Add main queue child contexts of the main queue MOC

[rcedwards]

Summary of Changes

Gives the API consumer the ability to create an NSManagedObjectContext with MainQueueConcurrencyType as a child of the Main Queue Context.

Addresses

#127

[wolfbrain]

👀 @Jobot

[Jobot]

Perhaps a guard would be a better choice here.

[Jobot]

Now that I better understand how your discriminating here I'm having trouble seeing how this would be written with guard. So, your first instinct was probably right.

[rcedwards]

hmm perhaps.

Do you think this reads better?

guard !(concurrencyType == .MainQueueConcurrencyType && !NSThread.isMainThread()) else {
    preconditionFailure("Main thread MOCs must be created on the main thread")
}

I don't like having to put a bang around the entire statement but I also like that it doesn't have a default: break

[lyricsboy]

I'd just use an if to avoid inverting the logic:

if concurrencyType == .MainQueueConcurrencyType && !NSThread.isMainThread() {
    preconditionFailure("Main thread MOCs must be created on the main thread")
}

In fact, you can just use precondition and put it all in one handy statement.

[Jobot]

Looks good.
👍

[lyricsboy]

Should the name of this function have something about Main in it for self-documenting purposes?

[rcedwards]

Well this function vends you a new managed object context that is a child of the main queue context, but doesn't' necessarily itself be a main queue context. In fact it defaults to a private queue context so I don't think main is necessary in the function name.