Question

I’m using Blocks instead of delegates for callbacks between viewControllers but i’m not able to figure out why this scenario is not working:

So I have a mainViewController which is calling a detailViewController, when returning the tableView which is a property on the mainViewController needs to be reloaded.

DetailViewController *actionDetail = [[DetailViewController alloc]initWithSaveBlock:^{
        [self.tableView reloadData]  //app crashes here
}];

in the detailViewController ‘save’ is called when the user taps the ‘Save button’

- (void)save
{
    if (self.saveBlock)
        self.saveBlock();

    [self dismissModalViewControllerAnimated:YES];
}

for some reason the [self.tableView reloadData] does not go along with [self dismissModalViewControllerAnimated:YES]. If I remove one of the two the app seems to work fine, obviously than I’m missing the intended behavior.

When the app crashes the message in the console is:

Previous frame inner to this frame (gdb could not unwind past this frame)

Anyone an idea why this is not working ?

Update: SaveBlock is defined like this

typedef void (^SaveBlock)();

@interface DetailViewController : UIViewController
{

}
@property(nonatomic,assign)SaveBlock saveBlock;

Answer 1

You need to declare your block property as copy:

@property(nonatomic, copy) SaveBlock saveBlock;

You can’t use assign or strong. The reason being that you need to ensure that you’ve got your own copy of the block. The one passed in might be on a stack frame which might be gone by the time you come to run the block. So you need to trigger a Block_copy() to ensure that the block gets copied to the heap (or just retained if it’s already there).

[Note: that explanation is very terse. I suggest you go read about the block runtime if you want to understand it fully.]