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A dependency injection framework for Swift


Firebolt is a dependency injection framework written for Swift. Inspired by Kotlin Koin. This framework is meant to be lightweight and unopinionated by design with resolutions working simply by good old functional programming.

Why use Firebolt?

To put it simply, Firebolt is nothing more than a Service Locator tool. It allows you to encapsulate layers of abstractions and exposes modules that are both easily testable and composable.

  1. Composability

Firebolt is designed to truly allow infinite composability with your dependency graph. Since each dependency can be made with multiple dependencies, and those dependencies can also be made with other dependencies, and so forth. You essentially have a inverted tree graph which ultimately can be accessed as simply as doing resolver.get() and not have to worry about how those dependencies are made. This is especially useful when architecting domain specific designs into your application which include Managers, Services, Repositories, etc.

  1. Testability

Firebolt is especially useful when trying to take control of the scope of your unit tests. Since each Resolver is the container of its dependencies, creating a new Resolver or better yet deallocating that Resolver will create an entire separate or completely remove those dependencies all together.

What does this mean? This means that you can scope out specific dependencies for each test suite, file, or method entirely and never have to share any state in between. Using the MockResolver, you are able to register dependencies that are really only needed by that test suite and then deallocate it once you're finished.


Firebolt is an open-source project, feel free to contact me if you want to help contribute to this codebase. You can also do a pull-request or open up issues.



Firebolt is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod 'Firebolt'

Swift Package Manager




  1. Instantiate a Resolver
let resolver = Resolver()
  1. Register dependencies.
class ClassA {}

resolver.register { ClassA() }
  1. Use the get() qualifier to resolve inner dependencies.
class ClassA {}
class ClassB { init(classA: ClassA) }
    .register { ClassA() }
    .register { ClassB(classA: $0.get()) } // <-- get() qualifier
  1. Start coding with dependency injection using the get() keyword.
let classA: ClassA = resolver.get()
let classB: ClassB = resolver.get()

// Or if you don't care about having more than one system
// you can access the global scope.
let classA: ClassA: = get()


You can pass in a scope qualifier during registration to tell the Resolver how you want to instance to be resolved.

The current supported forms of scope are:

enum Scope {
    case single // <- the same instance is resolved each time
    case factory // <- unique instances are resolved each time

You can set scope like this. .single is the default scope setting.

resolver.register(.single) { ClassA() } /// only a single instance will be created and shared when resolved
resolver.register(.factory) { ClassA() }  /// multiple instances are created each time when resolved
resolver.register { ClassA() } /// .single is the default

// now these two are of the same instances 
let classA: ClassA = resolver.get() 
let classA: ClassA = resolver.get()

Singleton resolutions also apply to protocols of concrete classes as well.

resolver.register(.single, expect: ClassAProtocol.self) { ClassA() }

let classA1: ClassAProtocol = resolver.get()
let classA2: ClassAProtocol = resolver.get()

// Both ClassA1 and ClassA2 are resolved from the same concrete instance


You can pass in arguments during registration like so.

let resolver = Resolver()
let environment: String = "stage"

reasolver.register { ClassD(environment: environment, classA: $0.get()) }

If the arguments need to be passed in at the call site. You can specify the expected type during registration.

resolver.register(arg1: String.self) { ClassD(environment: $0) }

Then you can pass in the argument afterwards.

let classD: ClassD = resolver.get("stage")

You can pass in multiple arguments as well.

resolver.register(arg1: String.self, arg2: Int.self) { ClassD(environment: $0, timestamp: $1) }

let classD: ClassD = resolver.get("stage", 1200)

You can also pass in optionals like so.

class ClassE { init(value: String?) {} }

let resolver = Resolver()
resolver.register(arg1: String?.self) { ClassE($0) }

// no arguments tells the resolver to pass nil instead
let classE: ClassE = resolver.get() 
let classE: ClassE = resolver.get("SOME_VALUE")

For shared non-registered arguments between dependencies, you can pass in arguments from within the register block using the upstream argument themselves.

let resolver = Resolver()

class ClassC {
    init(classA: ClassA, classB: ClassB) {}

    .register(arg1: ClassA.self) { ClassB(classA: $0) }
    .register(arg1: ClassA.self) { 
        // ClassA is now shared between ClassB and ClassC
        // without registration
        ClassC(classA: $0, classB: $0.get($0)) 

// Then call them like so
let classA: ClassA = ClassA()
let classC: ClassC = get(classA)

Protocol Conformance

Protocol conformance is also supported by the Resolver. Let's say you want to have a ClassA protocol and a ClassAImpl concrete type registered, you can use the expect argument.

protocol ClassA { func foo() }

class ClassAImpl: ClassA { func foo() {} }

let resolver = Resolver()

resolver.register(expect: ClassA.self) { ClassAImpl() }

Then when calling it in the callsite.

let classA: ClassA = get() // <- ClassAImpl will be returned

You are also able to have support for multiple protocols for the same concrete type.

protocol ClassAVariantA { func foo() }
protocol ClassAVariantB { func bar() }

class ClassA: ClassAVariantA, ClassAVariantB { 
    func foo() {} 
    func bar() {}

let resolver = Resolver()

resolver.register { ClassA() }

// multiple resolutions using the same concrete type with the expect qualifier
let variantA: ClassAVaraintA = get(expect: ClassA.self)
let variantB: ClassAVaraintB = get(expect: ClassA.self)

Or using a different method, passing multiple expects for the same concrete class.

let resolver = Resolver()

resolver.register(expects: [ClassAVaraintA.self, ClassAVaraintB.self]) { ClassA() }

let classA: ClassAVaraintA? = get()
let classA: ClassAVaraintB? = get()

If there are dependencies that require protocol conformance but you are only supporting a concrete class you can do the following:

class ClassA: ClassAVariantA {}

class ClassB { init(classAVariant: ClassAVariantA) {} }

let resolver = Resolver()

    .register { ClassA() }
    .register { ClassB(classAVariant: get(expect: ClassA.self)) }

// works
let classB: ClassB = get()

This works because ClassA is registered in the dependency scope but we are able to cast it to the expected type ClassAVaraintA by using the get() qualifier and the expect argument passed in during the callsite.

Opaque Conformance

With the some keyword, protocols with associative types can be generified.

Consider this example:

protocol OpaqueProtocol {
  associatedtype Value
  func getValue() -> Value

class OpaqueClassA: OpaqueProtocol {
  func getValue() -> String { "hello" }

class OpaqueClassAB: OpaqueProtocol {
  init(classA: OpaqueClassA) {}
  func getValue() -> Int { 1 }

With Firebolt, you are able to resolve opaque types.

let resolver = Resolver()
    .register(.single) { OpaqueClassA() }
    .register(.factory) { OpaqueClassB(classA: $0.get()) }

// this will work
let someClassA: some OpaqueProtocol = resolver.get(expect: OpaqueClassA.self)
let someClassB: some OpaqueProtocol = resolver.get(expect: OpaqueClassB.self)

// this will also work
let classA: OpaqueClassA = resolver.get()
let classB: OpaqueClassB = resolver.get()

// will print `true`
print(someClassA == classA)

Thread Safety

Firebolt has a internal global queue that makes sure dependencies and resolvers are registered/unregistered in the same sequence.

Global Resolver

Normally, when you initalize a Resolver you can optionally pass in a resolverId or a UUID().uuidString will be gererated for you, this ensures that all dependencies registered in that resolver are unique to that resolver's instance, they can never be shared amongst other resolvers.

If you want a globally scoped resolver, there is a special resolver that resides in the global scope which you can access by using the global static property of the Resolver class.

let resolver = Resolver.global // <-- resolves the GlobalResolver

resolver.register { ClassA() }

You can then globally inject dependencies without specifying a Resolver identifier.

// property scoped in another instance of the application 
// will resolve automatically for you.
let classA: ClassA = get()

Mock Resolver

There is another special Resolver subclass called the MockResolver, it is essentially a convenience class for creating quick dependency graphs for smaller scoped projects.

let mockResolver = MockResolver { resolver in
  resolver.register { ClassA() }
  resolver.register { ClassB(classA: resolver.get()) }

Multiple Resolvers

If you want to keep dependencies separate you can instantiate multiple resolvers with each having their own scope. When you deallocate these resolvers, the instances tied to the dependencies will deallocate as well.

When you initialize a Resolver you have to pass in a resolverId, Firebolt then registers this resolver in a cache.

  1. Instantiate a Resolver with a unique identifier.
let resolver1 = Resolver("Resolver_1")
resolver1.register { ClassA() }

let resolver2 = Resolver("Resolver_2")
resolver2.register { ClassA() }

// make sure to resolve using the Resolver itself using lamba
resolver2.register { ClassB(classA: $0.get()) }
  1. Then inject by referencing by their respective resolvers.
// resolves to `nil` because Resolver_1 never registered ClassB
let classB: ClassB = resolver1.get()

// resolves to ClassB 
let classB: ClassB = resolver2.get()

Here is an example of using a Resolver via an Interface like design.

let resolver: Resolver
init(resolver: Resolver) { self.resolver = resolver }

func viewDidLoad() {
    let classB: ClassB = resolver.get()

Objects not registered by the resolver won't be shared by other resolvers. This includes objects registered as .single as well unless they are registered by the GlobalResolver itself in which they become a true Singleton.

If you initailize two resolvers of the same identifier, they both will share the same cache of dependencies.

let resolverA = Resolver("SAME_IDENTIFIER")
resolverA.register { ClassA() }

let resolverB = Resolver("SAME_IDENTIFIER")

// This will successfully resolve since ResolverB shares the same 
// identifier as ResolverA - thus the same cache of dependencies.
let classA: ClassA = resolverB.get() 

Subclassing Resolvers

Resolvers are subclassable if you feel the need to create your own kind of a Resolver ex: MyAppResolver.

It is important that you pass in your own resolverId through an initializer witin your subclass. If you don't, your subclass will inheritely be a GlobalResolver since a standalone Resolver class with no identifier will essentiually access the singleton itself.

class MyAppResolver: Resolver {
    init() {

let myResolver = MyAppResolver()
myResolver.register { ClassA() }

// this will work
let classA: ClassA = myResolver.get()

// this will also work
let classA: ClassA = get(resolverId: "MyAppResolver")

// this will fail because it is accessing the Global Resolver
let classA: ClassA = get()

Unregister Dependencies

You can unregister dependencies like so.

resolver.register { ClassA() }

let classA: ClassA? = resolver.get() // will return ClassA


let classA: ClassA? = resolver.get() // will return nil

Unregsiter all dependencies.

    .register { ClassA() }
    .register { ClassB() }

let classA: ClassA? = resolver.get() // will return ClassA
let classB: ClassB? = resolver.get() // will return ClassB


let classA: ClassA? = resolver.get() // will return nil
let classB: ClassB? = resolver.get() // will return nil

Unregister all dependencies except these types.

    .register { ClassA() }
    .register { ClassB() }

let classA: ClassA? = resolver.get() // will return ClassA
let classB: ClassB? = resolver.get() // will return ClassB

resolver.unregisterAllDependencies(except: [ClassB.self])

let classA: ClassA? = resolver.get() // will return nil
let classB: ClassB? = resolver.get() // will return ClassB!

Drop Cached Dependencies

When a dependency is created via the .single scope, it is stored in it's respective Resolver's cache.

You can drop that cache like so.

    .register(.single) { ClassA() }

let classA1: ClassA? = resolver.get() 
let classA2: ClassA? = resolver.get() 

print(classA1.id == classA2.id) // will print true


let classA3: ClassA? = resolver.get() 

print(classA1.id == classA3.id) // will print false

Drop all cached dependencies.

    .register(.single) { ClassA() }
    .register(.single) { ClassB() }

let classA1: ClassA? = resolver.get() 
let classA2: ClassA? = resolver.get() 

print(classA1.id == classA2.id) // will print true

let classB1: ClassB? = resolver.get() 
let classB2: ClassB? = resolver.get() 

print(classB1.id == classB2.id) // will print true


let classA3: ClassA? = resolver.get() 
let classB4: ClassA? = resolver.get() 

print(classA1.id == classA3.id) // will print false
print(classB1.id == classB3.id) // will print false

Or drop all excluding some types.

    .register(.single) { ClassA() }
    .register(.single) { ClassB() }

let classA1: ClassA? = resolver.get() 
let classA2: ClassA? = resolver.get() 

print(classA1.id == classA2.id) // will print true

let classB1: ClassB? = resolver.get() 
let classB2: ClassB? = resolver.get() 

print(classB1.id == classB2.id) // will print true

resolver.dropAllCachedDependencies(except: [ClassB.self])

let classA3: ClassA? = resolver.get() 
let classB4: ClassA? = resolver.get() 

print(classA1.id == classA3.id) // will print false
print(classB1.id == classB3.id) // will print true


Storyboard Resolution

Firebolt can be used to resolve storyboards as well. Given this example,

// There are multiple ways to initialize a storyboard view code but in this case
// we will use a static initializer for the sake of allowing external parameters
class ViewController {
    class func initialize(userManager: UserManager): ViewController {
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        return storyboard.instantiateViewController(identifier: "ViewController") as! ViewController 

// .. then register
  .register { UserManager() }
  .register { ViewController.initialize(userManager: $0.get()) }
// ... when resolving it
let vc: ViewController = resolver.get()

// ... or if you're using the Global Resolver
  .register { UserManager() }
  .register { ViewController.initialize(userManager: $0.get()) }

let vc: ViewController = get()

Application Architecture

// UserManager.swift
class UserManager {}

// ViewController.swift
class ViewController: UIViewController {
    public init(userManager: UserManager) {}

// AppDelegate.swift
class AppDelegate {

  let resolver = Resolver()

  func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    resolver.register { UserManager() }
    resolver.register { ViewController(userManager: $0.get()) }

    let viewController: ViewController = resolver.get()
    window?.rootViewController = viewController

Unit Tests

func testMe() {
  let resolver = MockResolver { resolver in
    resolver.register(expect: ClassA) { ClassAImpl() }
    resolver.register(expect: ClassB) { ClassBImpl(classA: $0.get()) }

  /// GIVEN

  let classB: ClassB = resolver.get()
  let userId = 1

  classB.onCallHandler = { userId in
    return "\(userId)"

  /// WHEN

  let result = classB.stringifyUserId(userId)

  /// THEN

  xcAssertEquals("\(userId"), result)


Andrew Aquino, andrewaquino118@gmail.com


Firebolt is available under the MIT license. See the LICENSE file for more info.


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