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Crafting Idiomatic APIs with Kotlin and Compose
Droidcon Berlin - September '25
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What is an API?
curl --request GET \
--url "https://api.github.com/octocat" \
--header "Authorization: Bearer YOUR-TOKEN" \
--header "X-GitHub-Api-Version: 2022-11-28"
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Protocols
SOAP / Rest / GraphQL / gRPC
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SDKs
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"Ordinary" Code
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Everything is an API
Droidcon Berlin - Oct '21
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Unit Tests
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Temporary Code
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There is nothing more permanent
than a temporary solution
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Who is your API for?
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Consumers
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Colleagues
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Future Self
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Consequences
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Consequences
Job security
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Consequences
Increased learning curve
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Consequences
Longer peer review
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Consequences
Slowed feature delivery
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Every existing thing is born without
reason, prolongs itself out of
weakness, and dies by chance
— Jean-Paul Sartre
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Every line of code is written without
reason, maintained out of
weakness, and deleted by chance
— Jean-Paul Sartre’s Programming in ANSI C.
programmingisterrible.com/post/139222674273/write-code-that-is-easy-to-delete-not-easy-to 20
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Code Maintenance
Photo by jessica rigollot 21
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Idioms
kotlinlang.org/docs/idioms.html 22
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Idioms
Default Parameters
fun foo(a: Int = 0, b: String = "") { /* ... */ }
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Idioms
Lazy Properties
val property: String by lazy { /* ... */ }
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Idioms
Null Coalescing
val fileSize = files?.size ?: run {
val someSize = getSomeSize()
someSize * 2
}
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Idioms
Expression return statements
fun getDisplayNameOrDefault(userId: String?): String = getDisplayName(userId ?: return "default")
kotlinlang.org/docs/whatsnew2220.html 26
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Idioms
Guard Conditions
fun feedAnimal(animal: Animal) {
when (animal) {
is Animal.Dog -> feedDog()
is Animal.Cat if !animal.mouseHunter -> feedCat()
else -> println("Unknown animal")
}
}
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Kotlin
Coding Conventions
kotlinlang.org/docs/coding-conventions.html 28
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Feb 2016
Kotlin 1.1
Type Aliases,
Bound
References,
Lambda
destructuring
Nov 2017
Kotlin 1.2
Array Literals,
lateinit
properties
Oct 2018
Kotlin 1.3
Coroutines,
Multiplatform
projects,
Contracts, when
subject
Aug 2020
Kotlin 1.4
Sam
conversions,
explicit API
mode, trailing
comma
May 2021
Kotlin 1.5.0
Sealed
interfaces,
improved inline
classes
Nov 2021
Kotlin 1.6.0
Exhaustive
when,
suspending
functions as
supertypes
Jun 2022
Kotlin 1.7.0
K2 compiler
alpha,
underscore
operator
Apr 2023
Kotlin 1.8.20
Wasm target,
data objects,
secondary
constructor
bodies
May 2024
Kotlin 2.0.0
Compose
compiler Gradle
plugin
Jun 2025
Kotlin 2.2.0
Context
parameters,
guard
conditions,
non-local break
and continue,
multi-dollar
interpolation
Kotlin
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Coroutines
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Reactive / Imperative
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jakewharton.com/the-state-of-managing-state-with-rxjava/ 33
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@Suppress("DEPRECATION")
class CallbackLoginPresenter(val service: SessionService, val goTo: (Screen) -> Unit) {
var onModel: (LoginUiModel) -> Unit = {}
var task: AsyncTask? = null
fun start() = onModel(Content)
fun stop() = task?.cancel(true)
fun onEvent(event: LoginUiEvent) = when (event) {
is Submit -> task = LoginAsyncTask().also {
it.execute(event)
}
}
inner class LoginAsyncTask : AsyncTask() {
private var username: String = ""
override fun doInBackground(vararg events: Submit?): LoginResult {
val event = events[0]!!
username = event.username
return runBlocking { service.login(event.username, event.password) }
}
override fun onPostExecute(result: LoginResult?) = when (result) {
is Success -> goTo(LoggedInScreen(username))
is Failure -> goTo(ErrorScreen(result.throwable?.message ?: ""))
else -> Unit
}
}
}
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Observable.just("Hey")
.subscribeOn(Schedulers.io())
.map(String::length)
.subscribeOn(Schedulers.computation())
.observeOn(AndroidSchedulers.mainThread())
.doOnSubscribe { doAction() }
.flatMap {
doAction()
Observable.timer(1, TimeUnit.SECONDS)
.subscribeOn(Schedulers.single())
.doOnSubscribe { doAction() }
}
.subscribe { doAction() }
proandroiddev.com/how-rxjava-chain-actually-works-2800692f7e13 35
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Coroutines
Structured Concurrency
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Declarative Architecture
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Declarative Architecture
Immutability
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Declarative Architecture
Idempotency / Determinism
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Declarative Architecture
Unidirectional Data Flow
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Jetpack, Compose, & Multiplatform
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fun Counter($composer: Composer) {
$composer.startRestartGroup(-1913267612)
/* ... */
$composer.endRestartGroup()
}
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Compose
Higher-Order Functions
@Composable
fun TopAppBar(
navigationIcon: @Composable (() -> Unit)? = null,
title: @Composable () -> Unit,
actions: @Composable (RowScope.() -> Unit)? = null,
// ...
)
TopAppBar(
navigationIcon = {
Image(/* ... */)
},
// ...
)
chrisbanes.me/posts/slotting-in-with-compose-ui 44
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Compose
Scopes
@Stable
interface WeightScope {
fun Modifier.weight(weight: Float): Modifier
}
@Composable
fun WeightedRow(
modifier: Modifier = Modifier,
content: @Composable WeightScope.() -> Unit
) {
// ...
// Usage:
WeightedRow {
Text("Hello", Modifier.weight(1f))
Text("World", Modifier.weight(2f))
}
android.googlesource.com/platform/frameworks/support/+/androidx-main/compose/docs/compose-api-guidelines.md#layout_scoped-modifiers 45
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Compose
Scopes
@Composable
fun TopAppBar(
actions: @Composable (RowScope.() -> Unit)? = null,
// ...
)
TopAppBar(
actions = {
ImageButton(onClick = { /* ... */ }) {
Image(/* ... */)
}
ImageButton(onClick = { /* ... */ }) {
Image(/* ... */)
}
},
)
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Compose
Property Delegates
inline operator fun State.getValue(thisObj: Any?, property: KProperty<*>): T = value
val conference by remember { mutableStateOf("Droidcon Berlin") }
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Compose
Architecture
downloadManager
.downloadFile("https://.../")
.flatMapLatest { state ->
when (state) {
is State.Loaded -> stateFileManager.saveFile(
name = "storage/file",
value = state.value,
)
else -> state
}
}
.onEach { state ->
when (state) {
is State.Saved -> println("Downloaded file successfully")
is State.Loading -> /* ... */
}
}
.launchIn(coroutineScope)
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RootNode
ChildNode ChildNode ChildNode ChildNode ChildNode
ChildNode ChildNode ChildNode ChildNode
ChildNode ChildNode
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val downloadState = downloadManager
.downloadFile("https://.../")
.collectAsState(State.Downloading)
val fileState = when(downloadState) {
is State.Loaded ->
stateFileManager
.saveFile("storage/file", downloadState.value)
else -> downloadState
}
when (fileState) {
is State.Loading -> /* ... */
is State.Saved -> LaunchedEffect(fileState) {
println("Downloaded file successfully")
}
}
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fun CoroutineScope.launchCounter(): StateFlow {
return launchMolecule(mode = ContextClock) {
var count by remember { mutableStateOf(0) }
LaunchedEffect(Unit) {
while (true) {
delay(1_000)
count++
}
}
count
}
}
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Beyond the UI
mDevCamp Prague - Jun '25
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Language
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!
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colour
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Luxemburg
Netherlands
Denmark
Belgium
Austria
Germany
Poland
UK
0 0.5 1 1.5 2 2.5 3 3.5 4
Average number of languages spoken per person in Europe
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fun showCancelBookingAndBackToVerbindungsuebersichtDialog()
fun handleLoadBuchungskundenDatenOrOpenRechnungFailure()
fun getAngebotsKonditionIconAndContentDescription()
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Teuton: Eine Deutsche Programmiersprache
# -*- coding: iso-8859-1 -*-
schön = Wahr
häßlich = Falsch
für bäh in [schön, häßlich]:
drucke bäh
def sovielwiemöglich():
"gib"
zurück "was mir gehört"
fiber-space.de/EasyExtend/doc/teuton/teuton.htm 58
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Business Specific Terminology
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Stornierung, Ersatzverkehr, Signalstörung...
Business Specific Terminology
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Business Specific Terminology
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What makes a "good" API?
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There is no such thing as perfect code
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Perfect Solution Fallacy
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Documentation
/**
* A group of *members*.
*
* This class has no useful logic; it's just a documentation example.
*
* @param T the type of a member in this group.
* @property name the name of this group.
* @constructor Creates an empty group.
*/
class Group(val name: String) {
/**
* Adds a [member] to this group.
* @return the new size of the group.
*/
fun add(member: T): Int { ... }
}
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Good code is easy to delete
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High Cohesion - Loose Coupling
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Dependency Control
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Tooling
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Tooling
explicitApiMode()
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Tooling
com.slack.lint.compose:compose-lint-checks
slackhq.github.io/compose-lints/ 71
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Tooling
io.nlopez.compose.rules:ktlint
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Tooling
@Suppress
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Multiline expression wrapping
standard:multiline-expression-wrapping
val foo =
foo(
parameterName =
"The quick brown fox "
.plus("jumps ")
.plus("over the lazy dog"),
)
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What makes a bad API?
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Thank You!
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