Core Data with a Single Shared UIManagedDocument

Note: This post is a bit out of date these days and I'd now recommend using a "standard" Core Data stack over UIManagedDocument whether you're planning on using iCloud or not. Many of the Xcode templates come with a basic Core Data setup to help you get started. I've also put together a very small library, a Core Data In-A-Box if you will, you might also find useful.

UIManagedDocument is a great way to set up a Core Data stack for your application. It's also the best way to enable iCloud support for your Core Data applications. For applications with a single view controller as the entry point, setting up the document and passing it on to subsequent view controllers works great. But if your application has multiple entry points, or a single entry point you cannot modify (such as a UITabBarController), it can be a little less straightforward.

While re-writing an existing app to use many of the new features in iOS 5, I ran into this very issue. The application used a tab bar controller as the entry point and my previous experience with letting the first view controller set up the managed document was not going to work. How could I provide access to the document, while insuring it was in the correct state, to all the view controllers in the app? I guess I could have set it up in the application delegate and then grabbed a reference to the tab bar controller through the window and then used respondsToSelector: to figure out which controllers had a document property and...

Blech.

One thing I really like about writing apps for iOS 5 is how little the application delegate is responsible for now. Previously it had become a dumping ground for setting up nearly everything for an application while handling application events and pretty much everything else that didn't fit into a view controller. Using it for setting up my managed document seemed like a step backward. Why not create a new class? A global singleton whose only job is to handle and provide a convenient interface to the managed document?

#import <Foundation/Foundation.h>

typedef void (^OnDocumentReady) (UIManagedDocument *document);

@interface MYDocumentHandler : NSObject

@property (strong, nonatomic) UIManagedDocument *document;

+ (MYDocumentHandler *)sharedDocumentHandler;
- (void)performWithDocument:(OnDocumentReady)onDocumentReady;

@end

There. Just one instance method. Define a block that takes a UIManagedDocument as its only argument and feel secure that it will only be executed once the document has been successfully loaded (if necessary).

Looking inside:

#import "MYDocumentHandler.h"

@interface MYDocumentHandler ()
- (void)objectsDidChange:(NSNotification *)notification;
- (void)contextDidSave:(NSNotification *)notification;
@end;

@implementation MYDocumentHandler

@synthesize document = _document;

static MYDocumentHandler *_sharedInstance;

+ (MYDocumentHandler *)sharedDocumentHandler
{
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        _sharedInstance = [[self alloc] init];
    });

    return _sharedInstance;
}

- (id)init
{
    self = [super init];
    if (self) {
        NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSLibraryDirectory inDomains:NSUserDomainMask] lastObject];
        url = [url URLByAppendingPathComponent:@"MyDocument.md"];

        self.document = [[UIManagedDocument alloc] initWithFileURL:url];

        // Set our document up for automatic migrations
        NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                                 [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
                                 [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];
        self.document.persistentStoreOptions = options;

        // Register for notifications
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(objectsDidChange:)
                                                     name:NSManagedObjectContextObjectsDidChangeNotification
                                                   object:self.document.managedObjectContext];

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(contextDidSave:)
                                                     name:NSManagedObjectContextDidSaveNotification
                                                   object:self.document.managedObjectContext];
    }
    return self;
}

- (void)performWithDocument:(OnDocumentReady)onDocumentReady
{
    void (^OnDocumentDidLoad)(BOOL) = ^(BOOL success) {
        onDocumentReady(self.document);
    };

    if (![[NSFileManager defaultManager] fileExistsAtPath:[self.document.fileURL path]]) {
        [self.document saveToURL:self.document.fileURL
           forSaveOperation:UIDocumentSaveForCreating
          completionHandler:OnDocumentDidLoad];
    } else if (self.document.documentState == UIDocumentStateClosed) {
        [self.document openWithCompletionHandler:OnDocumentDidLoad];
    } else if (self.document.documentState == UIDocumentStateNormal) {
        OnDocumentDidLoad(YES);
    }
}

- (void)objectsDidChange:(NSNotification *)notification
{
#ifdef DEBUG
    NSLog(@"NSManagedObjects did change.");
#endif
}

- (void)contextDidSave:(NSNotification *)notification
{
#ifdef DEBUG
    NSLog(@"NSManagedContext did save.");
#endif
}

@end

Pretty simple. First we setup our singleton using dispatch_once. Then on initialization we define our document, set up the persistent store options and register for a couple notifications (optional, of course). When the document is needed (when performWithDocument: is called) we look to see if the document exists, or is closed, creating it or opening it as required. Once we've loaded our document we execute the block we've received, passing in our now-ready document. Now we have a nice way of accessing our document:

#import "MYDocumentHandler.h"

@implementation MyViewController

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    if (!self.document) {
        [[MYDocumentHandler sharedDocumentHandler] performWithDocument:^(UIManagedDocument *document) {
            self.document = document;
            // Do stuff with the document, set up a fetched results controller, whatever.
        }];
    }
}

@end

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