捕获属性值导致通过_property样式保留周期

Question

I have a class, there's a block proper:

@property(nonatomic, strong) void (^hehe)();

In the init method, I do the following work:

__weak test *weakSelf = self;
self.hehe = ^{
    test *self = weakSelf;
    NSLog(@"%zd", self.a);
    NSLog(@"%zd", self->_a);
    NSLog(@"%zd", _a);
};

What is the difference between the last two line in the block.

I thought self->_a is equals to _a .

But the Xcode shows warning on the last line:

Caputuring self strongly in this block is likely to lead a retain cycle

---

Edit:

I know the local self and the global self is not the same. How the os distinguish the difference. I used clang to rewrite the code, and get the following:

static void __ViewController__init_block_func_0(struct __ViewController__init_block_impl_0 *__cself) {
  __Block_byref_weakSelf_0 *weakSelf = __cself->weakSelf; // bound by ref

    ViewController *self = (weakSelf->__forwarding->weakSelf);
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_gp_6ztdfl3n5919c3y4pb03gd340000gn_T_ViewController_ad5b98_mi_0, ((NSInteger (*)(id, SEL))(void *)objc_msgSend)((id)self, sel_registerName("a")));
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_gp_6ztdfl3n5919c3y4pb03gd340000gn_T_ViewController_ad5b98_mi_1, (*(NSInteger *)((char *)self + OBJC_IVAR_$_ViewController$_a)));
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_gp_6ztdfl3n5919c3y4pb03gd340000gn_T_ViewController_ad5b98_mi_2, (*(NSInteger *)((char *)self + OBJC_IVAR_$_ViewController$_a)));
}

The last two line is used the same self...

Answer 1

Because it means a different self . A quick rename will make everything clear :

__weak test *weakSelf = self;
self.hehe = ^{
    test *strongSelf = weakSelf;
    NSLog(@"%zd", strongSelf.a);
    NSLog(@"%zd", strongSelf->_a);
    NSLog(@"%zd", _a);
};

Now it is clear, that the last line captures self from outside the block. The way you had it named, made it harder to differentiate between the self that was a local variable declared inside a block, and the self that was captured from outside its scope.

Answer 2

Referring to _a is a direct reference to an instance variable from the enclosing scope. That implicitly captures self. It doesn't invoke the property's getter/setter. It points directly to the instance variable.

Don't do that from inside a block that persists. (An "escaping" block, as Apple has started calling it.) Use the weakSelf/strongSelf syntax @Losiowaty showed in his answer.