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Advanced Kotlin
for Android
DroidKaigi
Feb. 19, 2016
Taro @ngsw_taro
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● 長澤太郎 - Taro Nagasawa
● Twitter: @ngsw_taro
● Programmer at M3, Inc.
● Kotlin evangelist (unofficial)
● Japan Kotlin User Group
About me
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エムスリー株式会社 - M3, Inc.
● provide services for health care workers
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Agenda
● What is Kotlin
● How to use Kotlin in Android projects
● Real world Android Kotlin
● Aggressive Kotlin
● Summary
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Agenda
● What is Kotlin
● How to use Kotlin in Android projects
● Real world Android Kotlin
● Aggressive Kotlin
● Summary
interest you in Kotlin
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Agenda
● What is Kotlin
● How to use Kotlin in Android projects
● Real world Android Kotlin
● Aggressive Kotlin
● Summary
show tips and ideas
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What is Kotlin
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Kotlin
● target JVM, JavaScript, Android
● developed by JetBrains
● Statically typed
● Object-oriented
● Modern grammar & features
● The so-called “Better Java”
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Like this
package sample
fun main(args: Array) {
"world".hello()
}
fun String.hello() {
println("Hello, $this")
}
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Like this
package sample
fun main(args: Array) {
"world".hello()
}
fun String.hello() {
println("Hello, $this")
}
top level function
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Like this
package sample
fun main(args: Array) {
"world".hello()
}
fun String.hello() {
println("Hello, $this")
}
extension function
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Very easy
data class User(val id: String,
val name: String)
fun loadUser(id: String) {
apiClient.getUser(id)
.observeOn(AndroidSchedulers.main())
.subscribe { user ->
userNameView.text = user.name
}
}
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Very easy
data class User(val id: String,
val name: String)
fun loadUser(id: String) {
apiClient.getUser(id)
.observeOn(AndroidSchedulers.main())
.subscribe { user ->
userNameView.text = user.name
}
}
data class
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Very easy
data class User(val id: String,
val name: String)
fun loadUser(id: String) {
apiClient.getUser(id)
.observeOn(AndroidSchedulers.main())
.subscribe { user ->
userNameView.text = user.name
}
}
function literal
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Very easy
data class User(val id: String,
val name: String)
fun loadUser(id: String) {
apiClient.getUser(id)
.observeOn(AndroidSchedulers.main())
.subscribe { user ->
userNameView.text = user.name
}
}
property access
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Very easy
data class User(val id: String,
val name: String)
fun loadUser(id: String) {
apiClient.getUser(id)
.observeOn(AndroidSchedulers.main())
.subscribe { user ->
userNameView.text = user.name
}
}
like properties
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The time has come!!!
● 1.0 was released only a few days ago
● Some companies started to use Kotlin
● I am writing a Kotlin book now
○ It will be released by this late summer
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How to use Kotlin
in Android projects
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(1) start Kotlin in Android Projects
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Kotlin plugin installation
1. launch Android Studio
2. open “Preferences”
3. click “Install JetBrains plugin…”
4. find “Kotlin” from the list, and click “Install plugin”
5. wait for installation to complete
6. restart AS to enable the plugin
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convert MainActivity.java to Kotlin code
Code menu
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add config for Kotlin to build.gradle
Tools menu
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add config for Kotlin to build.gradle
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
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Finish
● rename a directory “java” to “kotlin”
● options
○ specify Kotlin version in gradle.properties
○ make project root depend on Kotlin plugin
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(2) use Realm and Dagger
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JSR269 Annotation processing
● for generating “Java” code by annotations in
compilation time
● Useful libraries
○ Dagger
○ Realm
○ Butter Knife
○ etc...
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kapt
dependencies {
// Realm
compile "io.realm:realm-android:$version"
kapt "io.realm:realm-android:$version"
}
kapt {
generateStubs = true
}
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RealmObject
open class User(
open var id: Long = 0,
@Required open var name: String = ""
): RealmObject()
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RealmObject
open class User(
open var id: Long = 0,
@Required open var name: String = ""
): RealmObject()
provide
setter/getter
provide
default constructor
provide
default constructor
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Real world Android Kotlin
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(1) Extensions
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Extension function
package sample
fun main(args: Array) {
"world".hello()
}
fun String.hello() {
println("Hello, $this")
}
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(Example) Toast
fun Context.toast(message: String,
duration: Int = Toast.LENGTH_SHORT) {
Toast.makeText(this, message, duration).show()
}
// in Activity
toast("Hello")
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(Example) Application object
val Activity.app: MyApp
get() = application as MyApp
// in Activity
app.component.inject(this)
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Scope functions provided by the stdlib
cf. Kotlin スコープ関数 用途まとめ
goo.gl/Ec1Gpi
name receiver parameters return
let T block: (T)->R R
run T block: T.()->R R
apply T block: T.()->Unit T
with - receiver: T, block: T.()->R R
* T and R are any types.
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(Example) execute Intent
startActivity(createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK)
.putExtra("key", "value"))
createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK)
.putExtra("key", "value")
.let { startActivity(it) }
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(Example) No longer need fluent interface
createIntent(this).run {
addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK)
if(value != null) {
putExtra("key", value)
}
startActivity(this)
}
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(Example) new a fragment with arguments
class MyFragment: Fragment() {
companion object {
fun new(foo: Int, bar: Int): MyFragment =
MyFragment().apply {
arguments = Bundle().apply {
putInt("foo", foo)
putInt("bar", bar)
}
}
}
}
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(2) Delegated properties
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delegate other objects to property access
class Greeter {
val message: String by lazy { "Hello" }
}
lazy init
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(Example) Extra
fun Activity.stringExtra(name: String): Lazy = lazy {
requireNotNull(intent.getStringExtra(name), {"missing"})
}
class MyActivity: Activity() {
val message: String by stringExtra("message")
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
Log.d("MyActivity", message)
}
}
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Kotter Knife
● View binding library for Kotlin
● developed by Jake Wharton
● use delegated properties
val submitButton: Button by bindView(R.id.submit)
val nameTextView: TextView? by bindOptionalView(R.id.name)
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(3) Nullable
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Null-Safety
● A mechanism to eliminate NullPointerException
val s1: String = null // NG
val s2: String? = null // OK
s2.toUpperCase() // NG
if(s2 != null)
s2.toUppserCase() // OK
s2?.toUpperCase() // OK
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let is map, flatMap, ifPresent for the Optional in Java
fun reverse(s: String): String {...}
val reverse: String? = str?.let { reverse(it) }
fun findById(id: Long): User? {...}
val user: User? = userId?.let { findById(it) }
user?.let { println(it }
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let is useful, but if here are multiple nullable...
foo?.let { foo ->
bar?.let { bar ->
baz?.let { baz ->
execute(foo, bar, baz)
}
}
}
deep nesting
ugly (´;ω;`)
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Use if expression
if (foo != null && bar != null && baz != null) {
execute(foo, bar, baz)
}
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When foo, bar, baz are “var” properies...
if (foo != null && bar != null && baz != null) {
// NG because they are stil nullable
execute(foo, bar, baz)
}
for {
foo <- foo
bar <- bar
baz <- baz
} yield execute(foo, bar, baz)
Kotlin dose not have
such syntax
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Answer 1: early returns
fun method() {
val foo = foo ?: return
val bar = bar ?: return
val baz = baz ?: return
execute(foo, bar, baz)
}
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Answer 2: early returns + “run”
fun method() {
run {
val foo = foo ?: return@run
val bar = bar ?: return@run
val baz = baz ?: return@run
execute(foo, bar, baz)
}
Log.d(TAG, "done!!!")
}
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Aggressive Kotlin
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(1) Extensions in a interface
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RxAndroid + RxLifecycle
apiClient.findArticle(id)
.compose(bindToLifecycle())
.observeOn(AndroidSchedulers.main())
.subscribe {
articleView.article = it
}
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RxAndroid + RxLifecycle
apiClient.findArticle(id)
.compose(bindToLifecycle())
.observeOn(AndroidSchedulers.main())
.subscribe {
articleView.article = it
}
cannot be
inferred
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A extension function is needed
apiClient.findArticle(id)
.bindToLifecycle()
.observeOn(AndroidSchedulers.main())
.subscribe {
articleView.article = it
}
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want to use a ActivityLifecycleProvider’s method, but how ?
// BOF
fun Observable.bindToLifecycle() =
compose(???)
// EOF
ActivityLifecycleProvider#
bindToLifecycle
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define a extension function in a interface
// BOF
interface RxLifecyclerFeature: ActivityLifecycleProvider {
fun Observable.bindToLifecycle() =
compose([email protected]())
}
// EOF
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implemtns the interface
class MyActivity: RxActivity(), RxLifecycleFeature {
fun go() {
apiClient.findArticle(id)
.bindToLifecycle()
.observeOn(AndroidSchedulers.main())
.subscribe { ... }
}
...
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(2) Extensions in a interface
which has abstract methods
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Can such a function be defined?
createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.putExtra("key", "value")
.startActivity()
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Can this function be defined?
createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.putExtra("key", "value")
.startActivity()
Intent’s method
AND
use Activity’s method
(i.e. startActivity)
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define a extension function in a interface
// BOF
interface IntentFeature {
fun Intent.startActivity() {
???
}
}
// EOF
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Additionally, define a abstract method
// BOF
interface IntentFeature {
fun startActivity(intent: Intent)
fun Intent.startActivity() {
startActivity(this)
}
}
// EOF
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implement the interface
class MyActivity: Activity(), IntentFeature {
fun go() {
createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.putExtra("key", "value")
.startActivity()
}
...
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(3) “Double context extensions”
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Without interfaces version
class MyActivity: Activity() {
fun go() {
createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.putExtra("key", "value")
.startActivity()
}
...
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provide a extension property
// BOF
val Activity.startActivity: Intent.()->Unit
get() = { intent ->
startActivity(intent)
}
// EOF
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provide a extension property
// BOF
val Activity.startActivity: Intent.()->Unit
get() = { intent ->
startActivity(intent)
}
// EOF
Activity’s property
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provide a extension property
// BOF
val Activity.startActivity: Intent.()->Unit
get() = { intent ->
startActivity(intent)
}
// EOF
Intent’s method
Intent’s method
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Usage
class MyActivity: Activity() {
fun go() {
createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.putExtra("key", "value")
.startActivity()
}
...
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Usage
class MyActivity: Activity() {
fun go() {
createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.putExtra("key", "value")
.startActivity()
}
...
Activity’s property,
which returns a
Intent’method
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Usage
class MyActivity: Activity() {
fun go() {
createIntent(this)
.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
.putExtra("key", "value")
.startActivity()
}
... Method invocation
Method invocation
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I named this pattern “double context extension”
● Pros
○ expressive
○ keep your code base simple
● Cons
○ Code completion dose not work
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Summary
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Summary
● Kotlin is a pragmatic language for Android
● The time has come!
● Getting started is easy
● Extensions & scope functions are your friends
● It’s easy to deal with nullable
● Interfaces make extensions more powerful
● “Double context extensions”, my idea
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Thank you,
Enjoy Kotlin