[swift-evolution] Proposal: Local identifier for the newValue in didSet and willSet blocks

[Adrian Zubarev]

I believe this is just a ‘nice to have’ shortcut. I wouldn’t mind to have this:

willSet(newValue, oldValue)
didSet(newValue, oldValue)
$0 for newValue and $1 for oldValue
It’s nothing for phase one, so we’ll have to defer this until next year where we’ll start talking about sugar/additional features for Swift 4.



-- 
Adrian Zubarev
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Am 27. August 2016 um 08:47:38, Charlie Monroe via swift-evolution (swift-evolution at swift.org) schrieb:

The newValue is already set in the property, so you just use that property. I don't see much benefit in having "newValue" instead of just accessing it:

var anonymousUserMode = false {
didSet {
if oldValue == self.anonymousUserMode {
return  
}

renderLoginOverlay(show: newValue)
}
}

And the same goes for willSet, where you have newValue, but not "oldValue", since oldValue is still in the stored property...

> On Aug 26, 2016, at 8:20 PM, Eric Miller via swift-evolution <swift-evolution at swift.org> wrote:
>  
> Just an idea.
>  
> I use didSet a lot, and it'd be nice to have a generic way to access the newValue.
>  
> For UI modes:
>  
> var anonymousUserMode = false { didSet {
> if oldValue == anonymousUserMode { return }
> renderLoginOverlay(show: anonymousUserMode)
> }}
>  
> For animated display strings:
>  
> var errorMessage: String? = nil { didSet {
> if oldValue == errorMessage { return }
> errorLabel.text = errorMessage
> // Do some sliding or hiding based on the new value
> }}
>  
> Has the idea of using $0 / $1 or oldValue / newValue been considered?
>  
> I'd like to be able to write this as:
>  
> var anonymousUserMode = false { didSet {
> if oldValue == newValue { return }
> renderLoginOverlay(show: newValue)
> }}
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