Little Helpers for Android Development with Kotlin

LITTLE HELPERS FOR ANDROID
DEVELOPMENT WITH KOTLIN
KAI KOENIG (@AGENTK)

AGENDA
▸ What is Kotlin?
▸ Common idioms and language concepts
▸ Kotlin and Android
▸ Anko
▸ Other libraries and tools for Kotlin and Android
▸ Final thoughts

WHAT IS KOTLIN?
SOME FUNDAMENTALS
▸ Statically typed programming language for the
JVM and Android as well as the browser
▸ Started as internal language “Project Kotlin” at
Jetbrains in 2010
▸ Now: Open-Source, Apache License - Kotlin 1.0
released in Feb 2016
▸ 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, in particular when one has to use Java 6
or 7 (if we’re lucky…) and can’t use all the new, shiny features from Java 8 and
soon Java 9 and 10.

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 = "}")
}

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

COMMON IDIOMS AND LANGUAGE 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

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

STRING TEMPLATES & ENUM CLASSES
▸ Kotlin Strings can contain template expressions
▸ Start with a $ character and
▸ can contain simple references:  
$s
▸ complex expressions in curly braces:  
${s.length}
▸ Kotlin has a dedicated enum class, very similar
to Java
val s = "abc"
val str = "$s.length is ${s.length}"
enum class Locale(val hello: String) {
DE_DE("Hallo"), EN_NZ("Hello"), MI_NZ("Kia Ora")
}
class Customer(val firstName:String,
val lastName:String,
val locale: Locale = Locale.DE_DE) {
fun sayHello() = println("${locale.hello},
$firstName $lastName")
}
fun main(args : Array<String>) {
val myCustomer = Customer("Sandra",
"Musterfrau",
Locale.MI_NZ)
myCustomer.sayHello()
}

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 or use Safe Call operator ?. or
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)

PROPERTIES AND FIELDS
▸ Kotlin classes have mutable or immutable
properties
▸ An automated backing ﬁeld can be provided
by the compiler (if deemed necessary)
▸ Default getter/setters for properties, can be
customised
▸ lateinit modiﬁer to deal with non-nullable
properties that can’t be initialised in the
constructor
var counter = 0
set(value) {
if (value >= 0)
field = value
}
public class MyTest {
lateinit var subject: TestSubject
@SetUp fun setup() {
subject = TestSubject()
}
@Test fun test() {
subject.method()
}
}

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 = 1L + 3

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

DATA CLASSES
▸ The POJOs or Beans of other languages…
▸ Data classes implicitly create:
▸ getters/setters (the latter if a property is var)
▸ equals(), hashCode(), toString(), copy() - can
be overwritten by custom implementations
▸ copy() has default parameters and can be
used to alter a copy
▸ parameterless constructors need default
parameters speciﬁed
data class ChromeEncryptedPayload(
val encryptedPayload: String,
val encryptionHeader: String,
val cryptoKeyHeader: String)

EXTENSION FUNCTIONS
▸ Allow adding new functionality to a class
without inheritance or Decorators
▸ Kotlin allows extension functions and extension
properties
▸ Resolved statically, do not actually modify
the class (excellent example why this has to
be the case on https://kotlinlang.org/docs/
reference/extensions.html)
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")
...
}

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://github.com/blob0815/kotlin-android-sample

KOTLIN & 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 1.5.x/2.x or IntelliJ 15/2016
▸ Install Kotlin plugin (comes with “Jetbrains plugins” nowadays)
▸ Gradle dependencies project-level:
▸ classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.0.2"
▸ Gradle dependencies module-level:
▸ compile 'org.jetbrains.kotlin:kotlin-stdlib:1.0.2'
▸ apply plugin: 'kotlin-android'
▸ main.java.srcDirs += 'src/main/kotlin'

KOTLIN AND ANDROID
KOTLIN EXTENSIONS FOR ANDROID (I)
▸ 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>.view.*
▸ usage: <componentid>.doSomething()
▸ Integrates nicely with build ﬂavors, too

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"
}
}

KOTLIN AND ANDROID
KOTLIN EXTENSIONS FOR ANDROID (II)
▸ Used to be part of a separate plugin (https://plugins.jetbrains.com/plugin?
pluginId=7717)
▸ obsolete since December 2015
▸ functionality rolled into the main Kotlin plugin
▸ still needs a very particular setup (not properly documented but in this place:
https://youtrack.jetbrains.com/issue/KT-10336) 
 
TLDR: move dependencies in app module(s), not top-level Gradle ﬁle.

KOTLIN AND ANDROID
KOTLIN EXTENSIONS FOR ANDROID (III)
▸ The current assumption is that Jetbrains plan to add more to the Kotlin
Extensions for Android, but we don’t really know for sure at this stage.
▸ Current feature of view binding:
▸ Fresh take on view binding
▸ Very simple to setup and use
▸ Overall less powerful than Butter Knife, Kotter Knife

ANKO
A DSL FOR LAYOUTS
▸ Developed and maintained by Jetbrains, under Apache 2.0 license
▸ 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 truly programmatic sense
▸ Modular - as we’re talking about UI/Layout, it’s very important to select the
right library for your minSDKVersion
▸ Extensible - you can add your own DSL elements for custom UI controls

ANKO
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:gravity="center"
android:text="@string/empty_todos_message"
android:layout_weight="7"
android:layout_height="wrap_content" />
<Button
android:layout_weight="1"
android:text="Say Hello"
android:layout_height="0dp" />
</LinearLayout>

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

ANKO
BUT THERE’S MORE (ANKO “ADVANCED TOPICS”)
▸ Intent Wrappers for various purposes: e.g. sendSMS(number, [text])
▸ Service shortcuts
▸ Configuration qualifiers: configuration(screenSize = ScreenSize.LARGE,
orientation = Orientation.LANDSCAPE) { … }
▸ Asynchronous tasks
▸ SQLLite
▸ Removes all the tragic cursor handling and lot of the try/catch blocks necessary

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

OTHER LIBRARIES AND TOOLS FOR KOTLIN AND ANDROID
OVERVIEW
▸ Kotter Knife
▸ Butter Knife
▸ KAndroid
▸ Kovenant
▸ Quickies: Fuel, Injekt, Spek, Kotson

KOTTER KNIFE
▸ We don’t like ﬁndByViewId(…) - read: https://ragunathjawahar.wordpress.com/
2015/03/23/kotlin-ﬁndviewbyid-dead-as-dinosaurs
▸ Kotter Knife provides view binding in a similar way to Butter Knife for Android/
Java
▸ Why Kotter Knife (runtime library) over Kotlin Android Extensions (compiler
plugin)?
▸ Porting code from Java/Butter Knife to Kotlin
▸ Features like listener binding and resources binding that don’t exist in KAE.

BUTTER KNIFE
▸ Not much to say about it, probably one of the most famous libraries in the
Android world.
▸ Butter Knife was difﬁcult to use with Kotlin in the beginning
▸ Since Kotlin 1.0 RC you can in fact just use Butter Knife in your Kotlin code
▸ Sample code for Butter Knife/Kotlin in the ofﬁcial Jetbrains Kotlin Examples
repo (https://github.com/JetBrains/kotlin-examples/tree/master/gradle/
android-butterknife)

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")
}

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") })

KANDROID
▸ KAndroid is an extension library for Kotlin/Android
▸ Contains a variety of absolutely distinct and unconnected functionality
▸ Common theme: let’s get rid of boilerplate code
▸ Apache 2.0 license

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

KANDROID EXAMPLES
runDelayed(1000) {
// delayed execution
}
runDelayedOnUiThread(5000) {
// delayed UI update
}
toApi(16, inclusive = true) {
// handle devices running older APIs
}

KOVENANT
▸ In a nutshell: Promises for Kotlin
▸ Very modular built, you can essentially pick and choose the artifacts of
Kovenant that you’d like to use - Kovenant is not an Android-speciﬁc library
▸ Good staring set for Android: core, android, combine, jvm, functional
▸ MIT license

KOVENANT FEATURES
▸ 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

KOVENANT EXAMPLES (I)
task {
// some long-running thing
} success {
println("result: $it")
} fail {
println("problem: ${it.message}")
} always {
// this will always happen
}

KOVENANT EXAMPLES (II)
promiseOnUi {
// do some UI preparation
} then {
// the actual work
} successUi {
// update UI
}

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 is 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
▸ Refreshing language that makes both Android and JVM development signiﬁcantly
more pleasant and fun
▸ Issue: Tooling around static analysis is non-existent at the moment (some basic listing
for Android is available since 1.0.2)
▸ Ready for prime-time yet?

FINAL THOUGHTS
WHAT DID WE LEARN?
▸ What is Kotlin?
▸ Common idioms and language concepts
▸ Kotlin and Android
▸ Anko
▸ Other libraries and tools for Kotlin and Android

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

Little Helpers for Android Development with Kotlin

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