Developing Android with Kotlin

Antonio Leiva Android Engineer @ Plex, Inc http://antonioleiva.com http://plus.google.com/+AntonioLeivaGordillo @lime_cl

http://plex.tv

Bandhook Kotlin http://github.com/antoniolg/Bandhook-Kotlin

Index 1. What is Kotlin? 2. Configure your project 3. Kotlin on Android 4. Conclusions

What is Kotlin? 1. Introduction 2. Expressive 3. Type safe 4. Functional

What is Kotlin? • JVM based • Object-oriented functional language • Created by JetBrains (IntelliJ, Android Studio) • Simple,lightweight, interoperable

Expressive public class Artist {
 private long id;
 private String name;
 private String url;
 private String mbid;
 
 public long getId() {
 return id;
 }
 
 public void setId(long id) {
 this.id = id;
 }
 
 public String getName() {
 return name;
 }
 
 public void setName(String name) {
 this.name = name;
 }
 
 public String getUrl() {
 return url;
 }
 
 public void setUrl(String url) {
 this.url = url;
 }
 
 public String getMbid() {
 return mbid;
 }
 
 public void setMbid(String mbid) {
 this.mbid = mbid;
 }
 
 @Override public String toString() {
 return "Artist{" +
 "id=" + id +
 ", name='" + name + '\'' +
 ", url='" + url + '\'' +
 ", mbid='" + mbid + '\'' +
 '}';
 }
 }
 JAVA

Expressive data class Artist(
 var id: Long,
 var name: String,
 var url: String,
 var mbid: String) KOTLIN

Type safe Artist artist = null;
 artist.print(); JAVA Null Pointer Exception

Type safe KOTLIN var artist: Artist? = null
 artist.print() var artist: Artist? = null
 artist?.print() won´t compile will do nothing if (artist != null) {
 artist.print()
 } Smart cast var artist: Artist? = null
 artist!!.print() will crash

Functional • Functional support (lambdas) view.setOnClickListener { toast("Hello world!") }

Configure your project 1. Kotlin plugin install and configuration 2. Build.grade tweaks 3. Code transformation

Configure your project 1. Create a new Android Project

Configure your project 2. Download Kotlin plugin for Android Studio

Configure your project 3. Create new Kotlin folder

Configure your project 4. Parent build.grade buildscript {
 ext.kotlin_version = "0.11.91"
 repositories {
 jcenter()
 }
 dependencies {
 classpath "com.android.tools.build:gradle:1.1.3"
 classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
 }
 }

Configure your project 5. App build.grade apply plugin: 'com.android.application'
 apply plugin: 'kotlin-android' Apply plugin dependencies {
 compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
 } Kotlin library dependency android {
 …
 sourceSets {
 main.java.srcDirs += 'src/main/kotlin'
 }
 } Add kotlin folder to srcDirs

Configure your project 4b. Modify build.gradle files (alternative)

Configure your project 5. Transform main activity to kotlin code

Configure your project 5. Transform main activity to kotlin code

Kotlin on Android 1. Extension functions 2. Default values for arguments 3. Custom Views 4. Kotlin Android Extensions 5. Lambda Expressions 6. Extending language 7. Partial mult-inheritance 8. Collections

Extension functions fun Activity.toast(message: CharSequence, duration: Int = Toast.LENGTH_SHORT){
 Toast.makeText(this, message, duration).show()
 } override fun onCreate(savedInstanceState: Bundle?) {
 super.onCreate(savedInstanceState)
 toast("This is onCreate!!")
 }

Extension functions inline public fun Activity.navigate(id: String) {
 val intent = Intent(this, javaClass())
 intent.putExtra("id", id)
 startActivity(intent)
 } navigate("2")

Default values for arguments fun Activity.toast(message: CharSequence, duration: Int = Toast.LENGTH_SHORT){
 Toast.makeText(this, message, duration).show()
 } toast(“Short Toast!!”) toast("Long Toast!!", Toast.LENGTH_LONG)

Default values for arguments inline public fun Activity.navigate(
 id: String,
 sharedView: View? = null,
 transitionName: String? = null) {
 
 ...
 } navigate("2") navigate("2", sharedView, TRANSITION_NAME) navigate( id = "2", transitionName = TRANSITION_NAME)

Custom Views • By default, classes only have a unique constructor class MyClass(param: Int, optParam1: String = "", optParam2: Int = 1) {
 
 init {
 // Initialization code
 }
 }

Custom Views • Since Kotlin M11, multiple constructors class SquareImageView : ImageView {
 
 public constructor(context: Context) : super(context) {
 }
 
 public constructor(context: Context, attrs: AttributeSet) : super(context, attrs) {
 }
 
 public constructor(context: Context, attrs: AttributeSet, defStyleAttr: Int) : super(context, attrs, defStyleAttr) {
 }
 
 override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
 super.onMeasure(widthMeasureSpec, heightMeasureSpec)
 val width = getMeasuredWidth()
 setMeasuredDimension(width, width)
 }
 }

Kotlin Android Extensions • A extension of Kotlin library for Android apps • Currently includes a view binder • A new plugin buildscript {
 …
 dependencies { …
 classpath “org.jetbrains.kotlin:kotlin-android-extensions:$kotlin_version”
 }
 }

Kotlin Android Extensions Direct access to XML views using its id as property name main.xml 
 
 
 


Kotlin Android Extensions Import synthetic properties import kotlinx.android.synthetic.main.* Use the properties override fun onCreate(savedInstanceState: Bundle?) {
 super.onCreate(savedInstanceState) setContentView(R.id.main)
 frameLayout.setVisibility(View.VISIBLE)
 welcomeText.setText("I´m a welcome text!!")
 }

Lambda Expressions Palette.generateAsync(bitmap, new Palette.PaletteAsyncListener() {
 @Override public void onGenerated(Palette palette) {
 …
 }
 }); Palette.generateAsync(bitmap) { palette ->
 …
 } JAVA KOTLIN

Lambda Expressions view.setOnClickListener(object : View.OnClickListener{
 override fun onClick(v: View) {
 toast("Click")
 }
 }) Regular anonymous class (similar to Java)

Lambda Expressions view.setOnClickListener({
 toast("Click")
 }) If trait (interface) only has a function, substitute by function view.setOnClickListener({ view ->
 toast("Click")
 })

Lambda Expressions view.setOnClickListener(){
 toast("Click")
 } If last parameter is a function, can go out of parenthesis view.setOnClickListener{
 toast("Click")
 }

Lambda Expressions view.setOnClickListener(object : View.OnClickListener{
 override fun onClick(v: View) {
 toast("Click")
 }
 }) view.setOnClickListener { toast("Click") }

Extending language ‘With’ function fun with(t: T, body: T.() -> Unit) { t.body() } with(titleText) {
 setPivotX(0f)
 setPivotY(getHeight().toFloat())
 updateTitleScale(this, minHeight, maxHeight, 0)
 }

Extending language Check Android version inline fun supportsLollipop(code:() -> Unit){
 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP){
 code.invoke()
 }
 } supportsLollipop {
 getWindow().setStatusBarColor(Color.BLACK)
 }

Partial multi-inheritance Inheritance through Traits • Most similar thing to Interfaces in Java • Can contain code • Can´t contain state • Can declare properties • Properties are abstract for children • A class can extend several traits • A trait can extend another class • Can use parent functions • Can´t call ‘super’ on overriden functions

Partial multi-inheritance trait NavigationActivity : ActionBarActivity {
 val navigationDrawer: DrawerLayout
 fun initDrawer() { "..." }
 }
 
 trait SearchActivity : ActionBarActivity {
 var searchView: SearchView
 fun initSearch() { "..." }
 }
 
 trait ScrollHeaderActivity : ActionBarActivity {
 fun initHeader(){
 val image = findViewById(R.id.image) as ImageView
 val title = findViewById(R.id.title) as TextView
 }
 }

Partial multi-inheritance class MyActivity : ActionBarActivity(), NavigationActivity,
 SearchActivity, ScrollHeaderActivity {
 
 override val navigationDrawer: DrawerLayout by bindView(R.id.drawer)
 override var searchView: SearchView by Delegates.notNull()
 
 override fun onCreate(savedInstanceState: Bundle?) {
 super.onCreate(savedInstanceState)
 initDrawer()
 initHeader()
 initSearch()
 }
 }

Collections Operations filter sort map zip dropWhile first firstOrNull last lastOrNull fold …

Collections Operations fun transform(artists: List): List {
 return artists filter { isValid(it) } map { transform(it) }
 }

Collections Operations // Inmutable
 val list = listOf(1, 2, 3, 4, 5)
 
 // Mutable
 val mutableList = ArrayList()
 mutableList[0] = 1
 val first = mutableList[0]
 
 // Map
 val map = HashMap()
 map["key"] = 1
 val value = map["key"]

Conclusions • Kotlin in numbers • Limitations • Production ready?

Kotlin in numbers Library Jar Size Dex Size Method Count Field Count kotlin-runtime-0.10.195 354 KB 282 KB 1071 391 kotlin-stdlib-0.10.195 541 KB 835 KB 5508 458 play-services- base-6.5.87 773 KB 994 KB 5212 2252 support-v4-21.0.3 745 KB 688 KB 6721 1886 Using project Kotlin in Android (Jake Wharton): http://goo.gl/ZFtbBw

Limitations • Customs Views (Available since M11) • Interoperability with auto generated code • Many libraries use $ for auto generated names • Not possible to use Dagger 1 or Butterknife • jUnit testing in Android projects

Production ready? • Stable enough • Not final: 1.0 -> Summer 2015 • Language definition still changing

Bibliography • Kotlin reference
 http://kotlinlang.org • Kotlin official blog
 http://blog.jetbrains.com/kotlin/ • Kotlin: The swift of Android
 https://www.youtube.com/watch?v=dJscNr1silY • Using project Kotlin in Android
 http://goo.gl/ZFtbBw


Questions? http://antonioleiva.com http://plus.google.com/+AntonioLeivaGordillo @lime_cl