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 <owe...@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@swift.org <mailto:swift-users@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.
>
>
> _______________________________________________
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> swift-users@swift.org <mailto:swift-users@swift.org>
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>
>
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