How to store Blocks in NSDictionary

Are you getting a crash trying to store your block in a NSDictionary instance?


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. You still need to use [^{} copy] when passing “down” the stack into arrayWithObjects: and other methods that do a retain.


As an optimization, block storage starts out on the stack—just like blocks themselves do. If the block is copied using Block_copy (or in Objective-C when the block is sent a copy), variables are copied to the heap. Thus, the address of a __block variable can change over time.

This means that the block variables in your function are stored in the stack, and when the function returns, they get wiped out. If you’re using ARC, your crash is caused by block variables being over released. Calling copy on a block makes block variables stick around.

Here’s what it looks like:

        __weak id weakSelf = self;  // Weak block variable prevents retain cycle.
        void (^myBlock)() = ^() { 
            NSLog(@"Called from %@!", weakSelf);
        // Copy the block so the block variable sticks around!
        NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:[myBlock copy], @"MyBlock", nil];