Defining Objective-C blocks as properties - best practice

Question

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

@interface XYZObject : NSObject
@property (copy) void         


        

Answer 1

The "best practices" you refer to simply say, "Seeing as ARC is going to magically copy your block no matter what you write here, it's best you explicitly write 'copy' so as not to confuse future generations looking at your code."

Explanation follows:

Typically, you shouldn’t need to copy (or retain) a block. You only need to make a copy when you expect the block to be used after destruction of the scope within which it was declared. Copying moves a block to the heap.
–Blocks Programming Topics: Using Blocks, Copying Blocks

Clearly, assigning a block to a property means it could be used after the scope it was declared in has been destroyed. Thus, according to Blocks Programming Topics that block should be copied to the heap with Block_copy.

But ARC takes care of this for you:

Blocks “just work” when you pass blocks up the stack in ARC mode, such as in a return. You don’t have to call Block Copy any more.
–Transitioning to ARC

Note that this isn't about the retain semantics the block. There's simply no way for the block's context to exist without being moved off the (soon-to-be-popped) stack and on to the heap. So regardless of what attributes you qualify your @property with, ARC is still going to copy the block.