Core Data Ensembles

Author: Drew McCormack
Date: 29th September, 2013

Ensembles extends Apple's Core Data framework to add peer-to-peer synchronization for Mac OS and iOS. Multiple SQLite persistent stores can be coupled together via a file synchronization platform like iCloud or Dropbox. The framework can be readily extended to support any service capable of moving files between devices, including custom servers.

Incorporating Ensembles in an iOS Project

To add Ensembles to your App's Xcode Project...

1. Drag the Ensembles iOS.xcodeproj project from the Framework directory into your Xcode project.
2. Select the App's project root, and the App target.
3. In the General tab, click the + button in the Linked Frameworks and Libraries section.
4. Choose the libensembles_ios.a library and add it.
5. Select the Build Settings tab. Locate the Header Search Paths setting.
6. Add a path to the Ensembles directory Framework, and make the search path recursive. (You may want to form the path relative to your Xcode project, eg, $(SRCROOT)/../../Framework).
7. Locate the Other Linker Flags setting, and add the flag -ObjC.
8. Drag the Framework/Resources directory from the Ensembles project into your App project. Make sure the Create groups for any added Folders option is selected.

Idiomatic App

Idiomatic is a relatively simple example app that incorporates Ensembles and works with iCloud to sync across devices. The app allows you to record your ideas, and add tags to group them. The Core Data model of the app includes two entities, with a many-to-many relationship.

The Idiomatic project is a good way to get acquainted with Ensembles, and how it is integrated in a Core Data app. Idiomatic can be run in the iPhone Simulator, or on a device, but in order to test it, you need to follow a few preparatory steps.

1. Register an App ID for Idiomatic (eg com.yourcompany.idiomatic) in the Certificates, Identifiers & Profiles section of the iOS Developer Center, and make sure the iCloud service is enabled.

2. Select the Idiomatic Project in the source list of the Xcode project, and then select the Idiomatic target.

3. In the General section, set the bundle identifier (eg com.yourcompany.idiomatic).

4. Select the Capabilities section, turn on the iCloud switch, and replace the existing Ubiquity Container with your own.

5. In the IDMAppDelegate class, locate this code in application:didFinishLaunchingWithOptions:

    // Setup Ensemble
    cloudFileSystem = [[CDEICloudFileSystem alloc] initWithUbiquityContainerIdentifier:@"P7BXV6PHLD.com.mentalfaculty.idiomatic"];
   

6. Fill in the Ubiquity Container Identifier appropriate for your bundle identifier and team identifier. You can find this on the iOS Developer Center under the App ID you registered. Just combine the Prefix entry with the ID.

7. Build and install on devices and simulators that are logged into the same iCloud account.

8. Add notes, and tag them as desired. The app will sync when it becomes active, but you can force a sync by tapping the button under the Groups table.

Getting to Know Ensembles

The most important class in the Ensembles framework is CDEPersistentStoreEnsemble. You create one instance of this class for each NSPersistentStore that you want to sync. This class monitors saves to your SQLite store, and merges in changes from other devices as they arrive.

You typically initialize a CDEPersistentStoreEnsemble around the same point in your code that your Core Data stack is initialized. It is important that the ensemble is initialized before data is saved.

There is one other family of classes that you need to be familiar with. These are classes that conform to the CDECloudFileSystem protocol. Any class conforming to this protocol can serve as the file syncing backend of an ensemble, allowing data to be transferred between devices. You can use one of the existing classes (eg CDEICloudFileSystem), or develop your own.

The initialization of an ensemble is typically only a few lines long.

// Setup Ensemble
cloudFileSystem = [[CDEICloudFileSystem alloc] 
	initWithUbiquityContainerIdentifier:@"P7BXV6PHLD.com.mentalfaculty.idiomatic"];
ensemble = [[CDEPersistentStoreEnsemble alloc] initWithEnsembleIdentifier:@"MainStore" 
	persistentStorePath:storeURL.path 
	managedObjectModel:model 
	cloudFileSystem:cloudFileSystem];
ensemble.delegate = self;

After the cloud file system is initialized, it is passed to the CDEPersistentStoreEnsemble initializer, together with the NSManagedObjectModel and path to the NSPersistentStore. An ensemble identifier is used to match stores across devices. It is important that this be the same for each store in the ensemble.

Once a CDEPersistentStoreEnsemble has been initialized, it can be leeched. This step typically only needs to take place once, to setup the ensemble and perform an initial import of data in the local persistent store. Once an ensemble has been leeched, it remains leeched even after a relaunch. The ensemble only gets deleeched if you explicitly request it, or if a serious problem arises in the cloud file system, such as an account switch.

You can query an ensemble for whether it is already leeched using the isLeeched property, and initiate the leeching process with leechPersistentStoreWithCompletion:. (Attempting to leech an ensemble that is already leeched will cause an error.)

if (!ensemble.isLeeched) {
    [ensemble leechPersistentStoreWithCompletion:^(NSError *error) {
        if (error) NSLog(@"Could not leech to ensemble: %@", error);
    }];
}

Because many tasks in Ensembles can involve networking or long operations, most methods are asynchronous and include a block callback which is called on completion of the task with an error parameter. If the error is nil, the task completed successfully. Methods should only be initiated on the main thread, and completion callbacks are sent to the main queue.

With the ensemble leeched, sync operations can be initiated using the mergeWithCompletion: method.

[ensemble mergeWithCompletion:^(NSError *error) {
    if (error) NSLog(@"Error merging: %@", error);
}];	

A merge involves retrieving new changes for other devices from the cloud file system, integrating them in a background NSManagedObjectContext, merging with new local changes, and saving the result to the NSPersistentStore.

When a merge occurs, it is important to merge the changes into your main NSManagedObjectContext. You can do this in the persistentStoreEnsemble:didSaveMergeChangesWithNotification: delegate method.

- (void)persistentStoreEnsemble:(CDEPersistentStoreEnsemble *)ensemble didSaveMergeChangesWithNotification:(NSNotification *)notification
{
    [managedObjectContext performBlock:^{
        [managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
    }];
}

Note that this is invoked on the thread of the background context used for merging the changes. You need to make sure the mergeChangesFromContextDidSaveNotification: method is invoked on the thread corresponding to the main context.

There is one other delegate method that you will probably want to implement, in order to provide global identifiers for managed objects.

- (NSArray *)persistentStoreEnsemble:(CDEPersistentStoreEnsemble *)ensemble 
	globalIdentifiersForManagedObjects:(NSArray *)objects
{
    return [objects valueForKeyPath:@"uniqueIdentifier"];
} 

This method is also invoked on a background thread. Care should be taken to only access the objects passed on this thread.

It is not compulsory to provide global identifiers, but if you do, the framework will automatically ensure that no objects get duplicated due to multiple imports on different devices. If you don't provide global identifiers, the framework has no way to identify a new object, and will assign it a new unique identifier.

If you do decide to provide global identifiers, it is up to you how you generate them, and where you store them. A common choice is to add an extra attribute to entities in your data model, and set that to a uuid on insertion into the store.

Unit Tests

Unit tests are included for the Ensembles framework on the Mac. To run the tests, open the Xcode project for the Mac platform, choose the Ensembles Mac target in the toolbar at the top, and select the menu item Product > Test.