>     public var coordinate: CLLocationCoordinate2D { get }
>
>
>     // Title and subtitle for use by selection UI.
>     optional public var title: String? { get }
>     optional public var subtitle: String? { get }
> }
>
> I added the appropriate extension to “ManifestItem” and promptly got
> reminded with an error that MKAnnotation requires title be an optional.
>
> To do this, I converted “title” in my ManifestItem class to be an
> implicitly unwrapped optional. This value can never be nil, and should be
> treated as such throughout my code. However, the compiler still emits a
> warning. "Type of 'title' has different optionality than expected by
> protocol ‘MKAnnotation' "
>
> I’m wondering if this should be considered a bug? I am actually comforming
> correctly to the protocol - title is optional, though implicitly unwrapped.
> Should I have to make “title” optional, and every use of it optional or a
> force unwrap, when my code verifies that the manifest item is legitimately
> invalid if title property is nil?
>
> I think this is valid use of the implicitly unwrapped optional. It allows
> you to say “this is optional for some reason, but should generally always
> be assumed to be non null.” This sounds remarkably similar to my use case "this
> is optional for conformance reasons, but should generally always be assumed
> to be non null.”
>
> Thanks for your opinions,
>
> Rod
>
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>
>
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