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

[Eric Miller]

The benefit I see is that the logic internal to didSet blocks is often
pretty generic in terms of how it deals with old VS new.

Eg:
* always bail if they match
* always render the new value
* always animate from the old value to the new value

The internal logic may repeat these names a lot, and it is unconcerned with
the often lengthy or descriptive name that may be exposed to the outside
world.

Often I rename the property, and have to rename 3 or 4 places inside the
didSet which should in my view be unnecessary.

Conceptually this is similar to the benefit of having different internal
and external parameter names for functions. In this case we have a great
default internal parameter name available.

I'm not sure whether some good examples or some other form of reasoning
best advances my point.

I'll try a sort of philosophical approach though. Why not have not didSet
and willSet always both have newValue and oldValue, since keeping the API
the same removes something the developer has to think about. In this way we
actually simplify the language, since all change observing blocks have the
same preconfigured variables available.
On Sat, Aug 27, 2016 at 12:47 AM Charlie Monroe <charlie at charliemonroe.net>
wrote:

> 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)
> > }}
> > _______________________________________________
> > swift-evolution mailing list
> > swift-evolution at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
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