NSManagedObjectContext performBlockAndWait: doesn't execute on background thread?

Question

I have an NSManagedObjectContext declared like so:

- (NSManagedObjectContext *) backgroundMOC {
    if (backgroundMOC != nil) {
        return backgroundMOC;         


        

Answer 1

The performBlockAndWait: call only makes sure that you execute the code in such a way that you don't introduce concurrency (i.e. on 2 threads performBlockAndWait: will not run at the same time, they will block each other).

The long and the short of it is that you can't depend on which thread a MOC operation runs on, well basically ever. I've learned the hard way that if you use GCD or just straight up threads, you always have to create local MOCs for each operation and then merge them to the master MOC.

There is a great library (MagicalRecord) that makes that process very simple.