Generic typeof for weak self references

问题:

I am trying to figure out a way to use typeof to create a weak reference to self for use in blocks to avoid retain cycles.

When I first read about this it seems that the convention was to use __block typeof(self) bself = self;, which compiles but using __block to avoid retain cycles doesn't work anymore and __weak should be used instead.

However __weak typeof(self) bself = self; results in an error:

The type 'typeof (self)' (aka 'TUAccountsViewController *const __strong') already has retainment attributes set on it

Is there a way to use typeof or another call to generically create a weak reference to self?

回答1:

In the latest clang version Apple clang version 4.0 (tags/Apple/clang-421.1.48) (based on LLVM 3.1svn), i.e. Xcode 4.4+, the __typeof__((__typeof__(self))self) trick is not necessary anymore. The __weak typeof(self) bself = self; line will compile just fine.

回答2:

This works!

__typeof__(o) __weak

Which I've defined in my BBlock project as BBlockWeakSelf which can be used like this:

BBlockWeakSelf wself = self;

https://github.com/kgn/BBlock/blob/master/BBlock.h

Edited based on Aleph7's response.

回答3:

The correct way to do this is

__weak ActualClassName* weakSelf = self;

Macros only make it unclear what the variable actually is, and what you're actually doing with it, in addition to adding non-portable meta-language to your code.

If you need a more generic version of the class than ActualClassName provides, you aren't dealing with self anymore, since where self is defined, so is the class of self defined.

In those cases, you should use the closest base class name in your inheritance tree, NSObject or better, never id, e.g.

__weak MyBaseClassName* weakObject = object;

回答4:

Generic Weakself Reference (No Import Required + Snippet)

---

In my experience, the way to go is to use:

__typeof__(self) __weak weakSelf = self;

Note how the ownership qualifier belongs in front of the actual variable.

It's very apparent what's happening when it is used and it can be made into a handy code snippet in Xcode which makes it even easier to use in any project or class where this is needed. (I use "ws" as the snippet's completion shortcut)

Hmm.. I need a weak reference here..

ws{return}

Done. No need to ever include a header in future projects for this, just use the snippet.

---

Xcode Snippet

---

Title: Generic Weak Self Reference
Platform: All
Language: Objective-C
Completion Shortcut: ws
Completion Scopes: Function or Method
Code: __typeof__(self) __weak weakSelf = self;

---

Edit: Added note about ownership qualifier position based on comments, and Xcode Snippet Info

回答5:

why don't just use

__weak id bself = self;

回答6:

i think use this to be ok:

__weak __typeof(&*self)weakSelf = self;

it reference AFNetworking's AFURLConnectionOperation.m codes.

回答7:

I had a similar error but I fixed it differently:

I went to Project Navigator -> Project -> Target -> Build Settings

There I looked for C Language Dialect. I changed it from c11 to GNU99 and it solved my problem.

I hope it helps :)

回答8:

declareBlockSafe( self ) then blk( self ) inside the block. Self can be any variable or instance variable. Use declareBlockSafeAs for properties and method returns.

Also works with non-ARC if you import Mike Ash's splendid MAZeroingWeakRef. https://github.com/mikeash/MAZeroingWeakRef

#if __has_feature(objc_arc)

#define declareBlockSafe(__obj__) __weak typeof(__obj__) __tmpblk##__obj__ = __obj__
#define blockSafe(__obj__) __tmpblk##__obj__
#define blk(__obj__) blockSafe(__obj__)

#define declareBlockSafeAs(__obj__, __name__) \
__weak typeof((__obj__)) __tmpblk##__name__ = (__obj__)

#else

#define declareBlockSafe(__obj__) MAZeroingWeakRef *__tmpblk##__obj__ = [MAZeroingWeakRef refWithTarget:__obj__]
#define blockSafe(__obj__) ((typeof(__obj__))__tmpblk##__obj__##.target)
#define blk(__obj__) blockSafe(__obj__)

#define declareBlockSafeAs(__obj__, __name__) \
MAZeroingWeakRef *__tmpblk##__name__ = (__obj__)
#endif

You don't REALLY need blk() for ARC, it's just so that the macros can be used in the same way for non-ARC.

回答9:

I have this macro

#define weaken(object) __typeof__(self) __weak weakSelf = object

And i use it like this

weaken(self);
//The block referencing weakSelf goes here

回答10:

What about __unsafe_unretained? That's not as safe as __weak but it's the only thing I could think of. Also, why do you use typeof()?