Core Data data access pattern?

It seems crazy to me that I have all of these NSFetchRequests for the same NSManagedObjects spread out throughout different view controllers in my app, is ther开发者_开发技巧e a good pattern for data access that puts what I need in a single place?

I agree it is a bit much, fortunately there is Active Record for Core Data. This makes fetching less tedious, for example, fetching all Person objects from core data would be as simple as

NSArray *people = [Person findAll];

Yes there is, it is called a facade pattern. Simply define a public method on your NSManagedObject subclass like so:

@interface Group : NSManagedObject { }
// … cruft here …
-(NSArray*)peopleSortedByName;
@end

And hide the nasty implementation like so:

-(NSArray*)peopleSortedByName;
{
    NSFetchRequest* request = // … bla bla, lots of code here
    return [[self managedObjectContext] executeFetchRequest:request
                                                      error:NULL];
}

Then use the method just as if the it was any other class in your code. Write once, relief everywhere.

Define a category method for NSManagedObject context which wrappers up a general query into a one-liner.

@interface NSManagedObjectContext(MyQueryAdditions)

-(NSArray *)queryEntityForName:(NSString *)name predicateFormat:(NSString *)pstring argumentArray:(NSArray *)arr;

@end


@implementation NSManagedObjectContext(MyQueryAdditions)

-(NSArray *)queryEntityForName:(NSString *)name predicateFormat:(NSString *)pstring argumentArray:(NSArray *)arr
{
    NSEntityDescription *entity = [NSEntityDescription entityForName:name inManagedObjectContext:self];
    NSFetchRequest *fetch = [[[NSFetchRequest alloc] init] autorelease];

    [fetch setEntity:entity];
    NSPredicate *pred;

    if(pstring)
    {
        if(arr) pred = [NSPredicate predicateWithFormat:pstring argumentArray:arr];
        else    pred = [NSPredicate predicateWithFormat:pstring];
        [fetch setPredicate:pred];
    }

    NSError *error = nil;
    [self retain];
    [self lock];
    NSArray *results = [self executeFetchRequest:fetch error:&error];

    if (error) {
        NSLog(@"MOC Fetch - Unresolved error %@, %@", error, [error userInfo]);
        results =  [NSArray array];
    }

    [self unlock];
    [self release];

    return results;

}

@end

Means a basic all items query can be as simple as

NSArray *cres = [managedObjectContext queryEntityForName:@"Person" predicateFormat:nil argumentArray:nil];