Corridor: A Coreader-like Dependency Injection μFramework

by iOS Example

Corridor

A Coreader-like Dependency Injection μFramework




Why

In order to write tests we must substitute parts of our code that we do not have control over such as:

  • Network
  • File system
  • Creating dates
  • Keychain

We need to substitute them in tests in order to verify assumptions.

The purpose of Corridor is to:

  • Provide a common interface for things that need to be replaced in TestCases
  • Simplify setup in TestCases without manually providing mocks etc
  • Transparently provide the current context to all your Types
  • Separate any kind of test related logic from production code

In an ideal World a Coeffect is under control.

class Controller: UIViewController {

  override func viewWillAppear(_ animated: Bool) {
    print(Date())
  }
}

The Date in the above example is out of control.

Running a test for that Controller will always result in a different Date.
In that case the Date is just a placeholder for any Coeffect.

Corridor tries to solve this problem by taking the concept of a Coreader and turning it into a Swift friendly implementation via protocols and a single property.

What will it look like?

class Controller: UIViewController, HasInstanceContext {

  var resolve = `default`

  override func viewWillAppear(_ animated: Bool) {
     print(now)
  }
}

The idea behind Corridor was part of my talk at the Functional Swift Conference 2017 in Berlin.

Usage

Implement a Protocol

Either one of the two protocols provided by Corridor: HasInstanceContext or HasStaticContext.
Or any convenience protocol that extends one of those.

Add a Property

Any type that needs access to an injected value also needs to know how to resolve it. This is done by providing a property called resolve.

By default it should be set to var resolve = `default`.

Why the backticks?
default is a swift keyword and by using the backticks the property looks
more config-ish.

Setup Context

A base protocol that defines your dependencies:

protocol AppContext {

  /// The current Date
  var now: Date { get }
}

Context Implementation

An implemention of a Context.
Usually we use two implementations.
One for the running application, one for the test cases.

struct DefaultContext: AppContext {

  var now: Date {
    // The real current Date
    return Date()
  }
}

struct MockContext: AppContext {

  var now: Date {
    // We assume way earlier
    return Date.distantPast
  }
}

Resolver

In order to provide the default resolver you must extend the base protocol in Corridor. This will provide a static variable called default of Type Resolver to your Type in order to provide access.
This extension is done once in your app.

extension HasContext {

    typealias Context = AppContext

    static var `default`: Resolver<Self, AppContext> {
       return Resolver(context: DefaultContext())
    }
}

The visibility of any property in the context is controlled by extending either HasInstanceContext or HasStaticContext or any derived protocol.

By using protocols we can constrain access in a granular way. Additionally it allows for the injection of functions.

See example CorridorDemo.playground.

extension HasInstanceContext where Self.Context == AppContext  {

    /// Injected now property
    var now: Date {
        return resolve[\.now]
    }
}

Changing the Context

In your actual code everything resolves to the DefaultContext.
But in your Tests you need to make sure to switch to the mock context.
The simplest way is:

var myController = withContext(Controller(), MockContext())

Setting up the context in the Tests can easily be simplified by making the TestCase itself Context aware. Additionally you can build functions on top of that to make instantiation automagically have the correct Context.

extension HasContext {

    static var mock: Resolver<Self, AppContext> {
        return Resolver(context: MockContext())
    }
}

/// Extension for TestCase (e.g. subclass of XCTestCase)
/// to provide easy access to get controller with mock context
extension HasInstanceAppContext where Self: TestCase {

    func withController<V>() -> V?
        where V: UIViewController, V: ManagedByStoryboard, V: HasInstanceAppContext {
        /// A simplified function that will make sure your context is set
        return self.controller()
    }
}

Examples from Playground

See the provided Playground in the workspace.

Intro

import Foundation
import UIKit
import PlaygroundSupport
import Corridor

// 1. Protocol for Context 
// e.g. AppContext.swift
public protocol AppContext {
  var now: Date { get }
}

// 2. Context Implementation for Running App
// e.g. DefaultContext.swift
struct DefaultContext: AppContext {
  var now: Date { return Date() }
}

// 3. HasContext is Corridor base protocol
// e.g. Resolver.swift
extension HasContext {
  typealias Context = AppContext
  // provide default resolver
  static var `default`: Resolver<Self, AppContext> {
    return Resolver(context: DefaultContext())
  }
}

// 4. Add resolvable values
// e.g. Resolver.swift
extension HasInstanceContext
where Self.Context == AppContext {
  var now: Date {
    return resolve[\.now]
  }
}

// 5. Usage
final class Controller: LabelController, HasInstanceContext {
  var resolve = `default`
  override func viewWillAppear(_ animated: Bool) {
    label.text = "Now is \(now)"
  }
}

PlaygroundPage.current.liveView = Controller()

Test

import Foundation
import PlaygroundSupport
import Corridor

// 1. Context Implementation for Testing App
struct MockContext: AppContext {
  var now: Date { return Date.distantPast }
}

// 2. Usage
let test = withContext(Controller(), MockContext())

PlaygroundPage.current.liveView = test

Api

This example combines a Reader composition for chained REST calls.
Defining the Api in AppContext and by implementing ContextAware it
will have access to the current context.
Therefore we don't need to pass additional params to the network calls, and
it is ensured that all injected valus are correctly resolved.

import Foundation
import UIKit
import PlaygroundSupport
import Corridor

// 1. Protocol for Context
protocol AppContext {
  var now: Date { get }
  var api: Api { get }
}

// 2. Context Implementation for Running App
struct DefaultContext: AppContext {
  var now: Date { return Date() }
  var api: Api { return Api(connection: ServerConnection()) }
}

// 3. Context Implementation for Test App
struct MockContext: AppContext {
  var now: Date { return Date.distantPast }
  var api: Api { return Api(connection: MockConnection()) }
}

// 4. Extend Corridor
extension HasContext {
  typealias Context = AppContext
  static var `default`: Resolver<Self, AppContext> {
    return Resolver(context: DefaultContext())
  }
}

// 5. Convenience protocol
protocol ContextAware: HasInstanceContext
where Self.Context == AppContext {}

// 6. Define API for (String) -> Future<T>
struct Api: ContextAware {
  var resolve = `default`
  let connection: Connection
  init(connection: Connection) {
    self.connection = connection
  }
  var endpoint: Endpoint {
    return connection.endpoint
  }
  func getResponse<T: Codable>(_ s: String) -> Future<T> {
    return connection.getResponse(s)
  }
}

// 7. Fake ReSwift Store
var dispatched: Set<String> = Set()

// 8. Extend Corridor
extension ContextAware {
  // Extract
  var now: Date {
    return resolve[\.now]
  }
  // Extend
  var api: Api {
    return resolve[\.api]
  }
  var dispatch: Dispatch {
    return { dispatched.insert($0) }
  }
  var messages: String {
    return dispatched.sorted().joined(separator: "\n")
  }
}

// 9. Define API Operations
typealias ApiAware<O> = Reader<Api, O>
typealias ApiFuture<O> = ApiAware<Future<O>>
typealias ApiBind<I, O> = (Future<I>) -> ApiFuture<O>

func bind<I, O>(_ urlFrom: @escaping (I) -> String) -> ApiBind<I, O>
  where O: Codable {
    return { future in
      ApiAware { api in
        future.flatMap { input in
          api.dispatch("\(api.now.formatted) \n -\(input)")
          return api.getResponse(urlFrom(input))
        }
      }
    }
}

let apiEntrypoint: ApiFuture<Endpoint> = ApiAware { api in
  Future<Endpoint>(value: api.endpoint)
}
let usersEndpoint: ApiBind<Endpoint, UsersEndpoint> = bind {
  $0.usersEndpoint
}
let firstUserEndpoint: ApiBind<UsersEndpoint, UserEndpoint> = bind {
  $0.firstUserEndpoint
}
let addressEndpoint: ApiBind<UserEndpoint, AddressEndpoint> = bind {
  $0.addressEndpoint
}

let apiCall = usersEndpoint >=> firstUserEndpoint >=> addressEndpoint

// 10. Controller
final class Controller: LabelController, ContextAware {

  var resolve = `default`

  override func viewWillAppear(_ animated: Bool) {

    let address = apiEntrypoint
      .flatMap(apiCall)
      .run(api)

    address.onSuccess { (s: Address) in
      label.text = messages + "\n" + s

    }
  }
}

// 11. Run
let app = Controller()
let test = withContext(Controller(), MockContext())
PlaygroundPage.current.liveView = app

FAQ

What if a property needs to be resolved to the context?

You can use lazy var to resolve properties directly.
See Tests.


final class MyClass: HasInstanceContext {
  var resolve = `default`

  lazy var contextAwareProperty = resolve[AnotherContextAwareClass()]
}

Installation

Carthage

To integrate Corridor into your project using Carthage, add to your Cartfile:

github "symentis/Corridor"

See Carthage for further inststructions.

Requirements

Swift 4

Credits & License

Corridor is owned and maintained by Symentis GmbH.

Developed by: Elmar Kretzer

All modules are released under the MIT license. See LICENSE for details.

GitHub

https://github.com/symentis/Corridor