问题
I have to fix some existing code that builds just fine with LLVM (on iOS) so that it builds with llvm-gcc-4.2 too. I'm done with pretty much everything, except this pattern which is found at a few places:
@property (nonatomic, retain) __block id myProperty;
I suspect the intent here is to allow access to the property from inside a block without retaining self. How can I remove the __block attribute, which gcc doesn't support here, but still achieve the same effect?
回答1:
I'll suggest you've found a compiler bug, the declaration:
@property (nonatomic, retain) __block id myProperty;
is meaningless. The __block qualifier is used on local (stack allocated) variables so they are passed by reference to blocks, so they can be updated, and are usually[*] stored on the heap rather than the stack.
Therefore the qualifier __block has no meaning on a property declaration which is concerned with object instances, which are heap allocated at all times in Obj-C.
Just drop the __block from the property declarations - for every compiler.
[*] If a block is never copied to the heap a compiler may optimize __block variables and not move those to the heap either.
回答2:
just before you use the variable in a block, create a local pointer of type __block. Don't ever use __block in @property declarations.
EG: TypeOfVariable __block *bock_pointer = self.property;
^{ inside the block use bock_pointer }
来源：https://stackoverflow.com/questions/11515926/block-attribute-on-property-declarations