CohesionKit - Single Source of Truth

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Keep your models synchronized in your app and never have any inconsistency anymore. Designed using latest Swift features.

Why using CohesionKit?

  • 🔁 You need realtime synchronisation (websockets)
  • 🌐 You have multiple data sources (REST, CoreData, websocket, phone Contacts, Google Maps, etc…)
  • 🪶 You look for a full Swift lightweight tool
  • 🗃️ You want to use structs

Features

  • [x] 🦺 Thread safe
  • [x] 🪶 Lighweight (< 600 lines of code)
  • [x] 🪪 Working with plain Swift struct and Identifiable objects
  • [x] 🔀 Support for Combine
  • [x] 🧠 In-memory storage
  • [x] 🐾 Low memory footprint
  • [x] 🐪 Strongly typed

Where to put CohesionKit in my stack?

CohesionKit being a Single Source of Truth solution it handles your objects lifecycle and synchronization from any source.

You should put CohesionKit in front of your data sources (REST API, GraphQL, …) before returning data to your app.

sequenceDiagram
    autonumber

        YourApp ->>DataSource: findBooks
        DataSource ->>GraphQL: query findBooks
        GraphQL -->>DataSource: FindBooksQueryResult
        DataSource ->>CohesionKit: store books [A,B,C]
        CohesionKit -->> YourApp: Publisher<[A,B,C]>

        WebSocket ->> WebSocketListener: book A updated
        WebSocketListener ->> CohesionKit: update book A
        CohesionKit -->> YourApp: Publisher<[A,B,C]>

Installation

  • Swift Package Manager
dependencies: [
    .package(url: "https://github.com/pjechris/CohesionKit.git", .upToNextMajor(from: "0.7.0"))
]

Examples

Library comes with an example project so you can see a real case usage. It mostly shows:

  • How to store data in the library
  • How to retrieve and update that data for realtime
  • How data is synchronised throughout multiple screens

Getting started

Storing an object

First create an instance of EntityStore:

let entityStore = EntityStore()

EntityStore let you store Identifiable objects:

struct Book: Identifiable {
  let id: String
  let title: String
}

let book = Book(id: "ABCD", name: "My Book")

entityStore.store(book)

Then You can retrieve the object from anywhere in your code:

// somewhere else in the code
entityStore.find(Book.self, id: "ABCD") // return Book(id: "ABCD", name: "My Book")

Observing changes

Every time data is updated in EntityStore triggers a notification to any registered observer. To register yourself as an observer just use result from store or find methods:

func findBooks() -> some Publisher<[Book], Error> {
  // 1. load data using URLSession
  URLSession(...)
  // 2. store data inside our entityStore
    .store(in: entityStore)
    .sink { ... }
    .store(in: &cancellables)
}

entityStore.find(Book.self, id: 1)?
  .asPublisher
  .sink { ... }
  .store(in: &cancellables)

CohesionKit has a weak memory policy you should read about. As such, returned value from entityStore.store must be strongly retained to not lose value.

For brievety, next examples will omit .sink { ... }.store(in:&cancellables).

Relational objects

To store objects containing nested identity objects you need to make them conform to one protocol: Aggregate.

struct AuthorBooks: Aggregate {
  var id: Author.ID { author.id }

  var author: Author
  var books: [Book]

  // `nestedEntitiesKeyPaths` must list all Identifiable/Aggregate this object contain
  var nestedEntitiesKeyPaths: [PartialIdentifiableKeyPath<Self>] {
    [.init(\.author), .init(\.books)]
  }
}

CohesionKit then handles synchronisation for the three entities:

  • AuthorBook

Author

Author
  • Book

Only writable keypath are accepted. Using a KeyPath (let) will result in error: “Key path value type KeyPath cannot be converted to contextual type WritableKeyPath”

This gives you the ability to retrieve them independently from each other:

let authorBooks = AuthorBooks(
    author: Author(id: 1, name: "George R.R Martin"),
    books: [
      Book(id: "ACK", title: "A Clash of Kings"),
      Book(id: "ADD", title: "A Dance with Dragons")
    ]
)

entityStore.store(authorBooks)

entityStore.find(Author.self, id: 1) // George R.R Martin
entityStore.find(Book.self, id: "ACK") // A Clash of Kings
entityStore.find(Book.self, id: "ADD") // A Dance with Dragons

You can also modify any of them however you want. Notice the change is visible from the object itself AND from aggregate objects:

let newAuthor = Author(id: 1, name: "George R.R MartinI")

entityStore.store(newAuthor)

entityStore.find(Author.self, id: 1) // George R.R MartinI
entityStore.find(AuthorBooks.self, id: 1) // George R.R MartinI + [A Clash of Kings, A Dance with Dragons]

You might think about storing books on Author directly (author.books). In this case Author needs to implement Aggregate and declare books as nested entity.

However I strongly advise you to not nest Identifiable objects into other Identifiable objects. Read Handling relationships article if you want to know more about this subject.

Storing vs Updating

For now we only focused on entityStore.store but CohesionKit comes with another method to store data: entityStore.update.

Sometimes both can be used but they each have a different purpose:

  1. store is suited for storing full data retrieved from webservices, like GET /user for instance
  2. update is usually used for partial data. It’s also the preferred method when receiving events from websockets.

Advanced topics

Enum support

Starting with 0.13 library has support for enum types. Note that you’ll need to conform to EntityWrapper and provide computed getter/setter for each entity you’d like to store.

enum MediaType: EntityWrapper {
  case book(Book)
  case game(Game)
  case tvShow(TvShow)

  func wrappedEntitiesKeyPaths<Root>(relativeTo parent: WritableKeyPath<Root, Self>) -> [PartialIdentifiableKeyPath<Root>] {
    [.init(parent.appending(\.book)), .init(parent.appending(\.game)), .init(parent.appending(\.tvShow))]
  }

  var book: Book? {
    get { ... }
    set { ... }
  }

  var game: Game? {
    get { ... }
    set { ... }
  }

  var tvShow: TvShow? {
    get { ... }
    set { ... }
  }
}

struct AuthorMedia: Aggregate {
  var author: Author
  var media: MediaType

  var nestedEntitiesKeyPaths: [PartialIdentifiableKeyPath<Self>] {
    [.init(\.author), .init(wrapper: \.media)]
  }
}

Aliases

Sometimes you need to retrieve data without knowing the object id. Common case is current user.

CohesionKit provides a suitable mechanism: aliases. Aliases allow you to register and find entities using a key.

extension AliasKey where T == User {
  static let currentUser = AliasKey("user")
}

entityStore.store(currentUser, named: .currentUser)

Then request it somewhere else:

entityStore.find(named: .currentUser) // return the current user

Compared to regular entities, aliased objects are long-live objects: they will be kept in the storage even if no one observes them. This allow registered observers to be notified when alias value change:

entityStore.removeAlias(named: .currentUser) // observers will be notified currentUser is nil.

entityStore.store(newCurrentUser, named: .currentUser) // observers will be notified that currentUser changed even if currentUser was nil before

Stale data

When storing data CohesionKit actually require you to set a modification stamp on it. Stamp is used as a marker to compare data freshness: the higher stamp is the more recent data is.

By default CohesionKit will use the current date as stamp.

entityStore.store(book) // use default stamp: current date
entityStore.store(book, modifiedAt: Date().stamp) // explicitly use Date time stamp
entityStore.store(book, modifiedAt: 9000) // any Double value is valid

If for some reason you try to store data with a stamp lower than the already stamped stored data then the update will be discarded.

Weak memory management

CohesionKit has a weak memory policy: objects are kept in EntityStore as long as someone use them.

To that end you need to retain observers as long as you’re interested in the data:

let book = Book(id: "ACK", title: "A Clash of Kings")
let cancellable = entityStore.store(book) // observer is retained: data is retained

entityStore.find(Book.self, id: "ACK") // return  "A Clash of Kings"

If you don’t create/retain observers then once entities have no more observers they will be automatically discarded from the storage.

let book = Book(id: "ACK", title: "A Clash of Kings")
_ = entityStore.store(book) // observer is not retained and no one else observe this book: data is released

entityStore.find(Book.self, id: "ACK") // return nil

let book = Book(id: "ACK", title: "A Clash of Kings")
var cancellable = entityStore.store(book).asPublisher.sink { ... }
let cancellable2 = entityStore.find(Book.self, id: "ACK") // return a publisher

cancellable = nil

entityStore.find(Book.self, id: "ACK") // return "A Clash of Kings" because cancellable2 still observe this book

License

This project is released under the MIT License. Please see the LICENSE file for details.