[swift-evolution] [Discussion] Enforcing Calling Super

[Rod Brown]

I agree.

While there may be specific cases where super must be called first or last, I think the examples provided were exceptions, rather than rules. The dealloc method in Obj-C for example was one case, and it was so overarching that the compiler took control of that with ARC.

I fail to see in mainstream code why it would ever be necessary to force a programmer not to do a single line of work prior, or prior to the end of, a super call. Could there be exceptions? I’m sure we could eventually find a use case. But I don’t think this is prolific enough that we need to start adding to the language for it. We can document the requirement on the method if we need to.

> On 26 Feb 2016, at 7:38 AM, Vanderlei Martinelli via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I vote (again) to only specify if a func requires super. When to call super is up to the developer after reading the docs.
> 
> -Van
> 
> On Thu, Feb 25, 2016 at 5:02 PM, Jean-Daniel Dupas via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> Le 25 févr. 2016 à 20:28, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> a écrit :
>> 
>> 
>>> On Feb 25, 2016, at 1:23 PM, Jean-Daniel Dupas <mailing at xenonium.com <mailto:mailing at xenonium.com>> wrote:
>>> 
>>>> 
>>>> Le 25 févr. 2016 à 20:19, Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>>> 
>>>> 
>>>> 
>>>> Sent from my iPad
>>>> 
>>>>> On Feb 25, 2016, at 1:17 PM, Jean-Daniel Dupas via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> 
>>>>> 
>>>>>> Le 25 févr. 2016 à 16:47, Jeremy Pereira via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>>>>> 
>>>>>> 
>>>>>>> On 17 Feb 2016, at 22:26, Kyle Sherman via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>>> 
>>>>>>> Thanks for the replies.
>>>>>>> 
>>>>>>> Kenny: After thinking about it more, discussing with Peter, and looking Haravikk’s comments, I think the best thing would be for this to be a warning as suggested. I respectfully disagree that as a library creator you would not be able to know that a call to super should be required.
>>>>>> 
>>>>>> I disagree. You can’t possibly know all the use-cases in which your class might be subclassed. 
>>>>>> 
>>>>>> In particular, it is absurd to enforce having the call to super as the first or last line of the method. That would stop me doing things like this:
>>>>>> 
>>>>>> override func viewDidLoad()
>>>>>> {
>>>>>>     print(“About to run super.viewDidLoad()”)
>>>>>>     super.viewDidLoad()
>>>>>>     print(“Finished super.viewDidLoad()”)
>>>>>> }
>>>>>> 
>>>>>> Then there’s the perfectly reasonable case like this:
>>>>>> 
>>>>>> override func viewDidLoad()
>>>>>> {
>>>>>>     functionThatCallsSuperViewDidLoad()
>>>>>> }
>>>>>> 
>>>>>> Why shouldn’t I be allowed to do that?
>>>>> 
>>>>> +1 with your concern. I’d be curious to see a single real world use case where enforcing first or last is required.
>>>> 
>>>> I posted several examples from Apple frameworks in an old thread about this.  You might want to look for that message in the archives.
>>> 
>>> And not a single one has a strong requirement about prohibiting code to be call before or after the super class implementation.
>> 
>> It depends what you mean by “strong”.  Sure, a log statement won’t make much difference.  But the examples I gave do have semantic requirements that super should either go first or last in performing real work to be done by the method.
> 
> For instance:
> // If you override this method, you must call super first to get the invalidation context object to return. After getting this object, set any custom properties and return it.
> func invalidationContextForBoundsChange(_ newBounds: CGRect) -> UICollectionViewLayoutInvalidationContext
> 
> Why must I call super first ? Why can’t I don’t a bunch of things that are related to my subclass first ?
> 
> How am I supposed to do if I want to compute a new bound to pass to super instead of forwarding it naively.
> 
> ditto for 
> // call super first to retrieve the item’s existing attributes and then make your changes to the returned structure.
> layoutAttributesForInteractivelyMovingItemAtIndexPath(_ indexPath: NSIndexPath, withTargetPosition position: CGPoint) -> UICollectionViewLayoutAttributes
> Why would you prevent a subclass to compute an other indexPath or target position before calling super ?
> 
> 
> 
> 
> 
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