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Core Data — best practices Daniel Eggert NSSpain, Logroño September 18, 2013

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Daniel Eggert Me & Core Data

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Agenda • What is Core Data? • Core Data Stack & Data Model • Relationships • Getting Data from Core Data • Performance • Advanced Core Data

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What is Core Data?

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What is Core Data? • Objective-C • Mature Code Base — 8 years in the wild • OS X since 10.4 • iOS since iOS 3 • No UI

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What is Core Data? • (Very powerful) object graph management • Persistency technology • Not an SQL wrapper • Not a Object-relational mapping (O/RM)

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Meet the Stack

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Core Data Stack NSManaged Object NSManaged Object NSManaged Object NSManaged Object

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NSPersistent StoreCoordinator SQLite File System NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store NSPersistent Store NSPersistent Store SQLite File System SQLite File System Core Data Stack

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Core Data Stack NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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Core Data Stack NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System { { Object Graph Management Persistence

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Core Data Stack NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object

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Core Data Stack NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object

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Core Data Stack NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object

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Meet the Model

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© Jimmy Rodriguez www.jimmy-rodriguez.book.fr

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Core Data Model

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Core Data Model Berlin Bilbao Logroño Paris San Francisco Copenhagen

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Core Data Model

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain “Route” Entity “City” Entity

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entity |ˈɛntɪti| noun ( pl. entities ) a thing with distinct and independent existence: Church and empire were fused in a single entity. “Route” Entity “City” Entity

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Model & Relationships Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Data

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Relationships

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Boring Part

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain Where the Fun Happens

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Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain

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Context Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain

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NSManagedObjectContext Berlin Bilbao Logroño Paris San Francisco Copenhagen Denmark WWDC NSSpain NSManagedObjectContext

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Relationships

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Relationships

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Relationships Object Graph Management

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Relationships NSSpain Route *nsspain = ...

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Relationships NSSpain Route *nsspain = ... cities = nsspain.cities;

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Berlin Bilbao Logroño NSSpain Relationships NSSpain Route *nsspain = ... cities = nsspain.cities;

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Relationships cities = nsspain.cities; Berlin Bilbao Logroño NSSpain

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Relationships cities = nsspain.cities; Berlin Bilbao Logroño NSSpain berlin = cities[0];

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Relationships cities = nsspain.cities; Berlin Bilbao Logroño NSSpain berlin = cities[0]; otherRoutes = berlin.routes;

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Berlin Bilbao Logroño Denmark WWDC NSSpain Relationships cities = nsspain.cities; Berlin Bilbao Logroño NSSpain berlin = cities[0]; otherRoutes = berlin.routes;

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Relationships Route *nsspain = ... NSOrderedSet *cities = nsspain.cities; City *berlin = cities[0]; NSSet *otherRoutes = berlin.routes;

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Getting Data from Core Data © Phil Hobgen http://500px.com/philhobgen

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Use Relationships

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Use Relationships Really!

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Use Relationships Really! Really!

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NSFetchRequest

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NSFetchRequest What does a do?

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NSFetchRequest NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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NSFetchRequest NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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Traversing Relationship NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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Traversing Relationship NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System Expensive

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Never use NSFetchRequest, right?

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When to use NSFetchRequest • Searching • All objects • Pre-fetching

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Searching NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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Searching • NSFetchRequest allows you to set a predicate • NSPredicate specify what you want to fetch • Target your search • Less I/O → better performance • Only bring in those objects you want • Core Data doesn’t have to pull in data you don’t need

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Searching • Check Xcode documentation called “Core Data Snippets” for a lot of good info on predicates.

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All Objects • Showing a list of all objects to the user • UITableView • UICollectionView • You must not bring in all objects at once • You must not bring in each object on its own

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All Objects NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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All Objects • Use “batching”: fetchRequest.fetchBatchSize = 40; depends on your app

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All Objects fetchRequest.fetchBatchSize = 3; don’t use 3 in a real app 1 2 3 4 5 6 7 8 9 10 11 12

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All Objects fetchRequest.fetchBatchSize = 3; don’t use 3 in a real app 1 2 3 4 5 6 7 8 9 10 11 12

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All Objects fetchRequest.fetchBatchSize = 3; don’t use 3 in a real app 1 2 3 4 5 6 7 8 9 10 11 12

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All Objects fetchRequest.fetchBatchSize = 3; don’t use 3 in a real app 1 2 3 4 5 6 7 8 9 10 11 12

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All Objects fetchRequest.fetchBatchSize = 3; don’t use 3 in a real app 1 2 3 4 5 6 7 8 9 10 11 12

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Pre-Fetching • Round tripping to the Database is Expensive • Fetch as much as possible in 1 go NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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Pre-Fetching • Execute an NSFetchRequest without using its result. Just to bring objects into the context • Only do this when you need to. • Don’t fetch objects if they’re already in the context (or the persistent store coordinator’s row cache) • Measure • Don’t prematurely optimize

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Pre-Fetching NSManagedObject *mo = ...; mo.isFault [mo hasFaultForRelationshipNamed:]

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Performance © John Bender http://500px.com/bendersama

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Performance Issues Top 3 • Loading too much data • Not loading enough data • Inefficient queries These 3 are a problem in any database application, not just Core Data, but people like to blame Core Data.

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Loading too much data • Loading everything is BAD • batching • aggregating • Sorting and filtering in memory is BAD • predicates • sort descriptors

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Not loading enough data • Round trips to the file system are SLOW • Relationship prefetching

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Inefficient Queries • Use indexes

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Inefficient Queries • Unicode text searching is expensive, but necessary • normalize text (uppercase, diacritics) • ‘like’ is expensive! • beginswith and contains are a lot faster than like

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Inefficient Queries • Order of predicates matters! • Cheap stuff at the beginning • Relationships at the end • Xcode Documentation: “Predicates” Guide

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Performance Debugging -com.apple.CoreData.SQLDebug 1 and / or EXPLAIN QUERY PLAN ...

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© Derek Bruff http://500px.com/derekbruff Advanced Core Data

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Multi-Context Multi-Store Core Data support various, complex setups NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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Multi-Context NSPersistent StoreCoordinator SQLite File System NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store Contexts are distinct — distinct changes. NSManagedObject tied to their owning context.

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Multi-Store No relations across stores. NSPersistent StoreCoordinator SQLite File System NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store NSPersistent Store NSPersistent Store SQLite File System SQLite File System

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Multi-Stack NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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Mixing everything NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System NSPersistent StoreCoordinator SQLite File System NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store NSPersistent Store NSPersistent Store SQLite File System SQLite File System

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Complex Stacks • Very, very flexible • Also adds a lot of complexity • Threading is always complex • You NEED a merging policy • You can almost always do without • Save yourself the trouble — go for simple

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Parent-Child Context • Don’t use it — it’s more complicated than it seems • Implemented to facilitate deferred saving (UIManagedDocument) NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object

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Parent-Child Context • There’s a lot of locking that NEEDS to happen • NSManagedObjectID behaves differently NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object

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Incremental Store • Don’t use it — it’s (a lot) more complicated than it seems

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Incremental Store NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object NSPersistent Store SQLite File System

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NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object Your own code here ? Incremental Store

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NSPersistent StoreCoordinator NSManagedObjectContext NSManaged Object NSManaged Object NSManaged Object NSManaged Object Your own code here ? Incremental Store • Transactional ?!? • Threading / Async ?!?

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Why Core Data? • Relationships • Delete Propagation • Change Tracking and Undo • Scratchpad • Faulting • Uniquing

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Why Core Data? • Very mature code base — more than 8 years • Battle tested • Very performant if used right

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Covered Today • What is Core Data? • Core Data Stack & Data Model • Relationships • Getting Data from Core Data • Performance • Advanced Core Data

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More Information • Check the Xcode Documentation • “Core Data Starting Point” • “Core Data Programming Guide” • “Core Data Snippets” • http://www.objc.io/issue-4/

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Daniel Eggert @danielboedewadt http://www.objc.io/ Senior Software Developer Zeta Project GmbH, Berlin We’re hiring!

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A & Q NSSpain