[swift-users] Can you use @autoclosure in a setter?

[Slava Pestov]

You can have valueSource store a closure that captures the autoclosure value. For example,

set {
  valueSource = { newValue }
}

Slava

> On Sep 11, 2017, at 11:04 AM, Nevin Brackett-Rozinsky via swift-users <swift-users at swift.org> wrote:
> 
> Hi, quick question here:
> 
> I have a class with a property that needs to be really *really* lazy. So lazy, in fact, that when you assign to that property, the class actually stores a closure of what you assigned, which is only evaluated if and when you actually attempt to read the property.
> 
> Simplified:
> 
> class Foo {
>   private var valueSource: () -> Bar
>   private var valueCache: Bar?
>   
>   init(_ v: @escaping @autoclosure () -> Bar) {
>     valueSource = v
>   }
>   
>   var value: Bar {
>     get {
>       if let v = valueCache { return v }
>       let w = valueSource()
>       valueCache = w
>       return w
>     }
>     set {
>       /* ??? */
>     }
>   }
>   
>   // I want this function's logic to go in the setter above
>   func setValue(_ v: @escaping @autoclosure () -> Bar) {
>     valueSource = v
>     valueCache = nil
>   }
> }
> 
> The goal is to be able to write things like “someFoo.value = bar1 / bar2” (or even more complex expressions) and not evaluate them until/unless the result is actually needed.
> 
> Currently I am using “someFoo.setValue( bar1 / bar2 )”, which is not nearly as ergonomic as the assignment syntax. So, is there a way to make this work?
> 
> Nevin
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