a behind the scenes look at the core data objects added to your projects in xcode

1. Pulling back the curtain on
Core Data
Chris Mar
Spree Commerce
http://cmar.me

2. Chris Mar
• Spree Commerce
• iOS and Rails Developer
• @cmar
• Grassy Knoll Apps - Redskins
Radar

3. Goal
Pull back the curtain on Core Data so
you will use it on your next project
without fear!
(we’ll focus on concepts, not a tutorial)

4. Core Data
• Object Relational Mapping
• Gives you access to a database without using sql
or parsing results
• Cocoa API with XCode tooling to assist you
• Hibernate for Java, ActiveRecord for Rails

5. Storage Types
• SQLite Database
• XML
• In-Memory
we’ll focus on SQLite

6. NSManagedObject
• Row from the database
• Key-Value coding for fields
• Optionally subclass
• Fetched from NSManagedObjectContext
[employee valueForKey:@"name"];

7. Managed Objects
object1 object2
object1 object2
NSManagedObjectContext
NSPersistentStoreCoordinator
NSManagedObjectModel
object1
SQLite Database

8. FetchRequest for
NSManagedObjects
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"Order"
inManagedObjectContext:self.managedObjectContext]];
[request setPredicate:[NSPredicate
predicateWithFormat:@"name like %@", @"*7*"]];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
self.orders = [self.managedObjectContext
executeFetchRequest:request error:&error];
[sortDescriptor release];
[sortDescriptors release];
[request release];

9. The Big 3
NSManagedObjectContext
NSPersistentStoreCoordinator sqlite
NSManagedObjectModel .xcdatamodel

10. NSManagedObjectContext
• Object space for Managed Objects
• Fetches from Persistent Store
• Exclusive to a single thread
• Commit and Discard Changes

11. NSManagedObjectContext
(lazy loaded in AppDelegate)
- (NSManagedObjectContext *)managedObjectContext
{
if (__managedObjectContext != nil)
{
return __managedObjectContext;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil)
{
__managedObjectContext = [[NSManagedObjectContext alloc] init];
[__managedObjectContext setPersistentStoreCoordinator:coordinator];
}
return __managedObjectContext;
}

12. NSPersistentStoreCoordinator
• Manages connections to stores
• Maps data model to database
• Handles many stores
• Shared between threads

13. NSPersistentStoreCoordinator
(lazy loaded in AppDelegate)
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
if (__persistentStoreCoordinator != nil)
{
return __persistentStoreCoordinator;
}
NSURL *storeURL = [[self applicationDocumentsDirectory]
URLByAppendingPathComponent:@"MyApp.sqlite"];
NSError *error = nil;
__persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc]
initWithManagedObjectModel:[self managedObjectModel]];
if (![__persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType
configuration:nil URL:storeURL options:nil error:&error]) { }
return __persistentStoreCoordinator;
}

14. NSManagedObjectModel
• Definition of database schema
• Describes entity objects in store
• Predefined Queries - Fetch Requests
• Relationships between entities

15. NSManagedObjectModel
(lazy loaded in AppDelegate)
- (NSManagedObjectModel *)managedObjectModel
{
if (__managedObjectModel != nil)
{
return __managedObjectModel;
}
NSURL *modelURL = [[NSBundle mainBundle]
URLForResource:@"MyApp" withExtension:@"momd"];
__managedObjectModel = [[NSManagedObjectModel alloc]
initWithContentsOfURL:modelURL];
return __managedObjectModel;
}

16. NSManagedObjectModel
XCode Editor

17. Insert new
NSManagedObject
NSManagedObject *car = [NSEntityDescription insertNewObjectForEntityForName:@"Car”
inManagedObjectContext:self.managedObjectContext];
[car setValue:@"Jeep" forKey:@"name"];
//Save the whole context which will save all the objects in it
NSError *error = nil;
if ([self.managedObjectContext save:&error]) {
NSLog(@"Saved");
} else {
NSLog(@"Error saving %@", [error localizedDescription]);
}

18. Threads
Pass ID between threads
Thread1 Thread2
Object id=1 Object id=1
Object Object
NSManagedObjectContext NSManagedObjectContext
id=1 id=1
NSPersistentStoreCoordinator
SQLite Database
Object id=1

19. Memory Usage
self.orders = [self.managedObjectContext executeFetchRequest:request error:&error];
• Load into an Array
• All in memory
• Updates
• Fast and Easy for small tables

20. NSFetchedResultsController
• Efficiently backs a UITableView
• Reduces memory usage
• Caches results
• Updates table for changes

21. NSFetchedResultsController
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController
alloc]
initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext
sectionNameKeyPath:nil cacheName:@"Root"];

22. The Big Picture
UITableView
NSManagedObject
NSFetchedResultsController
NSManagedObject iPhone
NSManagedObject iPod touch
NSManagedObject
iPad
NSManagedObject
NSManagedObjectContext Steve Jobs Rocks
NSPersistentStoreCoordinator sqlite
NSManagedObjectModel .xcdatamodel

23. Relationships
• to-one
• NSManagedObject *manager = [employee valueForKey:@”manager”];
• to-many
• NSSet *directReports = [manager valueForKey:@”directReports”];
• Define inverses for data integrity
• Delete Rules - nullify, cascade, deny

24. Fetched Properties
• Lazy loaded query relationship
• Sorted
• Predicate - filtered
• “Give a list of direct reports that begin with the
letter C”

25. Fetch Requests
• Uses NSPredicate for Where clause
• Uses NSSortDescriptor for Order By clause
• Can be saved with Managed Object Model
[fetchRequestTemplateForName]

26. Predicates
• [NSPredicate predicateWithFormat:@"name CONTAINS[c] %@", searchText];
• @"name BETWEEN %@", [NSArray arrayWithObjects:@”a”,@”b”,nil]
• MATCHES for regex
• CONTAINS[c] for case insenstivity
• AND, OR, NOT

27. Default Project with Core
1. [awakeFromNib]
AppDelegate RootViewController
set managedObjectContext
2. [managedObjectContext] 3. [cellForRowAtIndexPath]
lazy load lazy load fetchedResultsController
NSManagedObjectContext fetchedResultsController

28. SQLite Tables

29. Z_PRIMARYKEY

30. Preloaded Database
• Load up database using Base
• Base lets you import csv
• Make sure you set the primary keys
• On start check for existence, then copy it over

31. Preloading Database
NSString *storePath = [[self applicationDocumentsDirectory]
stringByAppendingPathComponent: @"products.sqlite"];
NSURL *storeUrl = [NSURL fileURLWithPath:storePath];
// Put down default db if it doesn't already exist
NSFileManager *fileManager = [NSFileManager defaultManager];
if (![fileManager fileExistsAtPath:storePath]) {
NSString *defaultStorePath = [[NSBundle mainBundle]
pathForResource:@"preloaded_products" ofType:@"sqlite"];
if (defaultStorePath) {
[fileManager copyItemAtPath:defaultStorePath toPath:storePath
error:NULL];
}
}

32. FMDB
• Objective-C wrapper for SQLite
• Direct access
• Low overhead
• Great for simple data needs
• https://github.com/ccgus/fmdb

33. FMDB Example
FMDatabase *db = [FMDatabase databaseWithPath:@"/tmp/my.db"];
[db open]
FMResultSet *rs = [db executeQuery:@"select * from orders where customer_id = ?",
@"1000"];
[rs intForColumn:@"c"],
[rs stringForColumn:@"b"],
[rs dateForColumn:@"d"],
[rs doubleForColumn:@"e"]);
}
[rs close];
[db close];

34. Links
• http://www.raywenderlich.com/934/core-data-
tutorial-getting-started
• http://menial.co.uk/software/base/
• https://github.com/ccgus/fmdb

35. Thank You!
@cmar on twitter
http://cmar.me