Simpler Core Data with RubyMotion

Stefán Hafliðason ! http://stefan.haflidason.com

" @styrmis

Why RubyMotion?• Promises increased developer productivity

• Brings the flexibility of Ruby to iOS and OSX development

• Bridges directly to Obj-C libraries: no intermediate glue code

• A REPL for working with your app live!

• Make tweaks quickly

• Build whole views programmatically on the fly

Why Core Data?

• Optimised for low-memory/embedded (iOS) devices

• Mature data access/persistence framework

• Also available on OSX

• Works with iCloud—free cloud syncing for your app

Core Data is Difficult• Provided boilerplate code unnecessarily complex

• An object graph that’s persisted to an SQLite database

• Suggests relational access, which is not quite the case

• Typical patterns for working with relational data are not optimal here

RubyMotion is “Easy”

• Friendliness of Ruby

• An ARC equivalent is included

• Lots of work done to abstract complexity away

• More concepts similar to other OO languages

Core Data and RubyMotion

• No equivalent of Xcode’s visual data modeller

• How do I define my data model?!

• What about versioning?!

• How will I handle migrations?

What we need• Our data model (NSEntityDescriptions +

NSRelationshipDescriptions forming our NSManagedObject)

• A Core Data Stack (NSManagedObjectModel + NSPersistentStoreCoordinator + NSManagedObjectContext)

• A workflow for versioning and migrating between versions

Defining Our Data Model• We would normally do this in Xcode

• Visual Editor for .xcdatamodel bundles

• Integrated handling of versioning and custom migration code

• Automatic lightweight (schema) migrations

• How do we achieve this with RubyMotion?

Options for RubyMotion

• Handle everything programmatically (low level) #

• Use Xcode to work with .xcdatamodel files, copy in each time #

• Use a Ruby library for creating .xcdatamodel files $

Handling Everything Programmatically

entity = NSEntityDescription.alloc.initentity.name = 'Task'entity.managedObjectClassName = 'Task'entity.properties = [ 'task_description', NSStringAttributeType, 'completed', NSBooleanAttributeType ].each_slice(2).map do |name, type| property = NSAttributeDescription.alloc.init property.name = name property.attributeType = type property.optional = false property end

Handling Everything Programmatically

entity = NSEntityDescription.alloc.initentity.name = 'Task'entity.managedObjectClassName = 'Task'entity.properties = [ 'task_description', NSStringAttributeType, 'completed', NSBooleanAttributeType ].each_slice(2).map do |name, type| property = NSAttributeDescription.alloc.init property.name = name property.attributeType = type property.optional = false property end

Not all that bad, but we want to use .xcdatamodel files

.xcdatamodel files are just XML

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>!<model name="" userDefinedModelVersionIdentifier="001" type="com.apple.IDECoreDataModeler.DataModel" documentVersion="1.0" lastSavedToolsVersion="2061" systemVersion="12D78" minimumToolsVersion="Xcode 4.3" macOSVersion="Automatic" iOSVersion="Automatic">! <entity name="Article" syncable="YES">! <attribute name="title" optional="YES" attributeType="String" syncable="YES"/>! <relationship name="author" optional="YES" minCount="1" maxCount="1" deletionRule="Nullify" destinationEntity="Author" inverseName="articles" inverseEntity="Article" syncable="YES"/>! </entity>! <entity name="Author" syncable="YES">! <attribute name="name" optional="YES" attributeType="String" syncable="YES"/>! <relationship name="articles" optional="YES" minCount="1" maxCount="1" deletionRule="Nullify" destinationEntity="Article" inverseName="author" inverseEntity="Author" syncable="YES"/>! </entity>!</model>

Using a library to generate .xcdatamodel files (ruby-xcdm)

1 schema "001" do! 2 entity "Article" do! 3 string :body, optional: false! 4 integer32 :length! 5 boolean :published, default: false! 6 datetime :publishedAt, default: false! 7 string :title, optional: false! 8 ! 9 belongs_to :author!10 end!11 !12 entity "Author" do!13 float :fee!14 string :name, optional: false!15 has_many :articles!16 end!17 end

Workflow• Create schema file in schemas directory, e.g. schemas/001_initial.rb

• Build the schema

• Add a new schema version, e.g. 002_add_new_fields.rb

• Rebuild the schema

• That’s it!

Workflow$ echo "gem 'ruby-xcdm', '0.0.5'" >> Gemfile$ bundle install$ rake schema:buildGenerating Data Model learn-xcdm Loading schemas/001_initial.rb Writing resources/learn-xcdm.xcdatamodeld/1.xcdatamodel/contents$ rake # The default rake task is to run the app in the simulator(main)> mom = NSManagedObjectModel.mergedModelFromBundles(nil)=> #<NSManagedObjectModel:0x8fa7690>(main)> mom.entities.count=> 2(main)> mom.entities.first.name=> "Article"(main)> mom.entities.first.propertiesByName=> {"body"=>#<NSAttributeDescription:0x8e5db30>, "title"=>#<NSAttributeDescription:0x8ea4770>}

Advantages of using ruby-xcdm

• No magic: generates XML from a schema

• Schema versions are fully text-based and readable, making them well-suited to version control

• Can compile our versions into .xcdatamodeld bundles, completely removing dependence on Xcode

Basic Core Data Stack 1 model = NSManagedObjectModel.mergedModelFromBundles(nil) 2 3 store = NSPersistentStoreCoordinator.alloc.initWithManagedObjectModel(model) 4 store_path = File.join(NSHomeDirectory(), 'Documents', 'LearnXcdm.sqlite') 5 store_url = NSURL.fileURLWithPath(store_path) 6 7 options = { NSMigratePersistentStoresAutomaticallyOption => true, 8 NSInferMappingModelAutomaticallyOption => true } 9 10 error_ptr = Pointer.new(:object)11 12 unless store.addPersistentStoreWithType(NSSQLiteStoreType,13 configuration: nil,14 URL: store_url,15 options: options,16 error: error_ptr)17 raise "[ERROR] Failed to create persistent store: #{error_ptr[0].description}"18 end19 20 @context = NSManagedObjectContext.alloc.init21 @context.persistentStoreCoordinator = store

Core Data Query

• From the developers of ruby-xcdm

• Abstracts away much of the complexity of Core Data

• All you need is your .xcdatamodeld bundle

Core Data Query in Action# app/models/task.rbclass Task < CDQManagedObjectend!# app/app_delegate.rbclass AppDelegate include CDQ! def application(application, didFinishLaunchingWithOptions:launchOptions) cdq.setup true endend

Core Data Query in Action(main)> Task.count=> 0(main)> t1 = Task.create(task_description: "Complete presentation")(main)> t2 = Task.create(task_description: "File tax return")(main)> cdq.save=> true(main)> exit$ rake...(main)> Task.count=> 2(main)> t1, t2 = Task.all.array(main)> t1.task_description=> "Complete chapter"(main)> t2.task_description=> "File tax return"(main)> t2.destroy=> #<NSManagedObjectContext:0x914cbe0>(main)> cdq.save=> true(main)> Task.count=> 1

Core Data in ActionAuthor.where(:name).eq("Emily")Author.where(:name).not_equal("Emily")Author.limit(1)Author.offset(10)Author.where(:name).contains("A").offset(10).first!# ConjuctionsAuthor.where(:name).contains("Emily").and.contains("Dickinson")Author.where(:name).starts_with("E").or(:pub_count).eq(1)!# Nested ConjuctionsAuthor.where(:name).contains("Emily").and(cdq(:pub_count).gt(100).or.lt(10))!# RelationshipsAuthor.first.publications.offset(2).limit(1)cdq(emily_dickinson).publications.where(:type).eq('poetry')!class Author < CDQManagedObject scope :prolific, where(:pub_count).gt(50)end

Takeaways

• Don’t be put off by the Xcode boilerplate: Core Data doesn’t have to be that hard

• With CDQ, Core Data is arguably easier to use with RubyMotion rather than harder

• XCDM, CDQ and RubyMotion Query (all by Infinitered) are all worth taking a look at

Next Steps• In the coming weeks I’ll be researching and writing

about:

• How to best handle heavyweight/data migrations in RubyMotion

• Deconstructing the ‘magic’ in Core Data Query

• RubyMotion development best practices

Stefán Hafliðason ! http://stefan.haflidason.com

" @styrmis