Objective-C - Blocks and memory management?

Question

__weak MyClass *selfReference = self;

dispatch_async(dispatch_get_main_queue(), ^{
        [selfReference performSomeAction];
    });

• When do you need to pass a weak reference to a block?
• Does this rule apply to dispatch_async as well as custom blocks?
• Does a block copy the iVars used in it or does it retain them?
• Who owns the variables initialized inside a block? Who should release them?

Answer 1

1, 2) Blocks retain the object-pointers in it (any blocks, dispatch_async blocks are nothing special). This usually isn't a problem, but can lead to retain-cycles, because the block can be associated with an owner object and that owner object (often self) might be retained by the block. In that case you should use a weak variable and then reassign it to a strong capture:

__weak MyClass *weakSelf = self;
self.block = ^{
    MyClass *strongSelf = weakSelf;
    ...
    [strongSelf ...];
    [strongSelf.property ...];
    [strongSelf->iVar ...];
 }

Note: If you access an iVar directly, the compiler will transform that into self->iVar and thus retains self!

3) Blocks only retain the pointers, they don't copy them.

4) Variables created inside a block belong to that block and will be released when that block goes out of scope.