Question

I’m getting into the use of blocks in Objective-C and haven’t really found a good explanation of why a block, if you’re going to assign it to an instance variable, has to be assigned with copy and not assign?

for example:

typedef void (^MyBlock)();

@interface SomeClass : NSObject
{
    MyBlock myblock;
    // Other ivars
}

@property (nonatomic, copy) MyBlock myblock;  // Why must this be 'copy'???

//  other declarations

@end

Answer 1

Well, let’s analyze this:

Let’s say you create a block inside of some method, assign it to some variable:

MyBlock block = ^{};

Then you simply assigned it to a property with assign:

self.myblock = block;

When the containing method returns, the block variable will go out of scope and deallocated. So, with this in mind, you must copy the block object and then store it in your instance variable. That way, you can own the block for the lifetime of the containing object.