Software engineer by profession, iOS developer by chance, freelancer by luck, Photographer by hobby and dad for my son

 

If the file is found but the parse simply produces a nil parse tree, the NS_VALUERETURN macro is used to return nil to the readDataFile: caller. Note that it is not allowed to simply write “return nil;” inside the NS_DURING block, owing to the nature of the behind-the-scenes C constructs implementing the mechanism (the setjmp() and longjmp() functions). If you are in a void function not returning a value, you may use simply “NS_VOIDRETURN” instead.

Backstage with Nested Managed Object Contexts — Florian Kugler

All other objects can just be ignored, because they cannot be in use anyway. When importing large amounts of data on a background context, this behavior makes total sense. After all, most of the changed objects won’t be of interest to the context in the main thread.