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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
817
Declaring Block VariablesStart with a return type followed by the variable name and a parameter list. You'll need a caret (^) before the variable name and two sets of parentheses. With that in mind, the following example declares a completion block, identical to the one you just saw, but without using custom typedefs.
^blockName: Always remember that the block name is preceded by the ^ symbol. The block name can be any string you like, just like you name any other variable or method. Remember that both the ^ and the block name are enclosed in parentheses ().
Objective-C properties offer a way to define the information that a class is intended to encapsulate. As you saw in Properties Control Access to an Object's Values, property declarations are included in the interface for a class, like this: @interface XYZPerson : NSObject.
For many of us, it's best practice to always use weak combined with self inside closures to avoid retain cycles. However, this is only needed if self also retains the closure. By adding weak by default you probably end up working with optionals in a lot of cases while it's actually not needed.
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.
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