A minimalistic, thread safe, non-boilerplate and super easy to use version of Active Record on Core Data

by iOS Example

Skopelos

A minimalistic, thread-safe, non-boilerplate and super easy to use version of Active Record on Core Data. Simply all you need for doing Core Data. Swift 4 flavour.

Objective-C version

General notes

This component aims to have an extremely easy interface to introduce Core Data into your app with almost zero effort.

The design introduced here involves a few main components:

  • CoreDataStack
  • AppStateReactor
  • DALService (Data Access Layer)

CoreDataStack

If you have experience with Core Data, you might know that creating a stack is an annoying process full of pitfalls. This component is responsible for the creation of the stack (in terms of chain of managed object contexts) using the design described here by Marcus Zarra.

<div class="snippet-clipboard-content position-relative" data-snippet-clipboard-copy-content=" Managed Object Model Persistent Store
^
|
Root Context (NSPrivateQueueConcurrencyType)
^
|
————> Main Context (NSMainQueueConcurrencyType) 

      Managed Object Model  Persistent Store
                                                ^
                                                |
                           Root Context (NSPrivateQueueConcurrencyType)
                                                ^
                                                |
              ------------> Main Context (NSMainQueueConcurrencyType) <-------------
              |                                 ^                                  |
              |                                 |                                  |
       Scratch Context                   Scratch Context                    Scratch Context
(NSPrivateQueueConcurrencyType)   (NSPrivateQueueConcurrencyType)    (NSPrivateQueueConcurrencyType)

An important difference from Magical Record, or other third-party libraries, is that the savings always go in one direction, from scratch contexts down (up direction in the above diagram) to the persistent store. Other components allow you to create scratch contexts that have the private context as parent and this causes the main context not to be updated or to be updated via notifications to merge the context. The main context should be the source of truth and it is tied the UI: having a much simpler approach helps to create a system easier to reason about.

AppStateReactor

You should ignore this one. It sits in the CoreDataStack and takes care of saving the in-flight changes back to disk if the app goes to background, loses focus or is about to be terminated. It’s a silent friend who takes care of us.

DALService (Data Access Layer) / Skopelos

If you have experience with Core Data, you might also know that most of the operations are repetitive and that we usually call performBlock/performBlockAndWait on a context providing a block that eventually will call save: on that context as last statement. Databases are all about readings and writings and for this reason our APIs are in the form of read(statements: NSManagedObjectContext -> Void) and writeSync(changes: NSManagedObjectContext -> Void)/writeAsync(changes: NSManagedObjectContext -> Void): 2 protocols providing a CQRS (Command and Query Responsibility Segregation) approach. Read blocks will be executed on the main context (as it’s considered to be the single source of truth). Write blocks are executed on a scratch context which is saved at the end; changes are eventually saved asynchronously back to the persistent store without blocking the main thread. The completion handler of the write methods calls the completion handler when the changes are saved back to the persistent store.

In other words, writings are always consistent in the main managed object context and eventual consistent in the persistent store. Data are always available in the main managed object context.

Skopelos is just a subclass of DALService, to give a nice name to the component.

How to use

Import Skopelos.

To use this component, you could create a property of type Skopelos and instantiate it like so:

<div class="highlight highlight-source-swift position-relative" data-snippet-clipboard-copy-content="self.skopelos = Skopelos(sqliteStack: " “)
“>

self.skopelos = Skopelos(sqliteStack: "")

or

<div class="highlight highlight-source-swift position-relative" data-snippet-clipboard-copy-content="self.skopelos = Skopelos(sqliteStack: " “, securityApplicationGroupIdentifier: “”)
“>

self.skopelos = Skopelos(sqliteStack: "", securityApplicationGroupIdentifier: "")

or

<div class="highlight highlight-source-swift position-relative" data-snippet-clipboard-copy-content="self.skopelos = Skopelos(inMemoryStack: " “)
“>

self.skopelos = Skopelos(inMemoryStack: "")

N.B. All the above methods also accept an extra optional argument allowsConcurrentWritings (which defaults to false) to allow using a dedicated scratch context per writing operation. For simple applications, reusing the same scratch context (i.e. using the default value) on writings helps avoiding race conditions when the changes are pushed to the main context.

While it would be acceptable to treat Skopelos as a singleton, it’s always best to not use such patter but rather explicitly instantiate a single instance and inject it to parts of the app via dependency injection. Generally speaking, we don’t like singletons. They are not testable by nature, clients don’t have control over the lifecycle of the object and they break some principles. For these reasons, the library comes free of singletons.

You could inherit from Skopelos to:

  • wrap it into an interface that is specific to you use-case
  • override handleError(_error: NSError) to perform specific actions whenever an error is encountered

Here is an example:

<div class="highlight highlight-source-swift position-relative" data-snippet-clipboard-copy-content="protocol SkopelosClientDelegate: class {
func handle(_ error: NSError)
}

class SkopelosClient: Skopelos {

static let modelURL = Bundle(identifier: " “).url(forResource: “”, withExtension: “momd”)!
weak var delegate: SkopelosClientDelegate?
class func sqliteStack() -> Skopelos {
return Skopelos(sqliteStack: modelURL)
}
class func inMemoryStack() -> Skopelos {
return Skopelos(inMemoryStack: modelURL)
}
override func handleError(_ error: NSError) {
DispatchQueue.main.async {
self.delegate?.handle(error)
}
}
}
“>

protocol SkopelosClientDelegate: class {
    func handle(_ error: NSError)
}

class SkopelosClient: Skopelos {

    static let modelURL = Bundle(identifier: "").url(forResource: "", withExtension: "momd")!

    weak var delegate: SkopelosClientDelegate?

    class func sqliteStack() -> Skopelos {
        return Skopelos(sqliteStack: modelURL)
    }

    class func inMemoryStack() -> Skopelos {
        return Skopelos(inMemoryStack: modelURL)
    }

    override func handleError(_ error: NSError) {
        DispatchQueue.main.async {
            self.delegate?.handle(error)
        }
    }
}

Readings and writings

Speaking of readings and writings, let’s do now a comparison between some standard Core Data code and code written with Skopelos.

Standard Core Data reading:

__block NSArray *results = nil;

NSManagedObjectContext *context = ...;
[context performBlockAndWait:^{

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:NSStringFromClass(User)
    inManagedObjectContext:context];
    [request setEntity:entityDescription];

    NSError *error;
    results = [context executeFetchRequest:request error:&error];
}];

return results;

Standard Core Data writing:

NSManagedObjectContext *context = ...;
[context performBlockAndWait:^{

    User *user = [NSEntityDescription insertNewObjectForEntityForName:NSStringFromClass(User)
    inManagedObjectContext:context];
    user.firstname = @"John";
    user.lastname = @"Doe";

    NSError *error;
    [context save:&error];
    if (!error)
    {
        // continue to save back to the store
    }
}];

Skopelos reading:

skopelosClient.read { context in
    let users = User.SK_all(context)
    print(users)
}

Skopelos writing:

// Sync
skopelosClient.writeSync { context in
    let user = User.SK_create(context)
    user.firstname = "John"
    user.lastname = "Doe"
}

skopelosClient.writeSync({ context in
    let user = User.SK_create(context)
    user.firstname = "John"
    user.lastname = "Doe"
    }, completion: { (error: NSError?) in
        // changes are saved to the persistent store
})

// Async
skopelosClient.writeAsync { context in
    let user = User.SK_create(context)
    user.firstname = "John"
    user.lastname = "Doe"
}

skopelosClient.writeAsync({ context in
    let user = User.SK_create(context)
    user.firstname = "John"
    user.lastname = "Doe"
}, completion: { (error: NSError?) in
    // changes are saved to the persistent store
})

Skopelos also supports chaining:

skopelosClient.writeSync { context in
    user = User.SK_create(context)
    user.firstname = "John"
    user.lastname = "Doe"
}.writeSync { context in
    if let userInContext = user.SK_inContext(context) {
        userInContext.SK_remove(context)
    }
}.read { context in
    let users = User.SK_all(context)
    print(users)
}

The NSManagedObject category provides CRUD methods always explicit on the context. The context passed as parameter should be the one received in the read or write block. You should always use these methods from within read/write blocks. Main methods are:

Self
static func SK_numberOfEntities(context: NSManagedObjectContext) -> Int
func SK_remove(context: NSManagedObjectContext)
static func SK_removeAll(context: NSManagedObjectContext)
static func SK_all(context: NSManagedObjectContext) -> [Self]
static func SK_all(predicate: NSPredicate, context:NSManagedObjectContext) -> [Self]
static func SK_first(context: NSManagedObjectContext) -> Self?
“>

static func SK_create(context: NSManagedObjectContext) -> Self
static func SK_numberOfEntities(context: NSManagedObjectContext) -> Int
func SK_remove(context: NSManagedObjectContext)
static func SK_removeAll(context: NSManagedObjectContext)
static func SK_all(context: NSManagedObjectContext) -> [Self]
static func SK_all(predicate: NSPredicate, context:NSManagedObjectContext) -> [Self]
static func SK_first(context: NSManagedObjectContext) -> Self?

Mind the usage of SK_inContext: to retrieve an object in different read/write blocks (same read blocks are safe).

Thread-safety notes

All the accesses to the persistence layer done via a DALService instance are guaranteed to be thread-safe.

It is highly suggested to enable the flag -com.apple.CoreData.ConcurrencyDebug 1 in your project to make sure that you don’t misuse Core Data in terms of threading and concurrency (by accessing managed objects from different threads and similar errors).

This component doesn’t aim to introduce interfaces with the goal of hiding the concept of ManagedObjectContext: it would open up the doors to threading issues in clients’ code as developers should be responsible to check for the type of the calling thread at some level (that would be ignoring the benefits that Core Data gives us). Therefore, our design forces to make all the readings and writings via the DALService and the ManagedObject category methods are intended to always be explicit on the context (e.g. SK_create).

Clients

Skopelos is used in production in the following products:

GitHub

https://github.com/albertodebortoli/Skopelos