[swift-evolution] Property Selectors

[Nicholas Maccharoli]

+1 Lenses would be awesome!

On Wed, Mar 15, 2017 at 1:59 AM, Robert Widmann via swift-evolution <
swift-evolution at swift.org> wrote:

> Lenses <https://github.com/typelift/Focus>!  My only concern is that
> arbitrary effects can be attached to setters and getters, which can lead to
> surprising results when using the property reference.  As a language
> feature, I’d be interested to see where discussion around this will lead.
>
> ~Robert Widmann
>
> On Mar 14, 2017, at 4:02 AM, Andrew Thompson via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Hello Swift Evolution Community,
>
> I’ve been thinking about a new language feature that would allow
> properties to be first class citizens. The basic idea is as follows:
>
> let x: PropertySelector<UIView, CGFloat> = #property(UIView.frame.origin.
> x)
> let view: UIView = …
> view.frame.origin.x = 20
> x.read(view) // returns 20
> x.write(view, value: 9091)
> view.frame.origin.x // now 9091
>
> This is a trivial example, but now we can do more interesting things in
> our code. For example, we can animate any property on a view (that is
> documented to be animatable of course):
>
> func animate(view: UIView, property: PropertySelector<UIView, CGFloat>,
> amount: Int) {
> let originalValue = property.read(view)
> func generateKeyFrames() {
> let step = 1.0 / Double(amount)
> for i in 0..<amount {
> let newValue = originalValue + CGFloat(i)
> let time = Double(i) / Double(amount)
> UIView.addKeyframe(withRelativeStartTime: time,
>  relativeDuration: step,
>  animations: { property.write(view, value: newValue) }
> )
> }
> }
>
> UIView.animateKeyframes(withDuration: 1.0,
>   delay: 0,
>   options: [],
>   animations: generateKeyFrames,
>   completion: nil)
> }
>
> let myView: UIView = …
> myView.frame = CGRect(x: 20, y: 100, width: 99, height: 120)
>
> // once this completes, myView.frame.origin.x == 120
> animate(view: myView, property: #property(UIView.frame.origin.x), amount:
> 100)
>
> // once this completes, myView.frame.size.width == 198
> animate(view: myView, property: #property(UIView.frame.size.width),
> amount: 99)
>
> I think this would be a pretty neat feature to have, what do you think?
>
> Cheers,
> - Andrew
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>
>
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