[swift-users] weak self

[Rien]

> On 01 May 2017, at 17:42, Dennis Weissmann <dennis at dennisweissmann.me> wrote:
> 
>> 
>> On May 1, 2017, at 5:32 PM, Rien <Rien at Balancingrock.nl> wrote:
>> 
>>> 
>>> On 01 May 2017, at 16:59, Dennis Weissmann <dennis at dennisweissmann.me> wrote:
>>> 
>>>> 
>>>> On May 1, 2017, at 4:46 PM, Rien via swift-users <swift-users at swift.org> wrote:
>>>> 
>>>> In my code I use a lot of queues. And (very often) I will use [weak self] to prevent doing things when ‘self’ is no longer available.
>>>> 
>>>> Now I am wondering: how does the compiler know that [weak self] is referenced?
>>>> 
>>>> I am assuming it keeps a reverse reference from self to the [weak self] in order to ‘nil’ the [weak self] when self is nilled.
>>>> 
>>>> But when a job is executing it has no control over the exclusive rights to [weak self].
>>>> 
>>>> I.e. [weak self] may be nilled by an outside event in the middle of say:
>>>> 
>>>> 	if self != nil { return self!.myparam }
>>>> 
>>>> The if finding [weak self] non nil, but the return finding [weak self] as nil
>>>> 
>>>> Is that correct? i.e. should we never use the above if construct but always:
>>>> 
>>>> 	return self?.myparam ?? 42
>>>> 
>>> 
>>> Yes, as you said, you never know when self will be nilled out, that's why you need to create a (temporary) strong reference to it, work on it, and when the block returns, your strong reference is released and self either goes away immediately (incase it was released elsewhere after the local binding to a strong variable and no other objects had a strong reference to it) or it will stay as long as no other object holds a strong reference to it.
>>> 
>>> When the closure is more involved than a view lines, I often do a guard let `self` = self else { return } to conveniently work with a strong self inside the rest of the closure.
>> 
>> I was aware of that practise (use it myself) but I was not sure if it would always work.
>> I.e. I have not found it documented that
>> 	let strongSelf = self
>> will actually retain ‘self’ an not create a strong reference to an intermediate reference to self.
>> 
>> It makes sense though, otherwise there would be massive failures ‘out there’. ;-)
>> 
> 
> Yes :) local variables (as others too) are strong by default. If you want them weak (you should very rarely need that though) you have to explicitly mark them weak.
> 
>> will actually retain ‘self’ an not create a strong reference to an intermediate reference to self.
> 
> Isn't that the same? I might misunderstand you but they would both reference the same object, no? So in the end the retain message would be sent to the same object, wouldn't it? 

Just to belabour this point unnecessarily :)

If you are familiar with the compiler innards, then this might be obvious. However I am not, and I can imagine (crazy) scenario’s where an optional woud be “unwrapped” by simply taking the reference to the self from the optional and storing it in a new non-optional variable… :D … go figure… the mind works in devious ways… lol.

Rien.


> 
>> Thanks,
>> Rien.
>> 
>>> 
>>>> Regards,
>>>> Rien
>>>> 
>>>> Site: http://balancingrock.nl
>>>> Blog: http://swiftrien.blogspot.com
>>>> Github: http://github.com/Balancingrock
>>>> Project: http://swiftfire.nl - A server for websites build in Swift
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
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>>> 
>>> - Dennis
> 
> - Dennis