Coding for Android on steroids with Kotlin

CODING FOR ANDROID ON STEROIDS WITH KOTLIN KAI KOENIG (@AGENTK)

AGENDA ▸ What is Kotlin? ▸ Language idioms & concepts ▸ Kotlin and Android ▸ Anko DSL ▸ Libraries and Tools ▸ Final thoughts

WHAT IS KOTLIN? SOME FUNDAMENTALS ▸ Statically typed programming language for the JVM and Android ▸ Started as internal language “Project Kotlin” at Jetbrains in 2010 ▸ Now: Open-Source, Apache License ▸ Kotlin SDK plus tool support for IntelliJ, Android Studio, Eclipse ▸ Named after an island in the Gulf of Finland

WHAT IS KOTLIN? MOTIVATION FOR KOTLIN ▸ The Java platform is awesome, but it has its issues: ▸ sometimes tied to backwards/legacy compatibility ▸ can be a very verbose language and produce bloated code ▸ type system has various ﬂaws ▸ Kotlin aims to ﬁx a lot of those issues ▸ compiles to Java 6 byte code therefore caters well for Android runtimes

WHAT IS KOTLIN? HOW DOES A SIMPLE CONVERSION LOOK LIKE? public String listConvert(Collection<Integer> collection) { StringBuilder sb = new StringBuilder(); sb.append("{"); Iterator<Integer> iterator = collection.iterator(); while (iterator.hasNext()) { Integer element = iterator.next(); sb.append(element); if (iterator.hasNext()) { sb.append(", "); } } sb.append("}"); return sb.toString(); } fun listConvert(collection: Collection<Int>): String { val sb = StringBuilder() sb.append("{") val iterator = collection.iterator() while (iterator.hasNext()) { val element = iterator.next() sb.append(element) if (iterator.hasNext()) { sb.append(", ") } } sb.append("}") return sb.toString() } fun listConvertKt(collection: Collection<Int>): String { return collection.joinToString(prefix = "{",postfix = "}") }

LANGUAGE IDIOMS & CONCEPTS https://www.ﬂickr.com/photos/geraldford/6976818221/

LANGUAGE IDIOMS & CONCEPTS OVERVIEW ▸ Immutability ▸ String templates & Enum classes ▸ Null safety ▸ Properties and Fields ▸ Type inference and casts ▸ Data classes ▸ Syntactic sugar (loops, ranges etc) ▸ Extension functions ▸ Lambdas ▸ Collection API ▸ Type-safe builders ▸ Java-Kotlin-Interop

LANGUAGE IDIOMS & CONCEPTS IMMUTABILITY ▸ Built-in support for mutable and immutable variables, properties and ﬁelds ▸ Keywords var and val ▸ val - immutable (recommended) ▸ var - mutable ▸ Similar concept applies for class properties, val creates getters, var creates getters and setters (more later) val a: Int = 1 val b = 1 val c: Int c = 1 var x = 23 x += 1

LANGUAGE IDIOMS & CONCEPTS STRING TEMPLATES & ENUM CLASSES (I) ▸ Kotlin Strings can contain template expressions ▸ String templates start with a $ character and ▸ can contain simple references such as $s, as well as ▸ complex expressions in curly braces:   ${s.length} val s = "abc" val str = "$s.length is ${s.length}" Output: abc.length is 3

LANGUAGE IDIOMS & CONCEPTS STRING TEMPLATES & ENUM CLASSES (II) ▸ Kotlin has a dedicated and expressive enum class ▸ Can be used very nicely in conjunction with String templates enum class Locale(val hello: String) { DE_DE("Hallo"), EN_NZ("Hello"), MI_NZ("Kia Ora") } class Customer(val firstName:String, val lastName:String, val locale: Locale = Locale.DE_DE) { fun sayHello() = println( "${locale.hello}, $firstName $lastName") } fun main(args : Array<String>) { val myCustomer = Customer("Sandra", "Musterfrau", Locale.MI_NZ) myCustomer.sayHello() }

LANGUAGE IDIOMS & CONCEPTS NULL SAFETY ▸ Motivation: A better way to deal with NPEs ▸ Kotlin differentiates nullable types from non- nullable types by adding a ? to the type: ▸ String: no nullable ▸ String?: nullable ▸ Handle manually ▸ Use Safe Call operator ?. ▸ Use the !! operator to allow/trigger a NPE // Won't compile var lastName: String = null // Will compile var lastNameNullable: String? = null // Will also not compile println(lastNameNullable.length) // Option 1 (-1) println(if (lastNameNullable != null) lastNameNullable.length else -1) // Option 2 (null) println(lastNameNullable?.length) // Option 3 (NPE) println(lastNameNullable!!.length)

LANGUAGE IDIOMS & CONCEPTS PROPERTIES AND FIELDS ▸ Kotlin classes have mutable or immutable properties ▸ Default getter/setters for properties, can be customised ▸ An automated backing ﬁeld can be provided by the compiler (if required) ▸ Alternative: use an explicit backing property var counter = 0 set(value) { if (value >= 0) field = value } public class Address { public var name: String = ... public var street: String = ... public var city: String = ... public var state: String? = ... public var zip: String = ... }

LANGUAGE IDIOMS & CONCEPTS TYPE INFERENCE AND CASTS (I) ▸ When possible, Kotlin will infer the type of variables ▸ Explicit conversions, type widening and inference ▸ Smaller types are not subtypes of bigger types, no implicit conversion ▸ Types are often inferred from the context val b: Byte = 1 // This won't work val i: Int = b // This will val i: Int = b.toInt() val l: Long = 1L + 3

LANGUAGE IDIOMS & CONCEPTS TYPE INFERENCE AND CASTS (II) ▸ is or !is checks if an object adheres to a certain type ▸ Smart cast: Compiler tracks is-expressions for immutable values ▸ works for val local variables and private, internal or in-module performed casts ▸ works for var local variables if the variable hasn’t been modiﬁed between check and usage, never for var properties fun whatIs(x: Any) { when (x) { is Int -> println(x + 42) is String -> println(x.length) is IntArray -> println(x.sum()) } } whatIs(4) // 46 whatIs("4") // 1 whatIs(intArrayOf(1,2,3,4,5)) // 15

LANGUAGE IDIOMS & CONCEPTS DATA CLASSES ▸ The POJOs/Beans of other languages ▸ Data classes implicitly create: ▸ getters/setters (non-data classes have those too) - recommend to use val as often as possible. ▸ useful implementations for equals(), hashCode(), toString(), copy() ▸ copy() has default parameters and can be used to alter a copy data class ChromeEncryptedPayload( val encryptedPayload: String, val encryptionHeader: String, val cryptoKeyHeader: String) val pl = ChromeEncryptedPayload( encryptedPayload = "...", encryptionHeader = "...", cryptoKeyHeader = "...") val anotherPl = pl.copy( encryptedPayload = "...")

LANGUAGE IDIOMS & CONCEPTS EXTENSION FUNCTIONS ▸ Allow adding new functionality to a class without inheritance or Decorators ▸ Kotlin has extension functions as well as extension properties ▸ Resolved statically, don’t actually modify the class (excellent example why this has to be the case at http:// goo.gl/EN6bTs fun Int.sum(otherInt: Int): Int = this + otherInt 3.sum(7) fun Activity.toast(message: CharSequence, duration: Int = TOAST.LENGTH_SHORT) { Toast.makeText(this, message, duration).show() } // In onCreate of an Activity override fun onCreate(...) { ... toast("Hi there") ... }

LANGUAGE IDIOMS & CONCEPTS LAMBDAS ▸ Anonymous function or “function literal”, closely related to the ideas of Higher-Order-Functions val sum = { x: Int, y: Int -> x + y } val sum: (Int, Int) -> Int = { x, y -> x + y } private fun convertPetListToDomain(list: List<Pet>): List<PetDomain> { return list.map { convertPetItemToDomain(it) } } private fun convertPetItemToDomain(pet: Pet): PetDomain { return PetDomain(pet.age,pet.size,pet.id,pet.name,pet.sex,pet.breed) }

LANGUAGE IDIOMS & CONCEPTS TYPE-SAFE BUILDERS (I) ▸ Builders are a very popular approach in Groovy to deﬁne data in a declarative way ▸ Often used for: ▸ generating XML or JSON ▸ UI layout (Swing components) etc ▸ In Kotlin, builders even can be type- checked JsonBuilder builder = new JsonBuilder() builder.records { car { name 'HSV Maloo' make 'Holden' year 2006 country 'Australia' } } String json = JsonOutput.prettyPrint (builder.toString())

LANGUAGE IDIOMS & CONCEPTS TYPE-SAFE BUILDERS (II) ▸ html() is a function with a lambda as an argument (init) ▸ init’s type is a function type with receiver, this allows you to: ▸ pass receiver (of type HTML) to function ▸ call members of instance inside the function fun result(args: Array<String>) = html { head { title {”HTML in Kotlin"} } body { ... } } fun html(init: HTML.() -> Unit): HTML { val html = HTML() html.init() return html }

LANGUAGE IDIOMS & CONCEPTS TYPE-SAFE BUILDERS (III) ▸ HTML class has functions to build the head and the body elements of the DSL. ▸ Not shown: classes further down in the hierarchy: ▸ Head, Body etc. ▸ Complete HTML builder example at: http://goo.gl/TndcC9 class HTML { ... fun head(headBuilder: Head.() -> Unit) { head = Head() head?.headBuilder() } fun body(bodyBuilder: Body.() -> Unit) { body = Body() body?.bodyBuilder() } }

LANGUAGE IDIOMS & CONCEPTS JAVA-KOTLIN-INTEROP ▸ Java and Kotlin are fully interoperable from an integration point of view ▸ Your Java code can call and use Kotlin code ▸ Your Kotlin code can call and use Java code ▸ The latter is in particular useful because it means you can continue to use pretty much any existing Android/Java library ▸ Check out Hendrik Kokocinski’s sample Kotlin app that uses all kinds of well known Android/Java libs: https://goo.gl/xdYqf5

KOTLIN AND ANDROID https://realm.io/assets/img/news/tutorial-kotlin-anko-cover.png

KOTLIN AND ANDROID TOOLCHAIN AND FLOW Java sourcesKotlin sources Bytecode javackotlinc Dalvik ART JVM

KOTLIN AND ANDROID PROJECT SETUP ▸ Use Android Studio 2.x or IntelliJ 2016 ▸ Install Kotlin plugin (comes within “Jetbrains plugins”) ▸ Gradle dependencies project-level: ▸ classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.0.4" ▸ Gradle dependencies module-level: ▸ compile 'org.jetbrains.kotlin:kotlin-stdlib:1.0.4' ▸ apply plugin: 'kotlin-android' ▸ main.java.srcDirs += 'src/main/kotlin'

KOTLIN AND ANDROID KOTLIN EXTENSIONS FOR ANDROID ▸ Provides of a set of synthetic properties that bind views to those properties ▸ Alternative to Butter Knife-style bindings, no need for additional runtime library (Kotlin Extensions for Android are a Kotlin compiler plugin) ▸ import kotlinx.android.synthetic.main.<layout>.* ▸ import kotlinx.android.synthetic.main.<layout> ▸ usage: <componentid>.doSomething()

KOTLIN AND ANDROID SYNTHETIC PROPERTIES package ventegocreative.co.nz.kotlindemo import android.support.v7.app.AppCompatActivity import android.os.Bundle import kotlinx.android.synthetic.main.activity_main.* class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) helloworld.text = "Hey, I'm dynamically set" } }

ANKO DSL WHAT IS A DSL? ▸ Domain-Speciﬁc Language ▸ Limited expressiveness: ▸ DSLs are usually not general-purpose languages ▸ strongly focussed on a particular domain ▸ examples: SQL, Ant XML, XSLT, Gradle etc.

ANKO DSL A DSL FOR LAYOUTS ▸ The most important element of Anko is the Layout DSL ▸ Idea: replace XML layout deﬁnitions by Kotlin code - without having to build the layout in a fully programmatic sense ▸ Anko DSL is modular and extensible

ANKO DSL LAYOUT XML <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="match_parent" android:layout_height="match_parent"> <EditText android:layout_width="match_parent" android:gravity="center" android:text="@string/empty_todos_message" android:layout_weight="7" android:layout_height="wrap_content" /> <Button android:layout_width="match_parent" android:layout_weight="1" android:text="Say Hello" android:layout_height="0dp" /> </LinearLayout>

ANKO DSL PROGRAMMATIC LAYOUT IN KOTLIN val act = this val layout = LinearLayout(act) layout.orientation = LinearLayout.VERTICAL val name = EditText(act) val button = Button(act) button.text = "Say Hello" button.setOnClickListener { Toast.makeText(act, "Hello, ${name.text}!", Toast.LENGTH_SHORT).show() } layout.addView(name) layout.addView(button)

ANKO DSL ANKO DSL verticalLayout { val name = editText() button("Say Hello") { onClick { toast("Hello, ${name.text}!") } } }

ANKO DSL ANKO DSL INTERNALS ▸ Anko DSL example from previous slide looks very similar to the earlier HTML builder example ▸ Anko uses extension functions arranged into type-safe builders and lambdas ▸ You don’t have to write all those extensions by hand though - Anko generates them based on the Android SDK’s android.jar

ANKO DSL GETTING STARTED WITH ANKO DSL (I) ▸ Depending on minSdkVersion of project, import:  compile "org.jetbrains.anko:anko-sdk{15|19|21|23}:0.9" ▸ If the project uses an Android Support library, import matching Anko library:  compile "org.jetbrains.anko:anko-recyclerview-v7:0.9"

ANKO DSL GETTING STARTED WITH ANKO DSL (II) ▸ General approach: ▸ Anko DSL automatically becomes available in onCreate() in an Activity ▸ no need for setContentView(), Anko DSL also automatically sets the content view for activities override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) verticalLayout { padding = dip(30) editText { hint = "Name" textSize = 24f } editText { hint = "Password" textSize = 24f } button("Login") { textSize = 26f } } }

ANKO DSL ANKO DSL IN FRAGMENTS ▸ In fragments, use   onCreateView(…) ▸ Returns UI {…}.view: ▸ mandatory before Anko 0.8 ▸ works well for Fragments ▸ createTodo() is a fragment-private method override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? { return UI { verticalLayout { padding = dip(24) var title = editText { id = R.id.title hintResource = R.string.hint } ... button { id = R.id.add textResource = R.string.add onClick { view -> createTodo(title, desc) } } } }.view }

ANKO DSL ANKO COMPONENTS class EditFragmentUI : AnkoComponent<EditFragment> { override fun createView(ui: AnkoContext<EditFragment>) = with(ui) { verticalLayout { padding = dip(24) var title = editText { id = R.id.title hintResource = R.string.hint } ... button { id = R.id.add textResource = R.string.add onClick { view -> ui.owner.createTodoFrom(title, desc) } } } } } override fun onCreateView(...)= EditFragmentUI().createView(AnkoContext.create(ctx,this))

ANKO DSL EXTENDING ANKOS DSL FUNCTIONALITY ▸ Multiple ways to do this: ▸ insert an existing XML layout into an Anko DSL declaration:  ▸ adding new DSL elements to the language include<View>(R.layout.mylayout) { ... } inline fun ViewManager.customView(theme: Int = 0) = customView(theme) {} inline fun ViewManager.customView(theme: Int = 0, init: CustomView.() -> Unit) = ankoView({ CustomView(it) }, theme, init) inline fun Activity.customView(theme: Int = 0) = customView(theme) {} inline fun Activity.customView(theme: Int = 0, init: CustomView.() -> Unit) = ankoView({ CustomView(it) }, theme, init)

ANKO DSL THERE’S MORE ▸ Intent wrappers for various purposes: e.g. sendSMS(number, [text]) ▸ Intent builders ▸ Service shortcuts ▸ Configuration qualifiers: configuration(screenSize = ScreenSize.LARGE, orientation = Orientation.LANDSCAPE) { … } ▸ Asynchronous tasks ▸ SQLLite

LIBRARIES AND TOOLS https://www.ﬂickr.com/photos/sillygwailo/5990089210/

LIBRARIES AND TOOLS OVERVIEW ▸ Kotter Knife ▸ Butter Knife ▸ KAndroid ▸ Kovenant ▸ Fuel, Injekt, Spek, Kotson

LIBRARIES AND TOOLS KOTTER KNIFE (AND BUTTER KNIFE) ▸ Kotter Knife provides view binding in a similar way to Butter Knife for Android/ Java ▸ Why *Knife over KAE? ▸ Porting code from Java/Butter Knife to Kotlin ▸ Features like listener binding and resources binding that don’t exist in KAE. ▸ Sample code for Butter Knife/Kotlin in the ofﬁcial Jetbrains Kotlin Examples repo (https://goo.gl/rH7vBu)

LIBRARIES AND TOOLS KOTTER KNIFE VS BUTTER KNIFE VS KAE (I) findViewById(R.id.send).setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Log.d("MainActivity", "onClick: send") } }); @OnClick(R.id.send) void clickSend(View v) { Log.d("MainActivity", "onClick: send") }

LIBRARIES AND TOOLS KOTTER KNIFE VS BUTTER KNIFE VS KAE (II) findViewById(R.id.send).setOnClickListener { view -> Log.d("MainActivity", "onClick: send") }; val btnSend: Button by bindView(R.id.send) btnSend.setOnClickListener({ view -> Log.d("MainActivity", "onClick: send") }) import kotlinx.android.synthetic.activity_main.* btn_send.setOnClickListener({ view -> Log.d("MainActivity", "onClick: send") })

LIBRARIES AND TOOLS KANDROID ▸ View binding (again) ▸ TextWatcher ▸ SeekBar Extension ▸ SearchView Extension ▸ Shortcuts to system services ▸ Logging ▸ Dealing with Intents ▸ SDK Version API (from/to) ▸ Thread management

LIBRARIES AND TOOLS KOVENANT ▸ Core: foundations of a Promise framework ▸ Tasks & Callbacks ▸ Chaining (Then, ThenApply) ▸ Lazy Promises ▸ Cancelling and Voiding ▸ Combine: combines 2-20 promises ▸ Functional: adds map, bind and apply to support more advanced HOF constructs in Kovenant ▸ JVM: Executors and Throttles (thread pools) ▸ Android: UI callbacks and interacting with UI Thread

FINAL THOUGHTS https://www.ﬂickr.com/photos/brickset/16099265973/

FINAL THOUGHTS PERFORMANCE ▸ Runtime is pretty much on-par with Java ▸ Pre Kotlin 1.0.2: Build process was slower than a comparable app in Java - mainly due to how the Kotlin compiler works (no partial builds/compilation) ▸ Kotlin libraries do add to the size of the application as well as to the method count ▸ Kotlin runtime + stdlib are similar in method count to support-v4 or play- services-base and add signiﬁcantly less than Scala or Groovy

FINAL THOUGHTS LANGUAGE AND MATURITY ▸ Kotlin 1.0 was a big step for the language ▸ Surprisingly mature for a 1.0 release (but 5+ years in the making), full of great concepts and idioms, getting better with each release ▸ Anko is very stable given it’s still a 0.x release ▸ Anko makes UI declaration actually pleasurable but offers way more than only a layout DSL

FINAL THOUGHTS WHAT DID WE LEARN? ▸ What is Kotlin? ▸ Language idioms & concepts ▸ Kotlin and Android ▸ Anko DSL ▸ Libraries and Tools ▸ Final thoughts

RESOURCES RESOURCES ▸ Kotlin: http://kotlinlang.org ▸ Anko: https://github.com/Kotlin/anko ▸ Kotter Knife: https://github.com/JakeWharton/kotterknife ▸ KAndroid: https://github.com/pawegio/KAndroid ▸ Kovenant: https://github.com/mplatvoet/kovenant ▸ Fuel: https://github.com/kittinunf/Fuel ▸ Injekt: https://github.com/kohesive/injekt ▸ Spek: http://jetbrains.github.io/spek/ ▸ Kotson: https://github.com/SalomonBrys/Kotson

RESOURCES GET IN TOUCH Kai Koenig Email: kai@ventego-creative.co.nz Twitter: @AgentK

Coding for Android on steroids with Kotlin

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