CohesionKit - Single Source of Truth

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

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

Store an object

First create an instance of IdentityMap:

let identityMap = IdentityMap()

IdentityMap let you store Identifiable objects:

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

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

identityMap.store(book)

Your can then retrieve the object anywhere in your code:

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

Observing changes

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

func findBooks() {
  // 1. load data using URLSession
  URLSession(...)
  // 2. store data inside our identityMap
    .store(in: identityMap)
    .sink { ... }
    .store(in: &cancellables)
}

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

CohesionKit has a weak memory policy you should understand.

Relational objects

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

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

  let author: Author
  let books: [Book]

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

CohesionKit will then handle synchronisation for the three entities:

• AuthorBook
• Author
• Book

This allow you 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")
    ]
)

identityMap.store(authorBooks)

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

You can also modify any of them however you want:

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

identityMap.store(newAuthor)

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

Advanced topics

Aliases

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

CohesionKit provide 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")
}

identityMap.store(currentUser, named: \.currentUser)

Then request it somewhere else:

identityMap.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 observe them. This allow registered observers to be notified when alias value change:

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

identityMap.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.

identityMap.store(book) // use default stamp: current date
identityMap.store(book, modifiedAt: Date().stamp) // explicitly use Date time stamp
identityMap.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 IdentityMap 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 = identityMap.store(book) // observer is retained: data is retained

identityMap.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")
_ = identityMap.store(book) // observer is not retained and no one else observe this book: data is released

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

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

cancellable = nil

identityMap.find(Book.self, id: "ADD") // 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.