Objective-C ARC - does a method retain 'self'?

Question

Is this a safe thing to do?

__weak typeof (self) welf = self;
dispatch_async(dispatch_get_main_queue(), ^{
    [welf doStuff];
})

....

-(void)doStuff {
    [_member1 someMethod];
    ....
    [_member2 someMethodWithParam:_someCArray[1];];
}

As far as I understand ARC, the actual call to [welf doStuff] should be fine, as welf will either be valid (and the method call will proceed), or it will be nil - calling any method on nil is defined safe behaviour in Objective-C so that seems ok.

Once we get into doStuff however, does ARC cause self to be retained for the duration of the method call? If it does, I think we should be ok... If not, then self could get dealloc'ed halfway through the method call and then we'd be in a bad spot.

Note: I googled for this but couldn't find anything obvious. Perhaps it's just hard to search for?

Thanks!

This question already has bean answered http://stackoverflow.com/q/19018456/978568 — Yurii Kolesnykov, Sep 08 '15 at 23:53
@YuriiKolesnykov I don't see how that questions answers this one. — Rupert, Sep 09 '15 at 00:12
This is not a duplicate of the other question. The other question focuses on retain cycles. — John K, Feb 21 '17 at 18:31

score 1 · Answer 1 · edited May 23 '17 at 12:16

1

As it turns out I found the answer eventually, both here: https://stackoverflow.com/a/18011581/234 and here: http://clang.llvm.org/docs/AutomaticReferenceCounting.html#self

self is always unsafe unretained under ARC, so in my example, it would NOT be safe, and there would be nothing to stop it getting deallocated partway through my doStuff method. Ouch

edited May 23 '17 at 12:16

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1
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answered Sep 09 '15 at 00:29

Orion Edwards

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Your example is ok because welf is essentially retained once doStuff is called until doStuff returns. – John K Feb 21 '17 at 18:24

score 0 · Answer 2 · edited May 23 '17 at 12:24

0

You need to create a strong reference to self inside a block and call a method on it.

    __weak typeof (self) weakSelf = self;
    dispatch_async(dispatch_get_main_queue(), ^{
        typeof(self) strongSelf = weakSelf;
        [strongSelf doStuff];
    })

And yes, Orion, your question is answered by this question itself, and this particular anwswer

edited May 23 '17 at 12:24

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answered Sep 09 '15 at 00:32

Yurii Kolesnykov

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No you don't need to do strongSelf. The weak object is retained when doStuff is called until doStuff returns. – John K Feb 21 '17 at 18:24
According to http://clang.llvm.org/docs/AutomaticReferenceCounting.html#self the answer is correct and self isn't automagically retained while you are inside doStuff. – Steve Armstrong Mar 20 '20 at 22:29

Objective-C ARC - does a method retain 'self'?

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