Android Summit 2019: DIY Dependency Injection with Kotlin

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

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Why am I up here talking about this?
! Demystify the topic of Dependency Injection
! Show how you can implement DIY, if you choose

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Our Team’s Story
! Got tasked to build a reusable SDK
! This was the first time we were building a complex SDK

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Product Requirements
! SDK will be used by different apps, in different codebases
! Should not assume clients will be using a 3rd party library
! Stay lean and lightweight
! Make the SDK configurable

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Technical Requirements
! Reactive Programming
! Immutable Data
! Modular Code
! Single Responsibility Code
! Well Tested Code
! Avoid 3rd Party Libraries
! 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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Kotlin Language Features makes DIY Do-able
! Named Parameters
! Default Parameters
! “by lazy”
! () -> lambdas

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TL;DR - The DIY Recipe
! Kotlin ✅
! Constructor Injection ✅
! No Annotations or Code Generation ✅
! Some Hacks for Activities & Fragments

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Why Dependency Injection?
! 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
Use THIS instead of THAT.
“Swapping”

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

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Dependency Injection is Inversion of Control
Code
Dependency

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Code

Dependency
Logger

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Code

AndroidLogger
Logger

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Code

AndroidLogger SystemOutLogger
or
Logger

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Code

AndroidLogger SystemOutLogger
or
Logger
“Inversion of Control" aka “Swapping”

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Avoid Static Instances - “new” is Your Friend
Log.d("TAG", “message”)
! Global state
! Must be mutable to be “reset”
between tests
! Hard to mock in tests -
PowerMock

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No Dependency Injection
class NoInjectionRepository {
val log = AndroidLogger()
val database = SqlDelightDatabase()
fun updateUser(user: User) {
log.log("Updating User: $user")
database.save(user)
}
}
NoInjectionRepository().updateUser(user)

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Common Types
of Dependency Injection

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Setter Injection
class SetterInjectionRepository {
lateinit var log: Logger
lateinit var database: Database
database.save(user)
}
}
val setterInjection = SetterInjectionRepository()
setterInjection.log = AndroidLogger()
setterInjection.database = SqlDelightDatabase()

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Constructor Injection
class ConstructorInjectionRepository(
val logger: Logger,
val database: Database
) {
database.save(user)
}
}
ConstructorInjectionRepository(
logger = AndroidLogger(),
database = SqlDelightDatabase()
)

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class ConstructorInjectionRepository(
val log: Logger = AndroidLogger(),
val database: Database
) {
database.save(user)
}
}
ConstructorInjectionRepository(
database = SqlDelightDatabase()
)
Why Constructor Injection is Great for Kotlin
Default Parameters
Named Parameters

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Why DIY Constructor Injection Makes Sense
! Compile Time Safety
! No 3rd party library
! No magic
! Easy to “Find Usages” and understand

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

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DIY Dependency Injection
in Practice

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

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

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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 shoppingCart: ShoppingCart
val userPreferences: UserPreferences
}

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Multiple or Nested Graphs are
AppGraph
SessionGraph
NetworkGraph
! Each module project needs their own graph
! Each graph is similar to a Dagger @Component

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! Each Module can contain it’s own graph
! Doesn’t need a static instance unless there are Activities or
Fragments
Graphs in Modules

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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 as needed
! Create Graph during Application::onCreate() if required by Activity/
Fragment for possible onProcessRestore bugs
Setting up Your Graphs

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GRAPH BEING SETUP

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GRAPH BEING ACCESSED
IN ACTIVITY

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RECAP: The DIY Recipe

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The DIY Recipe
! Kotlin ✅
! Constructor Injection ✅
! No Annotations or Code Generation ✅
! Some Hacks for Activities & Fragments

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Challenges - Nothing is Perfect
! Static Availability for Activities and Fragments
! Onboarding
! Scoping

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Key Takeaways
! You don’t HAVE to use a Dependency Injection Library, Dagger
! There are no magic annotations. @Inject (JSR-330)
! Kotlin language features make this concise and straightforward
! Dependency Injection is critical for any app

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Thank you.

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Android Summit 2019: DIY Dependency Injection with Kotlin

Android Summit 2019: DIY Dependency Injection with Kotlin

Sam Edwards

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