Unsure if UIManagedDocument is giving me a managed object context? - ios

Question

EDIT: based on the (very helpful) comment below, after further review, I'd just like to add (for other rookies like myself) that when using UIManagedDocument to obtain your core data functionality, pay attention to WHEN your instance is being opened in your app.
The error I was getting was because my managed object context WAS nil when my fetched results controller was being set up. I moved the method call which set up the fetched results controller to after the UIManagedDocument instance was opened.
It might be very basic, and just a common-sense issue to most, but for us rookies, when core data is not being set up in the app delegate, we need to learn that the document has to be in a usable state before we can set the fetchedResultsController.
I'm working through Paul Hegarty's Stanford lectures on Core Data, specifically the demo lecture for the "Photomania" app.
Instead of using NSDictionaries of photos, I modified the app to include only Model objects that store NSStrings (for example, a person's name, etc.) It's just a learning exercise for me.
I believe I've successfully recreated a UIManagedDocument using his code, and set the view controller's managed object property to that of the document's managed object context through the two methods below (they're his methods, not mine, of course).
I put this code in a tableview controller that will appear on screen so I could test in viewDidLoad if the managed object context exists (it's a subclass of his CoreDataTableViewController class). Is that even a valid test for a managed object context?
I understand the code that creates or opens the UIManagedDocument, but I don't understand why that managed object context is nil (if i modify the little test in viewDidLoad, it will tell me that the context is == nil ).
At this point, nothing has been written to the context, I haven't included any fetched results either.
I'm breaking it down into sections because my simplified version of the lecture app kept giving me the error that the managed object context and fetched results controller were nil.
I want to test, in this case, if I'm getting a valid managed object context before moving forward. I have a strong feeling there's some fundamental piece of info I'm missing in what's essentially copied code (for learning purposes).
Does anyone know why the managed object context is nil? Or SHOULD it be that way at this point because I've missed something when setting it? Or when I set the UIManagedDocument's context as the table view controller's ( self) context?
Any information would be greatly appreciated.
-(void)setManagedObjectContext:(NSManagedObjectContext *)managedObjectContext
{
_managedObjectContext = managedObjectContext;
}
-(void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
if (!self.managedObjectContext) [self useDemoDocument];
}
-(void)useDemoDocument
{
NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
url = [url URLByAppendingPathComponent:#"Demo Document"];
UIManagedDocument *document = [[UIManagedDocument alloc] initWithFileURL:url];
if (![[NSFileManager defaultManager] fileExistsAtPath:[url path]]) {
//create it
[ document saveToURL:url
forSaveOperation:UIDocumentSaveForCreating
completionHandler:^(BOOL success) {
if (success) {
self.managedObjectContext = document.managedObjectContext;
}
}];
} else if (document.documentState == UIDocumentStateClosed){
//open it
[document openWithCompletionHandler:^(BOOL success) {
if (success) {
self.managedObjectContext = document.managedObjectContext;
}
}];
} else {
//try to use it
self.managedObjectContext = document.managedObjectContext;
}
}
-(void)viewDidLoad
{
if (self.managedObjectContext) {
NSLog(#"there is a managed object context");
}
}