[swift-evolution] [Discussion] Enforcing Calling Super

[Jean-Daniel Dupas]

> Le 25 févr. 2016 à 20:28, Matthew Johnson <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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