[swift-users] Optional conformance warnings with Protocols

[Zhao Xin]

Implicitly unwrapped optionals and optionals are not equal.

struct X {

    let value:String = "aa"

}


let x:X? = nil

let y:X! = nil

x?.value // nil

y?.value // nil

y.value // fatal error: unexpectedly found nil while unwrapping an Optional
value. Unless you don't use this. As this is equal to y!.value.

Zhaoxin



On Sun, Jul 24, 2016 at 9:55 PM, Rod Brown <rodney.brown6 at icloud.com> wrote:

> The final code to test this is:
>
> import Foundation
> import MapKit
>
> class ManifestItem: NSObject {
>
>
>     let title: String!
>     let subtitle: String?
>     dynamic var coordinate: CLLocationCoordinate2D
>
>
>     init(title: String, subtitle: String?, coordinate:
> CLLocationCoordinate2D) {
>         self.title = title
>         self.subtitle = subtitle
>         self.coordinate = coordinate
>     }
> }
>
> extension ManifestItem: MKAnnotation {
> }
>
> This displays a warning in beta 3, as well as in Swift 2.2 in the current
> Xcode.
>
> I understand why: MKAnnotation declares my title must be optional (“?”)
> but I am declaring it as an implicitly unwrapped optional (“!”). Still, I
> believe the fact is that they are both optional, and are a legitimate way
> of declaring “a protocol I conform to requires this to be optional, but it
> can be treated as implicitly inwrapped because it will never be nil”.
>
> Thoughts?
>
> - Rod
>
>
>
> On 24 Jul 2016, at 11:09 PM, Zhao Xin <owenzx at gmail.com> wrote:
>
> 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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