Android Summit 2019: DIY Dependency Injection with Kotlin

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

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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit Why 100% Kotlin?

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@HandstandSam #AndroidSummit Why 100% Kotlin? makes coding easy and concise.

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@HandstandSam #AndroidSummit Why 100% Kotlin? makes dependency injection easy and concise.

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@HandstandSam #AndroidSummit Kotlin Language Features makes DIY Do-able ! Named Parameters ! Default Parameters ! “by lazy” ! () -> lambdas

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

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@HandstandSam #AndroidSummit Why Dependency Injection? ! It enables all the really cool things I like to talk about

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@HandstandSam #AndroidSummit It Enables Testing DevFest Florida

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@HandstandSam #AndroidSummit It Enables Custom Debug Features Droidcon Boston

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@HandstandSam #AndroidSummit It Enable Mock Flavors Droidcon NYC

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@HandstandSam #AndroidSummit Dependency Injection is Not Magical

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@HandstandSam #AndroidSummit Dependency Injection is Not Magical DI !=

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@HandstandSam #AndroidSummit Dependency Injection is Use THIS instead of THAT. “Swapping”

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@HandstandSam #AndroidSummit Dependency Injection is Passing in dependencies  instead of pre-defining them. Inversion of Control

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@HandstandSam #AndroidSummit Dependency Injection is Inversion of Control Code Dependency

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@HandstandSam #AndroidSummit Dependency Injection is Inversion of Control Code Dependency Logger

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@HandstandSam #AndroidSummit Dependency Injection is Inversion of Control Code AndroidLogger Logger

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@HandstandSam #AndroidSummit Dependency Injection is Inversion of Control Code AndroidLogger SystemOutLogger or Logger

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@HandstandSam #AndroidSummit Dependency Injection is Inversion of Control Code AndroidLogger SystemOutLogger or Logger

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@HandstandSam #AndroidSummit Dependency Injection is Inversion of Control Code AndroidLogger SystemOutLogger or Logger “Inversion of Control" aka “Swapping”

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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit Common Types of Dependency Injection

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@HandstandSam #AndroidSummit Setter Injection class SetterInjectionRepository { lateinit var log: Logger lateinit var database: Database fun updateUser(user: User) { log.log("Updating User: $user") database.save(user) } } val setterInjection = SetterInjectionRepository() setterInjection.log = AndroidLogger() setterInjection.database = SqlDelightDatabase()

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@HandstandSam #AndroidSummit Constructor Injection class ConstructorInjectionRepository( val logger: Logger, val database: Database ) { fun updateUser(user: User) { log.log("Updating User: $user") database.save(user) } } ConstructorInjectionRepository( logger = AndroidLogger(), database = SqlDelightDatabase() )

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@HandstandSam #AndroidSummit class ConstructorInjectionRepository( val log: Logger = AndroidLogger(), val database: Database ) { fun updateUser(user: User) { log.log("Updating User: $user") database.save(user) } } ConstructorInjectionRepository( database = SqlDelightDatabase() ) Why Constructor Injection is Great for Kotlin Default Parameters Named Parameters

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

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@HandstandSam #AndroidSummit Open Source DI Libraries ! Dagger ! Toothpick ! Koin ! Kodein

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@HandstandSam #AndroidSummit ! Dagger ! Toothpick ! Koin ! Kodein Code Generation YES YES NO NO

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@HandstandSam #AndroidSummit ! Dagger ! Toothpick ! Koin ! Kodein Code Generation YES YES NO NO ❌ ❌

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@HandstandSam #AndroidSummit ! Dagger ! Toothpick ! Koin ! Kodein Code Generation YES YES NO NO Single  Instance NO NO YES NO ❌ ❌

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@HandstandSam #AndroidSummit ! Dagger ! Toothpick ! Koin ! Kodein Code Generation YES YES NO NO Single  Instance NO NO YES NO ❌ ❌ ❌

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@HandstandSam #AndroidSummit ! 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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@HandstandSam #AndroidSummit ! 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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@HandstandSam #AndroidSummit ! 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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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit DIY Dependency Injection in Practice

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@HandstandSam #AndroidSummit ShoppingApp - Open Source on GitHub https:/ /github.com/handstandsam/ShoppingApp

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@HandstandSam #AndroidSummit Network Graph interface NetworkGraph { val categoryRepo: CategoryRepo val itemRepo: ItemRepo val userRepo: UserRepo }

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

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@HandstandSam #AndroidSummit Session Graph interface SessionGraph { val sessionManager: SessionManager val shoppingCart: ShoppingCart val userPreferences: UserPreferences }

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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit ! 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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@HandstandSam #AndroidSummit ! 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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@HandstandSam #AndroidSummit ! 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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@HandstandSam #AndroidSummit GRAPH BEING SETUP

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@HandstandSam #AndroidSummit GRAPH BEING ACCESSED IN ACTIVITY

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@HandstandSam #AndroidSummit RECAP: The DIY Recipe

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

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@HandstandSam #AndroidSummit Challenges - Nothing is Perfect ! Static Availability for Activities and Fragments ! Onboarding ! Scoping

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@HandstandSam #AndroidSummit 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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@HandstandSam #AndroidSummit Thank you.