Question

I’ve recently come across an Apple document that shows the following property declaration for a block:

@interface XYZObject : NSObject
@property (copy) void (^blockProperty)(void);
@end

Also, this article states:

Note: You should specify copy as the property attribute, because a block needs to be copied to keep track of its captured state outside of the original scope. This isn’t something you need to worry about when using Automatic Reference Counting, as it will happen automatically, but it’s best practice for the property attribute to show the resultant behavior. For more information, see Blocks Programming Topics.

I also read the suggested Blocks Programming Topics but haven’t found anything relevant there.

I’m still curious as to why defining a block property as “copy” is best practice. If you have a good answer, please try to distinguish between ARC and MRC differences if there are any.

Thank you

Answer 1

By default blocks are created on the stack. Meaning they only exist in the scope they have been created in.

In case you want to access them later they have to be copied to the heap by sending a copy message to the block object. ARC will do this for you as soon as it detects a block needs to be accessed outside the scope its created in. As a best practise you declare any block property as copy because that’s the way it should be under automatic memory management.

Read Stack and Heap Objects in Objective-C by Mike Ash for more info on stack vs. heap.