Question

In iOS5, I have a NSManagedObjectContext which I create with a NSPrivateQueueConcurrencyType, like so:

self.moc = [[[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType] autorelease];
moc.parentContext = rootContext;

This executes on the main thread, but if I understand the doc correctly that shouldn’t matter, since the MOC gets its own queue, right?

Now, at some point I perform a fetch request, like so:

[self.moc performBlockAndWait:^() { // (1)
    NSError* err = nil;

    result = [self.moc executeFetchRequest:request error:&err];

    NSLog(@"%@ %@", [NSThread currentThread], [NSThread isMainThread]?@"MAIN":@"");
}];

The invocation (1) is also called on the main thread. But I expect the fetch request inside the block to execute on the private MOC thread. Correct so far?

However, if I check the current thread inside the block, it actually is the main thread! The NSLog prints:

<NSThread: 0x6b10780>{name = (null), num = 1} MAIN

Checking the thread dump confirms this.

The fetch should NOT be executed on the main thread, because this does occasionally result in a deadlock with some other running code. So what am I doing wrong here?

Answer 1

Ok, I’m still not entirely sure about the mechanics here, i.e. how something posted on the context-private queue ends up on the main thread. I suspect it has to do with the performBlockAndWait, which executes the block synchronously. I have rewritten stuff so everything is now done asynchronously, and the block is now executed on its private thread.

Thanks for all suggestions.