Import AppDelegate into models?

I'm trying to unify all of my functions of a specific class within the model file. For instance, I would have a function fetchContactWithName:(NSString *)name in the model 'Contact.h/Contact.m', which my viewcontroller would subsequently call.

In this case, would it be a bad idea to import the AppDelegate.h file into the model file as a I need to access its managedObjectContext?

#import "AppDelegate.h"

@implementation Contact

...

+ (Contact *) fetchContactWithName:(NSString *) name {
  AppDelegate *delegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];

  NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
  NSEntityDescription *entity = [NSEntityDescription entityForName:@"Contact" inManagedObjectContext:delegate.managedObjectContext];
  [fetchRequest setEntity:entity];

  NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name == %@", name];
  [fetchRequest setPredicate:predicate];

  NSError *error = nil;
  NSArray *fetchedObjects = [delegate.managedObjectContext executeFetchRequest:fetchRequest error:&error];

  Contact *fetchedContact;
  for(Contact *contact in fetchedObjects) {
      fetchedContact = contact;
  }

  if(fetchedContact != nil) {
      return fetchedContact;
  } else {
      return nil;
  }

}
@end

---

In my opinion it's a bad idea to directly ask a different class to get your managed object context. Because

You can't really reuse your classes in different projects (think OS X app)

You can't fetch contacts in a different context (think background import)

You can't use unit tests if the method asks other class

You should tell this method in which context it should fetch.
Instead of + (Contact *) fetchContactWithName:(NSString *) name your method signature should look like this:

+ (Contact *)fetchContactWithName:(NSString *)name inManagedObjectContext:(NSManagedObjectContext *)context

each viewController should have a reference to the NSManagedObjectContext used in your app delegate. You can pass a reference of the context to each viewController in application:didFinishLaunchingWithOptions: , and each time you push or present a new viewController you pass the context instance to it.

This might look like a lot of work now, but some day you will benefit from the "tell, don't ask" approach.

---

I think, it is bad idea . Because it's OOP-ugly. What do I suggest? You should make a singleton class SharedStorage like this:

+ (SharedStorage*)sharedStorage
{
    static SharedStorage* _sharedStorage = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedStorage = [[self alloc] init];
    });
    return _sharedStorage;
}

- (SharedStorage*)init {
    self = [super init];
    if (self)
    {
        [self setupPersistentStoreCoordinator];
    }
    return self;
}

- (NSManagedObjectModel*)__managedObjectModel
{
    if (_managedObjectModel_ == nil)
    {
        NSBundle *mainBundle = [NSBundle mainBundle];
        NSURL *modelURL = [mainBundle URLForResource:@"yourURL" withExtension:@"momd"];
        _managedObjectModel_ = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    }
    return _managedObjectModel_;
}


- (void)setupPersistentStoreCoordinator
{
    if (_storeCoordinator != nil)
        return;
    NSString *storePath = [[self applicationDocumentsDirectory] stringByAppendingPathComponent:@"YourDB.sqlite"];
    NSURL *storeUrl = [NSURL fileURLWithPath:storePath];
    NSError *error;
    _storeCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self __managedObjectModel]];
    if (![_storeCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:nil error:&error])
    {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    }
}

And everywhere you want to use context you have to create a new NSManagedObjectContext with the same persistentStoreCoordinator :

self.context = [NSManagedObjectContext new];
    self.context.persistentStoreCoordinator = [[SharedStorage sharedStorage] storeCoordinator];

---

To access stuff's of AppDelegate class you need to

In header file

extern AppDelegate *appDelegate; 

and in AppDelegate.m file

AppDelegate *appDelegate = nil;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    appDelegate = (AppDelegate *)[[UIApplication sharedApplication]delegate];
}

Now in Model view controller you import AppDelegate file and you can access its methods through its object.

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