What exactly does '__weak typeof(self)weakSelf = self;' mean

Question

This is used in the weakify pattern of Objective-C

My guess is that it means: assign a weak reference to self with the name 'weakSelf' and the typeof self (e.g. MyViewController)

If it's correct and looks obvious to you: I want to be absolutely sure to get this right. Thanks.

@CrimsonChris Nice but one has to include `libextobjc/EXTScope`, it is not a part of LLDB. See: [weakify](http://aceontech.com/objc/ios/2014/01/10/weakify-a-more-elegant-solution-to-weakself.html) — zaph, May 19 '14 at 16:33
Weakify and Strongify just wrap all that typeof nonsense and suppress some compiler warnings about shadowing self. It's all the same, but with a few more tricks to make it readable. — CodaFi, May 19 '14 at 16:34

Sergey Kalinichenko · Accepted Answer · 2016-03-01T19:07:06.067

29

My guess is that it means: assign a weak reference to self with the name weakSelf and the typeof self (e.g. MyViewController)

Yes, that is almost exactly what it means. The type of self would be MyViewController* (with an asterisk) not MyViewController.

The idea behind using this syntax instead of simply writing

MyViewController __weak *weakSelf = self;

is making it easier to refactor the code. The use of typeof also makes it possible to define a code snippet that can be pasted anywhere in your code.

edited Mar 01 '16 at 19:07

answered May 19 '14 at 16:27

Sergey Kalinichenko

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It's supposed to be written `MyViewController * __weak weakSelf = self;` – CommaToast Mar 01 '16 at 17:54

score 12 · Answer 2 · edited Aug 16 '22 at 07:26

12

Using @weakify and @strongify from libExtObjC helps to simplify the "weak-strong dance" one has to do sometimes around blocks.

Example!

__weak __typeof(self) weakSelf = self;
__weak __typeof(delegate) weakDelegate = delegate;
__weak __typeof(field) weakField = field;
__weak __typeof(viewController) weakViewController = viewController;
[viewController respondToSelector:@selector(buttonPressed:) usingBlock:^(id receiver){
    __strong __typeof(weakSelf) strongSelf = weakSelf;
    __strong __typeof(weakDelegate) strongDelegate = weakDelegate;
    __strong __typeof(weakField) strongField = weakField;
    __strong __typeof(weakViewController) strongViewController = weakViewController;

versus...

@weakify(self, delegate, field, viewController);
[viewController respondToSelector:@selector(buttonPressed:) usingBlock:^(id receiver){
    @strongify(self, delegate, field, viewController);

edited Aug 16 '22 at 07:26

brainray

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answered May 19 '14 at 16:44

CrimsonChris

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I think you should mention that this answer requires 'libextobjc' – Logan May 19 '14 at 16:47
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@CrimsonChris why do you weakify the delegate? I guess already was. – Ricardo Dec 07 '15 at 17:41
If I created the delegate like so... `MyDelegate *delegate = [[MyDelegate alloc] init];` I would have a strong reference to it until the current method ended. It wouldn't matter if the property on `viewController` was weak, I would still be creating a retain cycle if I didn't weakify it. – CrimsonChris Dec 07 '15 at 17:56

score 5 · Answer 3 · answered May 19 '14 at 16:29

5

Your interpretation is correct. However, I find that when it is written that way, it is slightly confusing to read. I prefer it with an additional space after typeof(self):

__weak typeof(self) weakSelf = self;

answered May 19 '14 at 16:29

Zev Eisenberg

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How would you port this to swift- its being used in a library that I am trying to upgrade? – UKDataGeek Jan 03 '17 at 01:31
If it's for a callback block/closure, it's `doSomething() { [weak self] (other, parameters) in doOtherStuff() }`. – Zev Eisenberg Jan 03 '17 at 01:33

score 4 · Answer 4 · edited May 23 '17 at 11:33

4

Depending on your compiler settings, you may get a warning "Expected ';' after expression". You can fix this by changing it to use __typeof like this: __typeof(self) __weak weakSelf = self;

Thanks to Leo Natan and this question: https://stackoverflow.com/a/32145709/1758224.

edited May 23 '17 at 11:33

Community

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answered Jun 28 '16 at 19:50

Eli Burke

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What exactly does '__weak typeof(self)weakSelf = self;' mean

4 Answers4