Changing an instance variable in a block

I am quite confused about how to change an instance variable inside of a block.

The interface file (.h):

@interface TPFavoritesViewController : UIViewController {
    bool refreshing;
}

The implementation:

__weak TPFavoritesViewController *temp_self = self;
refreshing = NO;
[myTableView addPullToRefreshWithActionHandler:^{
    refreshing = YES;
    [temp_self refresh];
}];

As you might guess, I get a retain cycle warning when I try to change the refreshing ivar inside of the block. How would I do this without getting an error?