Simple protocol to inject dependencies when `init(with:)` is not available (e.g. XIB and Storyboard UIViewControllers)

Injectable.swift

//
//  Injectable.swift
//
//  Created by André Pinto on 06/05/17.
//  Copyright © 2017 André Pinto. All rights reserved.
//

protocol Injectable {
  
  /// Type which defines the dependencies to be injected
  ///
  /// e.g.:
  ///
  ///     typealias Dependencies = (detailURL: String, screenTitle: String, trackingId: Int?)a
  associatedtype Dependencies
  
  /// Instance with the injected dependencies
  var d: Dependencies! { get set }
  
  /// Dependency injector
  mutating func inject(_ dependencies: Dependencies)
  
  /// Assert that the dependencies are set.
  ///
  /// The default implementation just checks if var `d` is not nil.
  func assertDependencies()
}

// Default implementation of `inject()` and `assertDependencies()`.
extension Injectable {
  mutating func inject(_ dependencies: Dependencies) {
    d = dependencies
  }
  
  func assertDependencies() {
    assert(d != nil, "Dependencies are empty!")
  }
}

/* ** Instructions **

1. Conform to Injectable prototol:

  class SomeViewController: UIViewController, Injectable {

2. Define the dependencies:

  struct SomeDependencies {
    let detailURL: String
    let screenTitle: String
    let trackingId: Int?
  }
  typealias Dependencies = SomeDependencies
  var d: Dependencies!

3. Assert the dependencies (e.g. in viewDidLoad for UIViewControllers):

  override func viewDidLoad() {
    super.viewDidLoad()
    assertDependencies()
  }

4. Inject values after object is instantiated:

  let deps = SomeViewController.Dependencies(detailURL: "http://host.com",
                                             screenTitle: "Nice Title",
                                             trackingId: 11)
  someViewController.inject(deps)

 
*/