Kodein

KOtlin DEpendency INjection

My name is…​

Salomon BRYS

  • Kotliner for 3 years

  • Author & maintainer of Kodein

I work at…​

Dental Monitoring.

Dependency Injection

Dependency injection separates the creation of a client’s dependencies from the client’s behavior, which allows program designs to be loosely coupled and to follow the dependency inversion and single responsibility principles.

— Wikipedia

Dependency Injection

Dependency Injection

Source: Microsoft

What can you inject ?

  • Business managers

  • Presenters (in MVP)

  • OS services

  • Anything that can be abstracted!

DI libraries: Dagger 2

Warning

Dagger 2 is a valid solution, widely used and very reliable.

All views expressed here are those of the author only.
Please research the solution that’s right and do NOT trust my word only ;)

Dagger 2: Validation at runtime

Allows all dependencies to be checked before compilation :)

  • You need to declare all injected classes

  • Weird compilation errors (missing dependency, circular dependency, etc.)

  • Less flexibility

Dagger 2: Annotation based

  • Module configuration through annotations.

  • Unsemantic binding syntax.

    @Singleton
fun provideManager(net: Network, user: User): Manager
    = ManagerImpl(net, user)

DI libraries: Guice

  • Depends on Guava.

  • Adds 13k+ methods.

  • Annotation based.

Goals for DI in Kotlin

  • Semantic (simple declarative DSL).

  • Idiomatic (best use of the Kotlin language)

  • Easy (to use & bootstrap).

  • Debuggable (understandable errors).

  • Android friendly (duh).

  • Java friendly (for legacy code).

  • Documented (heavily).

DI library in Kotlin

How about optimized?

— A random #perfmatters activist

  • Yes, #perfmatters, but only in critical path (see the latest ART)

  • The very majority of times, DI is not a critical path!

  • In DI, readability and debuggability are way more important !

KODEIN

KODEIN

https://github.com/SalomonBrys/Kodein

Kodein: History

  • Version 1.x (from apr. 2015): Concept

  • Version 2.x (from oct. 2015): Robust API

  • Deprecation of Injekt (26 jun. 2016)

  • Version 3.x (jul. 2016): Core reshaping & lots of features.

  • Version 4.x (jun. 2017): Compatible with JS.

Kodein: External Contributors

Mainly

  • Jayson Minard (author of injekt) from Collokia, lots of help & insight.

  • Eliezer Graber from Insite Applications, huge investment in the Android module.

But also a lot of other smaller contributions from the community.

Kodein: Documentation

RTFM!

https://salomonbrys.github.io/Kodein/

A bit of Kotlin

To understand the magic

Kotlin: Infix functions

user registers event at noon

user.register(event).at(noon)

Kotlin: reified types

inline fun <reified T> instance(): T
    = container.getInstance(T::class.java)

val manager = instance<Manager>()

Kotlin: inferred (reified) types

inline fun <reified T> instance(): T
    = container.getInstance(T::class.java)

val manager: Manager = instance()
//           └─────────────────┘

KODEIN

KODEIN

Let’s code, dammit!

KODEIN

  • Semantic (simple declarative DSL).

  • Idiomatic (best use of the Kotlin language)

  • Easy (to use & bootstrap).

  • Debuggable (understandable errors).

  • Android friendly (duh).

  • Java friendly (for legacy code).

  • Documented (heavily).

KODEIN

Thank you!