Syncing CoreData and Web Apps with RestKit

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Rails + iOS with RestKit @andrewculver

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The Evolution of iOS Tools

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Before iCloud

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After iCloud

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Syncing is ... hard?

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Keep it simple.

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Topics • Rails API Versioning • RestKit Installation • Mapping Configuration • Pulling and Pushing Data • Offline Mode and Other Syncing • Authentication

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Goals • Understand some basics. • Keep it as simple as possible.

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Our Example App “ClipQueue”

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“ClipQueue” • Helps with workflow automation. • Detects content in the clipboard. • Prompts to save it as a “Clip”, categorize it. • Server processes it. • Dispatches webhooks, sends emails, etc.

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Rails JSON API

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Rails JSON API • Keep your API separate from your core app. • Doing this efficiently requires thin controllers. • Creating resources to represent actions that would otherwise be custom controller actions helps, too.

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class Clip < ActiveRecord::Base attr_accessible :content, :created_at end Clip Model

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Clipqueue::Application.routes.draw do namespace :api do namespace :v1 do resources :clips end end end Setting up a Router

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class Api::V1::ClipsController < ApplicationController load_and_authorize_resource :clip respond_to :json ... end API Controller

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class Api::V1::ClipsController < ApplicationController load_and_authorize_resource :clip respond_to :json ... def index respond_with @clips end ... end API Controller Actions

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class Api::V1::ClipsController < ApplicationController load_and_authorize_resource :clip respond_to :json ... def show respond_with @clip end ... end API Controller Actions

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class Api::V1::ClipsController < ApplicationController load_and_authorize_resource :clip respond_to :json ... def create @clip.save redirect_to [:api, :v1, @clip] end ... end API Controller Actions

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class Ability include CanCan::Ability // Guest users can do anything. def initialize(user) user ||= User.new can :manage, :all end end Single User System

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RestKit Installation

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Installation • Use CocoaPods. • This tutorial includes great installation steps for new projects. • https://github.com/RestKit/RKGist/blob/master/ TUTORIAL.md

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Rails API Versioning

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Rails JSON API Versioning • If your API changes, older apps will crash. • Your ranking in the App Store will crash, too. • Does anyone have a Gem they use for this? • Maintaining old versions of your API is easy: • Make a separate copy for each new version.

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$ cp -R v1/ v2/ Setting up a Router

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Clipqueue::Application.routes.draw do namespace :api do namespace :v1 do resources :clips end namespace :v2 do resources :clips resources :categories end end end Setting up a Router

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Pulling Data

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RESTKit ‘GET’ Lifecycle • Issues HTTP GET request for URL. • Receives XML/JSON response. • Converts to NSDictionary + NSArray structure. • Matches response URL to defined entity mapping. • Maps response data to entity.

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RKEntityMapping *clipMapping = [RKEntityMapping mappingForEntityForName: @"Clip" inManagedObjectStore: managedObjectStore]; [clipMapping addAttributeMappingsFromDictionary:@{ @"id": @"clipId", @"content": @"content", @"created_at": @"createdAt"}]; A Mapping

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RKResponseDescriptor *clipsResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping: clipMapping pathPattern: @"/api/v1/clips" keyPath: nil statusCodes: RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)]; [objectManager addResponseDescriptor:clipsResponseDescriptor]; ‘Index’ Response Descriptor

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RKResponseDescriptor *clipResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping: clipMapping pathPattern: @"/api/v1/clips/:id" keyPath: nil statusCodes: RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)]; [objectManager addResponseDescriptor:clipResponseDescriptor]; ‘Show’ Response Descriptor

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- (void)loadClips { [[RKObjectManager sharedManager] getObjectsAtPath:@"/api/v1/clips" parameters:nil success:nil failure:nil]; } Fetching Objects (UITableViewController)

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- (void)loadClips { [[RKObjectManager sharedManager] getObjectsAtPath:@"/api/v1/clips" parameters:nil success: ^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {} failure: ^(RKObjectRequestOperation *operation, NSError *error) {} ]; } Fetching Objects (UITableViewController)

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The View • How does this information get to the user? • A NSFetchedResultsController is handling the heavy lifting.

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Pull to Refresh

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// In your UITableViewController. UIRefreshControl *refreshControl = [UIRefreshControl new]; [refreshControl addTarget:self action:@selector(loadClips) forControlEvents:UIControlEventValueChanged]; self.refreshControl = refreshControl; Adding Pull to Refresh (UITableViewController)

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Pushing Data

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RESTKit ‘POST’ Lifecycle • Issues HTTP POST request to URL. • Rails creates the object and redirects to it. • From here it’s the same as before: • Receives XML/JSON response. • Converts to NSDictionary + NSArray structure. • Matches redirected URL to defined entity mapping. • Maps response data to entity.

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Clip *clip = (Clip *)[NSEntityDescription insertNewObjectForEntityForName:@"Clip" inManagedObjectContext:self.managedObjectContext]; clip.content = [[UIPasteboard generalPasteboard] string]; Creating a New Clip

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[[RKObjectManager sharedManager] postObject:clip path:@"/api/v1/clips" parameters:nil success:nil failure:nil]; Posting a New Clip

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[[RKObjectManager sharedManager] postObject:clip path:@"/api/v1/clips" parameters:nil success: ^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {} failure: ^(RKObjectRequestOperation *operation, NSError *error) {} ]; Posting a New Clip

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RKRequestDescriptor *newClipRequestDescriptor = [RKRequestDescriptor requestDescriptorWithMapping:[clipMapping inverseMapping] objectClass:[Clip class] rootKeyPath:@"clip"]; [objectManager addRequestDescriptor:newClipRequestDescriptor]; A Request Descriptor

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Mass-Assignment • Rails’ new mass-assignment defaults are good, but they cause issues for us here. • RESTKit sends the ‘id’ and ‘created_at’ attributes across in the request. • This triggers an exception in development by default, because of this: config.active_record.mass_assignment_sanitizer = :strict

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Offline Mode

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Offline Mode • The server is the authority, but is not always available. • We want to be able to work and save data locally even in the absence of an Internet connection. • We don’t want to lose our work. • When a connection is available, we’d like our local work to be pushed up to the server.

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A Very Simple Example • In this app, “Clips” are read-only. • Don’t have to worry about resolving conflicting changes from two sources. • How simple a solution can you get away with?

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A Very Simple Solution • Try saving to the server. • If that doesn’t work, just save it locally. • It won’t have a “Clip ID”, but that doesn’t matter for CoreData, even with relationships. • When a connection becomes available, push it up.

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A Very Simple Solution • The only question is, how will the new ID get mapped to the locally stored object? • It can’t be matched by primary key ID, because it didn’t have one locally. • We’ll use a globally unique ID.

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NSString *dataBaseString = [RKApplicationDataDirectory() stringByAppendingPathComponent:@"ClipQueue.sqlite"]; NSPersistentStore __unused *persistentStore = [managedObjectStore addSQLitePersistentStoreAtPath:dataBaseString fromSeedDatabaseAtPath:nil withConfiguration:nil options:nil error:nil]; Adding Local Storage

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clip.uuid = [self getUUID]; [[RKObjectManager sharedManager] postObject:clip path:@"/api/v1/clips" parameters:nil success: ^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {} failure: ^(RKObjectRequestOperation *operation, NSError *error) { NSError *localError = nil; if (![self.managedObjectContext save:&localError]) { // Show UIAlertView to user. [self showErrorAlert:[localError localizedDescription]]; } }]; Queuing Up Locally

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NSFetchRequest *request = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Clip" inManagedObjectContext:self.managedObjectContext]; [request setEntity:entity]; NSPredicate *predicate = [NSPredicate predicateWithFormat:@"clipId == 0"]; [request setPredicate:predicate]; ... for (Clip *clip in mutableFetchResults) { [[RKObjectManager sharedManager] postObject:clip path:@"/api/v1/clips" parameters:nil success:nil failure: nil]; } Syncing Up

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class Clip < ActiveRecord::Base ... before_validation do self.uuid = UUIDTools::UUID.random_create.to_s if uuid.nil? end end Updating API Logic

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Sync Friendly Deletes

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Sync Friendly Deletes • An object exists locally, but not on the server. • Was it created locally or deleted remotely? • An object exists remotely, but not locally. • Was it created remotely or deleted locally? • Let’s look at a really easy way to deal with this.

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class Clip < ActiveRecord::Base ... scope :not_deleted, where('deleted_at IS NULL') def destroy self.update_column(:deleted_at, Time.zone.now) end end Soft Delete

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- (NSFetchedResultsController *)fetchedResultsController { ... // Filter out deleted results. NSPredicate *predicate = [NSPredicate predicateWithFormat:@"deletedAt == nil"]; [fetchRequest setPredicate:predicate]; ... } Filtering Deleted Items

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Added Bonus • NSFetchedResultsController makes this an other UI updates really pleasant to the user by default, even when they’re triggered by results from the server.

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Authentication

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Authentication Goals • Require username and password. • Scope all data by user.

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Authentication • Prompt for credentials. • Store them in iOS Keychain. • Delay loading until credentials are provided. • Configure the credentials for RestKit. • Implement data scoping on the server based on the session.

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Devise.setup do |config| ... config.http_authenticatable = true ... end HTTP Authentication

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- (void)viewWillAppear:(BOOL)animated { [super viewWillAppear:animated]; if ([self shouldDisplayLoginModal]) { [self performSegueWithIdentifier:@"ShowLogin" sender:self]; } } Show Sign-In Modal

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- (void)viewDidAppear:(BOOL)animated { [super viewDidAppear:animated]; if (![self shouldDisplayLoginModal]) { [self loadObjectsFromDataStore]; } } Delay Loading

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RKObjectManager* objectManager = [RKObjectManager sharedManager]; objectManager.client.username = emailAddressField.text; objectManager.client.password = passwordField.text; Configure Credentials

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Add User-Clips Relationship • Add “user_id” attribute to Clip. • Add “has_one :user” relationship to Clip. • Add “has_many :clips” relationship to User.

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class Ability include CanCan::Ability def initialize(user) user ||= User.new if user.persisted? can :manage, :clip, {user_id: user.id} end end end HTTP Authentication

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class Api::V1::ClipsController < ApplicationController before_filter :authenticate_user! load_and_authorize_resource :clip, through: current_user ... end Scoping Controller Results

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What We’ve Covered • Rails API Versioning • RestKit Installation • Mapping Configuration • Pulling and Pushing Data • Offline Mode and Other Syncing • Authentication

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Goals • Understand some basics. • Keep it as simple as possible.

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Additional Resources Blake Watters’ RESTKit 0.20.0 “Gist” Tutorial https://github.com/RestKit/RKGist/blob/master/TUTORIAL.md (A work in progress.)