Block automatic retaining, does it affect even to ivars in self?

问题

If I have class:

@interface A : NSObject
{
    BOOL b;
    id   c;
}
@end

and reference b and c in a block, is the block retain self automatically? Or just b and c? About c, it may be retained itself, but how about b?

回答1:

Bill's answer isn't quite correct:

If you have an instance of A and create a block inside of that instance like so:

^{
  b = YES;
}

Then self is retained (when the block is copied). b is not const-copied, because b is strongly referenced by self, and only self is const within the scope of the block.

On the other hand, if you do:

BOOL aBool = YES;
^{
  aBool = NO;
  c = [[NSObject alloc] init];
}

Then again, self is const-copied (and retained when the block itself is copied) and the assignment to c is allowed. However, the assignment to aBOOL is not allowed, because the value of aBool is const-copied.

In other words, the compiler recognizes the b and c are ivars, and will retain self instead of the ivars directly.

---

One way to think about this that helps me understand what's going on is to remember that an object is really just a fancy struct, which means you can technically access ivars via the arrow operator: ->

So when you're accessing ivars:

b = YES;

is equivalent to:

self->b = YES;

In that light, it makes perfect sense why you have to retain self, but b is not const. It's because b is only a slim part of the "bigger picture", and in order to get b, you must necessarily include all of self as well (since copying part of a struct doesn't really make sense in this context).

回答2:

Code from the block would be helpful in answering but assuming you have something like this;

^(A *a) {
    [a doSomething];
}

a will be retained by the block and released when the block is released. Nothing outside the ordinary will happen with b and c.

If you have something like;

^(id c, BOOL b) {
    [c doSomethingElse];
}

Then c will be retained by the block and b will be captured by the block as a const value (i.e. you would get a compiler error for doing b = NO)

For more details see the docs;

http://developer.apple.com/library/ios/#documentation/cocoa/Conceptual/Blocks/Articles/bxVariables.html%23//apple_ref/doc/uid/TP40007502-CH6-SW1

来源：https://stackoverflow.com/questions/5807022/block-automatic-retaining-does-it-affect-even-to-ivars-in-self