> On Jul 22, 2017, at 12:12 AM, William Shipley via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> submodelObservation = observe(\.controller.submodel) { object, change
> in
> // code
> }
>
>
> @objc dynamic internal var controller: Controller?
>
> in the the “Controller" class:
>
> @objc dynamic internal var submodel: SubmodelForImporting?
I believe you're meant to actually construct a key path which looks through an
`Optional` like this:
\.controller?.submodel
But if you try that, you'll get a different, temporary error:
error: key path support for optional chaining components is not
implemented
My understanding is that optional chaining in key paths *is* planned for Swift
4; it just hasn't landed yet. If making `controller` an implicitly-unwrapped
optional would be a suitable workaround for you, I see that a couple pull
requests to support IUOs were merged to swift-4.0-branch recently[1]; they
might be in the open-source Swift 4 snapshot[2]. If not, my best suggestion
would be to temporarily add a computed `controller_submodel` property and use
the traditional `keyPathsForValuesAffecting…` mechanism to tie it to the
underlying properties.
[1] https://github.com/apple/swift/pull/11005
[2] https://swift.org/download/#snapshots
--
Brent Royal-Gordon
Architechies
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