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https://github.com/albertodebortoli/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 flavour.
https://github.com/albertodebortoli/Skopelos

coredata coredatastack ios persistence swift

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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 flavour.

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# Skopelos



![logo](./skopelos.png)



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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](https://github.com/albertodebortoli/Skiathos)



## 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](http://martiancraft.com/blog/2015/03/core-data-stack/) by Marcus Zarra.



```

      Managed Object Model <------ Persistent Store Coordinator ------> 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:



```swift

self.skopelos = Skopelos(sqliteStack: "<#ModelURL>")

```

or

```swift

self.skopelos = Skopelos(sqliteStack: "<#ModelURL>", securityApplicationGroupIdentifier: "<#GroupID>")



```

or

```swift

self.skopelos = Skopelos(inMemoryStack: "<#ModelURL>")

```



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:



```swift

protocol SkopelosClientDelegate: class {

    func handle(_ error: NSError)

}



class SkopelosClient: Skopelos {



    static let modelURL = Bundle(identifier: "<#com.mydomain.myapp>").url(forResource: "<#DataModel>", 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:



```objc

__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:



```objc

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: 



```swift

skopelosClient.read { context in

    let users = User.SK_all(context)

    print(users)

}

```



Skopelos writing:



```swift

// 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:



```swift

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:



```swift

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:



- [Just Eat](https://itunes.apple.com/gb/app/just-eat-food-delivery/id566347057?mt=8)