Where to use and initialize UIManagedDocument - iphone

Question

I am new to IOS and I am following CS193p Stanford's lecture to learn about Core Data and in the demo the Professor used a category with somehow similar to what actually to if we check core data application checkbox while creating application and it puts the core data stuff in appDelegate. That's what professor did in demo he created managedObjectContext in AppDelegate and used a Notification to Pass the Context to controller. Now for Assignment he asked to use UIManagedDocument but I am confused about where to put it. Should I declare it in the View Controller or in AppDelegate and pass context using Notification Centre. So I just want to know which one is better for UIManagedDocument. S

Answer 1

Best way I found is create a new project (just as a sample), check 'Use Core Data' when asked, and look inside the AppDelegate. Use that to see how it's done and possibly, copy the appropriate parts (some adjustments will be needed of course). But mostly, use it to learn how Apple recommends it to be used.
Create a custom class with a shared instance that has managedObjectContext property.
In your application didFinishLaunchingWithOptions in AppDelegate, create your sharedInstance and pass it the 'managedObjectContext`.
MyDataManager *dataManager = [MyDataManager sharedInstance];
dataManager.managedObjectContext = self.managedObjectContext;
Notice also that if you terminate the app by hitting the 'Stop' in Xcode, the data won't be saved (since it's done, as it should, when the AppDelegate is about to close the app properly).
Make sure to also have [self saveContext]; in applicationDidEnterBackground, it supposed to be inside applicationWillTerminate as well. It is a good practice to write the database to memory only when needed (and not at every change). In the AppDelegate, it also ask if the data has changed before committing.
Of course, you can also save manually if you have the managedObjectContext instance.
If you made changes to the database you might have to delete the app from the simulator before running again (otherwise app will crash since the older DB won't match the new one).
Edit: I can't remember where I read it, but the professor does not use CoreData as a database pre-se, only to manage data (images etc.).

Answer 2

You can initialize your UIManagedDocument where ever you want to initiate access to your database. When I did the assignment (last year), I put in the AppDelegate. In glancing back now I see there are two ViewControllers in a UITabBarController and they both need the context. So I used the NSNotification mechanism like Paul did in lecture.
The reasons he gives for using UIManagedDocument over "User Core Data" checkbox (when creating the project) was:
It was simpler for him to explain (UIManagedDocument takes care of lots of complexities)
UIManagedDocument puts you on a fast track to using iCloud
I was just reviewing this as well. If you want to hear from Paul, he talks about it in Lecture 12 (Fall 2013-14) at 14 minutes and 25 seconds (UIManagedContext) and at 14:50 he starts talking about the two ways to get the UIManagedContext -- being UIManagedDocument or the "Use Core Data" checkbox.
Here us a link to the course on iTunes: https://itunes.apple.com/us/course/developing-ios-7-apps-for/id733644550