[swift-evolution] [Proposal] Make `didSet` also available for `let` property, or something new such as `didInit`

[Michal Pearse]

I feel like this would be an unexpected way of doing things.

Why would I want to separate my set-up code from my init() without any
immediately obvious link between the two?

If your init() is getting unwieldy, perhaps a cleaner solution is to simply
decompose some of the setting up into smaller (private) methods that are
then called by the init()? At least when it is done this way, you can
immediately see that other stuff happens from within the init() method
without having to think about whether a will/didSet might have been called
on a property.


On Thu, Dec 24, 2015 at 6:39 AM, shengjia wang via swift-evolution <
swift-evolution at swift.org> wrote:

> Hi,
>
> You are saying: *"Because you can probably just put most of that code at
> the end of your initialiser"*. I see.. and this proposal is exactly about
> how to avoid this situation.
>
> I'm saying it would be neat if we can band some side effects once we set a
> `let` property. Example:
>
>     let view: UIView {
>
>         didSet {
>
>             view.background = UIColor.blackColor()
>
>             view.translatesAutoresizingMaskIntoConstraints
>
>         }
>
>     }
>
>     let scrollView: UIScrollView {
>
>         willSet {
>
>             scrollView.removeObserver(self, forKeyPath: "contentOffset")
>
>         }
>
>         didSet {
>
>             scrollView.addObserver(self, forKeyPath: "contentOffset",
> options: .New, context: nil)
>
>         }
>
>     }
>
>     init(targetScrollView: UIScrollView) {
>
>         view = UIView()
>
>         scrollView = targetScrollView
>
>         super.init()
>
>
>
>         // if we could put them into property observing ...
>
>         // view.background = UIColor.blackColor()
>
>         // view.translatesAutoresizingMaskIntoConstraints
>
>         // scrollView.addObserver(self, forKeyPath: "contentOffset",
> options:.New, context: nil)
>
>         // ...
>
>     }
>
>
> We can extract the "setup property" step from init method and separate
> them for all different properties just after the property be initialised.
> Otherwise, we have to either mix them into the init method or change `let`
> to `var`. Both are doable but not ideal in my opinion.
>
>
> By the way, the principal of this idea is similar to another approach for
> IBOutlet property
> <https://twitter.com/jesse_squires/status/626264940450480128> :
>
>
> Use *didSet* on your IBOutlets to configure views instead of cramming
>> code into viewDidLoad. Much cleaner. Still called only once.
>
>
> - Victor Wang
>
> On Wed, Dec 23, 2015 at 5:07 PM, Félix Cloutier <felixcca at yahoo.ca> wrote:
>
>> willSet and didSet are currently not even called from the init method.
>> This also goes counter to the current property behaviors proposal (didGet
>> and didSet would become part of that) because behaviors aren't planned for
>> let properties.
>>
>> You're saying you want it to happen as soon as it was set, but do you
>> really need it "as soon as that" or can you afford to wait a little bit?
>> Because you can probably just put most of that code at the end of your
>> initializer, where it's guaranteed that the property has been set.
>>
>> Félix
>>
>> Le 23 déc. 2015 à 10:22:46, shengjia wang via swift-evolution <
>> swift-evolution at swift.org> a écrit :
>>
>>
>> Since swift v1.2, we can initialize `let` property in `init()` instead of
>> being forced to give a value when declare it. This is great !
>>
>> But every time I run into the case such as the example below:
>>
>> let view: UIView {
>>   didSet {
>>     /**
>>      * This time, `view` did set ( a.k.a initialized in case of `let`
>> property), so I want to bind some other actions just after, such as
>> `setBackgroundColor`. But actually I can't, compiler will complain that
>> `let` declaration can not be observing properties. So I have to either move
>> all these "actions" to `init()` or change `view` to a `var` property which
>> is not necessary at all.
>>      */
>>   }
>> }
>>
>> Actually in swift, I think it's quite commun issue that people run into a
>> large `init()` method. This approach could make it way better in most cases.
>>
>> So, I'm wondering why not make `didSet` also available for `let`
>> property, or maybe even better to add new keyword such as "didInit" which
>> only get called for first set.
>>
>> - Victor Wang
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
>>
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20151224/45f51343/attachment.html>