Is it necessary to do Core Data reads within performBlock when using NSMainQueueConcurrencyType?

Question

According to Daniel Eggert's answer in this question, when using a managed object context with NSPrivateQueueConcurrencyType it's necessary to do anything that touches it or objects that belong to it within performBlock: or performBlockAndWait:

Is the same true for NSMainQueueConcurrencyType? Imagine the following code running on the main thread, in a UIViewController for instance:

self.moc = [[[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType] autorelease];
//moc setup

__block RHWidget *widget = nil;

[self.moc performBlockAndWait:^{
    widget = [(RHWidget *)[self.moc objectWithID:self.widgetObjectID] retain];
}];

self.labelView.text = widget.descriptionString;

[widget release];

Is it safe to use the widget outside the block, since we know we're on the main thread? Or it is it necessary to do this:

__block NSString *description = nil;

[self.moc performBlockAndWait:^{
    RHWidget *widget = (RHWidget *)[self.moc objectWithID:self.widgetObjectID];
    description = [widget.descriptionString copy];
}];

self.labelView.text = description;

[description release];

Do things change if there's another NSManagedObjectContext out there, possibly of the private queue type, doing work in blocks and pushing changes up to self.moc as a parentContext?

This is a slightly contrived example of course, but it'd be nice to safely pass that widget to, for instance, a modal view controller that needs to access some of the widget's properties. Should I be passing the objectID of the widget instead and refetching it within performBlock: in the new view controller?

Answer 1

Update: According to WWDC 2011 Session 303 (What's New in Core Data on iOS), NSMainQueueConcurrencyType is intended to allow normal messaging on the main thread; you only need to use -performBlock: when interacting with the context from a different thread. (Still-relevant parts of my original answer below.)

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I've made an app or two that modifies Xcode's default "Master-Detail" app template to make the "main" MOC (created by the app delegate and passed among view controllers) main-queue-only, and parent to a private-queue context that I use for background operations like importing data from a web fetch. Thus, most uses of the context and its objects happen without being wrapped in performBlock:. (The only time I do use performBlock: is to push changes from the background-task context back to the main one to update the UI.) Works just fine.