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Question

I took over project from another company and I often see this part of code when assigning value to variable:

// interface:
@property NSArray *foos;


// somewhere in implementation:
BOOL foosExist = ^BOOL {
    return self.foos.count > 0; // self inside block
}();

Moreover compiler claims when referencing inside block to foos property by underlying variable _foos :

Block implicitly retains "self"; explicitly mention "self" to indicate this is intended behavior.

Does this self inside block truly creates retain cycle? If not, why? Can someone elaborate?

Answer 1

There is no cycle in your example.

Your block is simply a value created during the evaluation of an expression and them immediately applied to produce a BOOL value.

While your situation is unusual, creating a block to immediately apply it in the same expression, a similar situation occurs when you pass a block to another method, either directly or by storing it in a local variable and passing the variables value - no cycle is created.

If instead you created the same block but stored it into an instance variable ( not a local variable), then self would reference the block, the block would reference self , and you would have a cycle. That is not in itself bad, it only becomes bad if the cycle is never broken, which causes a leak. However if at some point the cycle is broken, say by writing a different value into the instance variable, then the cycle never becomes an issue.

HTH