[swift-users] Conforming to the protocol requirements with implicitly unwrapped optional

[Zhao Xin]

You can try this.

protocol Container : class {

    var component: Component { get }

}


class ConcreteContainer : Container {

    lazy var component: Component = { return Component(container: self) }()

}


class Component {

    unowned let container: Container

    init(container: Container) {

        self.container = container

    }

}


Zhaoxin

On Mon, May 29, 2017 at 11:48 PM, Anton Bronnikov <anton.bronnikov at me.com>
wrote:

> Like I mentioned, it is necessary to use optional, but it can not be nil.
> Consider the following example where I have to use an optional in order to
> break retention cycle:
>
> protocol Container : class {
>     var component: Component { get }
> }
>
> class ConcreteContainer : Container {
>     var component: Component! = nil
>     init() {
>         component = Component(container: self)
>     }
> }
>
> class Component {
>     unowned let container: Container
>     init(container: Container) {
>         self.container = container
>     }
> }
>
> *Playground execution failed: error: scratchpad.playground:5:7: error:
> type 'ConcreteContainer' does not conform to protocol 'Container'*
> *class ConcreteContainer : Container {*
> *      ^*
>
> *scratchpad.playground:2:9: note: protocol requires property 'component'
> with type 'Component'; do you want to add a stub?*
> *    var component: Component { get }*
> *        ^*
>
> *scratchpad.playground:6:9: note: candidate has non-matching type
> 'Component!'*
> *    var component: Component! = nil*
> *        ^*
>
>
> Cheers,
> Anton
>
> P.S. Declaring protocol as
>
> protocol Container : class {
>     var component: Component! { get }
> }
>
>
> .. would also work of course, but my question is not about how to move
> forward, rather about whether such setup is deliberate.
>
>
> On 29 May 2017, at 08:20, Zhao Xin <owenzx at gmail.com> wrote:
>
> Why you have to use `unwrapped optional` at the first place? If you have
> to use it, it means it could be nil. So it won't conform the protocol,
> which requires the `value` never nil.
>
> Zhaoxin
>
> On Mon, May 29, 2017 at 12:37 PM, Anton Bronnikov via swift-users <
> swift-users at swift.org> wrote:
>
>> Hi All,
>>
>> If I have a protocol with a property requirement such as:
>>
>> protocol Foo {
>>     var value: Int { get }
>> }
>>
>>
>> .. and I need to conform to it using an implicitly unwrapped optional
>> like:
>>
>> struct Bar : Foo {
>>     let value: Int! = nil
>> }
>>
>>
>> .. then the compiler fails with an error:
>>
>> *Playground execution failed: error: scratchpad.playground:5:8: error:
>> type 'Bar' does not conform to protocol 'Foo'*
>> *struct Bar : Foo {*
>> *       ^*
>>
>> *scratchpad.playground:2:9: note: protocol requires property 'value' with
>> type 'Int'; do you want to add a stub?*
>> *    var value: Int { get }*
>> *        ^*
>>
>> *scratchpad.playground:6:9: note: candidate has non-matching type 'Int!'*
>> *    let value: Int! = nil*
>> *        ^*
>>
>> Technically, I understand why the error, and currently work around it
>> with a cumbersome:
>>
>> struct Bar : Foo {
>>     var value: Int { return _value! }
>>     let _value: Int? = nil
>> }
>>
>>
>> However, I keep wondering - would it not be making sense to accept
>> implicitly unwrapped optionals as a conformance to be base-type
>> requirements?  It sort of works just like that in all other parts of the
>> language.
>>
>> Or in other words, is this by design, or should I probably create an
>> issue for this?
>>
>> Thank you.
>> Cheers,
>> Anton
>>
>> P.S. The example is oversimplified for the sake of clarity, in practice
>> the property just has to be an optional - implicitly unwrapped or not -
>> because it’s a part of the composite that holds unowning reference to its
>> container object.
>>
>>
>> _______________________________________________
>> swift-users mailing list
>> swift-users at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>>
>>
>
>
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