DIY Dependency Injection with Kotlin @ DevFest Florida 2019

@HandstandSam #DevFestFL
DIY Dependency
Injection with Kotlin
Sam Edwards -

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Why am I talking about DIY Dependency Injection with Kotlin?

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! Share the story of how we implemented DIY

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! Demystify the topic of Dependency Injection

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! Demystify the topic of Dependency Injection
! Show how you can implement DIY, if you choose

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Our Story

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Our Story
! Our team previously built Capital One Wallet

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Our Story
! We got tasked to build a reusable SDK for Authentication

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Our Story
! We got tasked to build a reusable SDK for Authentication
! This was the first time we were building a complex SDK

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Product Requirements

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! SDK will be used by different apps, in different codebases

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! Should not assume clients will be using a 3rd party library

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! Stay lean and lightweight

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! Stay lean and lightweight
! Make the SDK configurable

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Technical Requirements

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! Reactive Programming

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! Immutable Data

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! Immutable Data
! Modular Code

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! Immutable Data
! Modular Code
! Single Responsibility Code

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! Immutable Data
! Modular Code
! Well Tested Code

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! Immutable Data
! Modular Code
! Well Tested Code
! Avoid 3rd Party Libraries

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! Immutable Data
! Modular Code
! Well Tested Code
! 100% Kotlin

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! Immutable Data
! Modular Code
! Well Tested Code
! 100% Kotlin
! Dependency Injection

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Why 100% Kotlin?

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Why 100% Kotlin?
makes coding easy and concise.

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Why 100% Kotlin?
makes dependency injection easy and concise.

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Why Dependency Injection?

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! Testing

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! Testing
! Mock Flavors

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! Testing
! Mock Flavors
! Custom Debug Features

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! Testing
! Mock Flavors
! Environment Switching (Test vs Production Servers)

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! Testing
! Mock Flavors
! Promotes building more reusable code

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! Testing
! Mock Flavors
! Discourages use of static singletons

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! Testing
! Mock Flavors
! Discourages use of static singletons
! It enables all the really cool things I like to talk about

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It Enables Testing
DevFest Florida

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It Enables Custom Debug Features
Droidcon Boston

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It Enable Mock Flavors
Droidcon NYC

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Dependency Injection is Not Magical

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Dependency Injection is Not Magical
DI !=

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Dependency Injection is

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Inversion of Control

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Passing in dependencies 
instead of pre-defining them.
Inversion of Control

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Dependency Injection is Inversion of Control
Code
execute
Dependencies
( )

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Dependency Injection is Inversion of Control
Code
execute
( )

Dependencies

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Without Dependency Injection
class NetworkConfig(
val fullUrl = "http://localhost:8080/"
val isLocalhostServer = true
}

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Setter Injection
var fullUrl: String,
var isLocalhostServer: Boolean
)
val config = NetworkConfig()
config.fullUrl = “http://localhost:8080/"
config.isLocalhostServer = true

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Constructor Injection
val baseUrl: String,
val port: Int
) {
val fullUrl = "$baseUrl:$port/"
val isLocalhostServer = baseUrl.contains("//localhost")
}

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Commonalities: Inversion of Control
! Inversion of Control is passing in dependencies instead of pre-
defining them.

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Aren’t Their Libraries That Do DI for You?
! Yes, let’s take a look.

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Open Source DI Libraries
! Dagger
! Toothpick
! Koin
! Kodein

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! Dagger
! Toothpick
! Koin
! Kodein
Code
Generation
YES
YES
NO
NO

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! Dagger
! Toothpick
! Koin
! Kodein
Code
Generation
YES
YES
NO
NO
❌
❌

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! Dagger
! Toothpick
! Koin
! Kodein
Code
Generation
YES
YES
NO
NO
Single 
Instance
NO
NO
YES
NO
❌
❌

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! Dagger
! Toothpick
! Koin
! Kodein
Code
Generation
YES
YES
NO
NO
Single 
Instance
NO
NO
YES
NO
❌
❌
❌

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! Dagger
! Toothpick
! Koin
! Kodein
Code
Generation
YES
YES
NO
NO
Single 
Instance
NO
NO
YES
NO
Available 
at the time
YES
YES
YES
NO
❌
❌
❌

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! Dagger
! Toothpick
! Koin
! Kodein
Code
Generation
YES
YES
NO
NO
Single 
Instance
NO
NO
YES
NO
Available 
at the time
YES
YES
YES
NO
❌
❌
❌
❌

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! Dagger
! Toothpick
! Koin
! Kodein
Code
Generation
YES
YES
NO
NO
Single 
Instance
NO
NO
YES
NO
Available 
at the time
YES
YES
YES
NO
Adds dependency
on another library
YES
YES
YES
YES
❌
❌
❌
❌

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So, How Much Work is DIY?

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! Not that much

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! Not that much
! Slightly more verbose

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! Not that much
! Have to manually build advanced DI features

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! Not that much
! Have to manually build advanced DI features
! Great for simple use cases when you don’t already know a DI library
really well

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ShoppingApp - Open Source on GitHub
https:/
/github.com/handstandsam/ShoppingApp

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Swapping Dependencies on a Graph

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! Mock Server vs. Testing Server vs. Live (Production Server)

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! Graph is where all your dependencies are configured.

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! Just swap out pieces of your graph you need to change.

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! Just swap out pieces of your graph you need to change.
! Or provide a brand new graph with the required fields.

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Network Graph
interface NetworkGraph {
val categoryRepo: CategoryRepo
val itemRepo: ItemRepo
val userRepo: UserRepo
}

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Network Graph
interface NetworkGraph {
val categoryRepo: CategoryRepo
val itemRepo: ItemRepo
val userRepo: UserRepo
}
class NetworkGraphImpl(
networkConfig: NetworkConfig
) : NetworkGraph {
private val retrofit: Retrofit=
Retrofit.Builder()
.baseUrl(networkConfig.fullUrl)
.addConverterFactory(moshiConverterFactory)
.addCallAdapterFactory(rxJava2CallAdapterFactory)
.client(okHttpClientBuilder.build())
.build()
private val shoppingService: ShoppingService =
retrofit.create(ShoppingService::class.java)
override val categoryRepo: CategoryRepo = NetworkCategoryRepo(shoppingService)
override val itemRepo: ItemRepo = NetworkItemRepo(shoppingService)
override val userRepo: UserRepo = NetworkUserRepo(shoppingService)
}

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Create Graphs of Dependencies
• Similar to Dagger @Component
OkHttpClient Services
Retrofit
NetworkGraph
Url
Repos

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Session Graph
interface SessionGraph {
val sessionManager: SessionManager
val checkoutCart: CheckoutCart
val userPreferences: UserPreferences
}

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Session Graph interface SessionGraph {
val sessionManager: SessionManager
val checkoutCart: CheckoutCart
val userPreferences: UserPreferences
}
class SessionGraphImpl(
appContext: Context
) : SessionGraph {
override val checkoutCart = CheckoutCart()
override val userPreferences = UserPreferences(appContext)
override val sessionManager = SessionManager(checkoutCart, userPreferences)
}

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Multiple or Nested Graphs are
AppGraph
SessionGraph
NetworkGraph

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AppGraph
SessionGraph
NetworkGraph
! Each module project needs their own graph

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AppGraph
SessionGraph
NetworkGraph
! Each module project needs their own graph
! Each graph is similar to a Dagger @Component

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! Use the `by lazy` language feature of Kotlin to delay initialization, but
cache result.
Leveraging `by lazy`

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! Use the `by lazy` language feature of Kotlin to delay initialization, but
cache result.
Leveraging `by lazy`
class AppGraph {
val networkGraph by lazy NetworkGraph(config = config)
}

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! Create a factory with a lambda
Factories

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! Create a factory with a lambda
Factories
class AppGraph {
val dbFactory: () -> Database =
{Database(logger = logger, utils = utils)}
}
val db = dbFactory()

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Scoping

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! Use a factory, but this is DIY, so you will have to manage scoping
yourself.
Scoping

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Setting up Your Graphs
class MyApplication : Application {
val graph by lazy { AppGraph(this) }
override fun onCreate() {
//Access Graph to Instantiate
graph
}
}
! Create Graph during Application::onCreate() and attach to Application

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Setting up Your Graphs
val graph by lazy { AppGraph(this) }
//Access Graph to Instantiate
graph
}
}
lateinit var graph: AppGraph
graph = AppGraph(this)
}
}
or
! Create Graph during Application::onCreate() and attach to Application

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Supporting Graphs in Modules
class AppGraph(context: Context) {
init { instance = this }
companion object {
lateinit var instance: AppGraph
}
}

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! Create a singleton since the module is unaware of the Application
Supporting Graphs in Modules
class AppGraph(context: Context) {
init { instance = this }
companion object {
lateinit var instance: AppGraph
}
}

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Dependency Injection on Activities & Fragments

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! Constructors of Application, Fragments, Activities & Services are
called by the platform

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! Constructors of Application, Fragments, Activities & Services are
called by the platform
! You have to wait do do any injection until after the `onCreate`
lifecycle method

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! lateinit var, grabbing from Application
Accessing Graph in Activities & Fragments
class MyActivity : Activity() {
lateinit private var graph: AppGraph
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
graph = (application as MyApplication).graph
//Access your fields now
graph.database
}
}

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! Simplify using an extension function.
lateinit private var graph: AppGraph
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
graph = application.graph
}
}
fun Application.graph(): AppGraph{
return (this as MyApplication).graph
}

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! OR use `by lazy` to defer access until the Activity has been created
private val graph: AppGraph get() = application.graph()
private val db: UserRepo by lazy {graph.networkGraph.userRepo}
}
}

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private val db: UserRepo get() = graph.networkGraph.userRepo
}
}

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! OR use `get() =` to defer access until the Activity has been created
private val db: UserRepo get() = graph.networkGraph.userRepo
}
}

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Pro Tips

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Pro Tips
! Try and avoid `lateinit var` for injected values

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Pro Tips
! `lateinit` can create runtime Exceptions

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Pro Tips
! `var` creates mutable state

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Pro Tips
! `by lazy` create an anonymous class

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Pro Tips
! `by lazy` create an anonymous class
! You can use `get() =` if your data is an immutable singleton

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Accessing Dependencies from the Graph
LoginPresenter(
view = loginView,
sessionManager = appGraph.sessionGraph.sessionManager,
userRepo = appGraph.networkGraph.userRepo
)

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Accessing Dependencies from the Graph
private val sessionManager: SessionManager
get() = graph.sessionGraph.sessionManager
private val userRepo: UserRepo
get() = graph.networkGraph.userRepo
LoginPresenter(
view = loginView,
sessionManager = sessionManager,
userRepo = userRepo
)

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Key Takeaways

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Key Takeaways
! You don’t HAVE to use a Dependency Injection Library, Dagger

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Key Takeaways
! There are no magic annotations. @Inject

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Key Takeaways
! Kotlin language features make this concise and straightforward

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Key Takeaways
! Kotlin language features make this concise and straightforward
! Dependency Injection is critical for any app

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Thank you.
My team is hiring in DC, Richmond & San Francisco.

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DIY Dependency Injection with Kotlin @ DevFest Florida 2019

DIY Dependency Injection with Kotlin @ DevFest Florida 2019

Sam Edwards

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