No, newValue is a closure here, and valueSource will capture this closure. When valueSource is evaluated, the original closure will be evaluated.
Slava > On Sep 11, 2017, at 3:11 PM, Hooman Mehr <hoo...@mac.com> wrote: > > But the expression that is assigned to the property will be eagerly evaluated > to produce `newValue`. So this will not accomplish what Nevin is trying to > do. > >> On Sep 11, 2017, at 3:08 PM, Slava Pestov via swift-users >> <swift-users@swift.org <mailto:swift-users@swift.org>> wrote: >> >> 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@swift.org <mailto:swift-users@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 >>> _______________________________________________ >>> swift-users mailing list >>> swift-users@swift.org <mailto:swift-users@swift.org> >>> https://lists.swift.org/mailman/listinfo/swift-users >>> <https://lists.swift.org/mailman/listinfo/swift-users> >> >> _______________________________________________ >> swift-users mailing list >> swift-users@swift.org <mailto:swift-users@swift.org> >> https://lists.swift.org/mailman/listinfo/swift-users >
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