Kotlin is a statically typed programming language that compiles to Java bytecode and runs on the Java Virtual Machine. It is designed to interoperate fully with Java, but aims to be less verbose and improve on some of Java's weaknesses. Anko is a Kotlin library that provides a domain-specific language for defining Android user interface layouts in a declarative manner, similar to XML layout files but using Kotlin code. The presentation introduces Kotlin language concepts like null safety, properties, type inference, and extension functions. It then demonstrates how to build a simple Android app using Kotlin and Anko to define UI layouts without requiring XML.
2. HELLO
HELLO
I’m Kai!
I’m a { Software Engineer | Pilot | Traveller |
Gamer | Husband | Dad to 1 cat and 3 chickens |
… }
Find me on Twitter: @AgentK
9. 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
10. 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
12. 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
13. 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
14. 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 ${s.length}"
Output:
abc.length is 3
15. LANGUAGE IDIOMS & CONCEPTS
STRING TEMPLATES & ENUM CLASSES (II)
▸ Kotlin has a dedicated and expressive
enum class
▸ Can be used very nicely in conjunction
with String templates
enum class Locale(val hello: String) {
DE_DE("Hallo"),
EN_NZ("Hello"),
MI_NZ("Kia Ora")
}
class Customer(val firstName:String,
val lastName:String,
val locale: Locale = Locale.DE_DE) {
fun sayHello() = println(
"${locale.hello}, $firstName $lastName")
}
fun main(args : Array<String>) {
val myCustomer = Customer("Sandra",
"Musterfrau",
Locale.MI_NZ)
myCustomer.sayHello()
}
16. 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)
17. 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 = ...
}
18. 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
19. 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
20. 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 = "...")
21. 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")
...
}
22. 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)
}
23. 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())
24. 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
}
25. 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()
}
}
26. 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
27. 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
39. 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.
40. 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
▸ 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
42. 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)
44. 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
45. 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"
46. 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
}
}
}
47. 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
}
48. ANKO DSL
ANKO COMPONENTS (I)
▸ Often it’s nicer to have UI code separated from onCreate or other livecycle
methods
▸ AnkoComponent helps organising your UI code in their own classes
49. ANKO DSL
ANKO COMPONENTS (II)
override fun onCreateView(...)= EditFragmentUI().createView(AnkoContext.create(ctx,this))
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) }
}
}
}
}
50. ANKO DSL
ANKO COMPONENTS (III)
▸ Anko Preview plugin for
AnkoComponent layouts
▸ Get it here: http://goo.gl/5CVmAA
▸ Can preview layouts and convert XML
to Anko DSL
▸ Can be fiddly to get going with AS,
Kotlin plugin versions
51. ANKO DSL
EXTENDING ANKOS DSL FUNCTIONALITY (I)
▸ 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)
52. ANKO DSL
EXTENDING ANKOS DSL FUNCTIONALITY (II)
▸ Extending Anko’s DSL to support your own custom view groups is currently rather
painful
▸ Essentially it comes down to .lparams (for layout purposes) not being easily accessible
▸ More here:
▸ http://goo.gl/qpb3SL
▸ http://goo.gl/tMHs2c
▸ Fix targeted for Anko 0.9.1
54. THE HIDDEN PARTS OF ANKO
BUT 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
55. THE HIDDEN PARTS OF ANKO
INTENT WRAPPERS AND SERVICE SHORTCUTS
▸ Anko provides wrappers for commonly used Intents:
▸ makeCall(number), sendSMS(number, text), browse(url) etc.
▸ Shortcuts to Android services:
▸ notificationManager, displayManager, sensorManager etc.
▸ Useful, but both not major features of Anko
56. THE HIDDEN PARTS OF ANKO
INTENT BUILDERS
▸ Syntactic sugar to cut down the amount of code needed to start a new activity
val intent = Intent(this, javaClass<MyActivity>())
intent.putExtra("customerId", 4711)
intent.putExtra("country", "NZ")
startActivity(intent)
startActivity(Intent(this, MyActivity::class.java).
putExtra("customerId", 4711).putExtra("country", "NZ"))
startActivity<MyActivity>("customerId" to 4711, "country" to "NZ")
57. THE HIDDEN PARTS OF ANKO
CONFIGURATION QUALIFIERS
▸ “CSS Media Queries for Android”
▸ Screen features:
▸ screenSize, density, orientation, long,
nightMode, rightToLeft, smallestWidth
▸ SDK versions:
▸ fromSdk, sdk
▸ Other:
▸ language, uiMode
configuration(screenSize = ScreenSize.LARGE,
orientation =
Orientation.LANDSCAPE)
{
/*
This code will be only executed
if the screen is large and its
orientation is landscape
*/
}
58. THE HIDDEN PARTS OF ANKO
ASYNC DSL
▸ Does anyone enjoy working with AsyncTasks?
▸ Anko: async and asyncResult (return future respectively future with result)
async() {
val result = PetFindCommand("90210","cat").execute()
uiThread {
animalList.adapter = AnimalListAdapter(result,
object: AnimalListAdapter.ItemClickListener{
override fun invoke(pet:Pet) {
toast(pet.name)
}
}
)
}
}
59. THE HIDDEN PARTS OF ANKO
SQLITE (I)
▸ Working with SQLite usually requires a lot of boilerplate code and try/catch-
structures, issues around concurrency, dealing with cached code and
references etc.
▸ Anko aims to make that easier
▸ Dependency: compile ‘org.jetbrains.anko:anko-sqlite:0.9'
▸ Setup class extending ManagedSQLiteOpenHelper in your code
60. THE HIDDEN PARTS OF ANKO
SQLITE (II)
▸ database.use ensures DB is opened
and closed correctly
▸ Asynchronous use of the DB is possible
through Anko’s async DSL
▸ Important: There can still be
SQLiteExceptions when handling SQL
code - Anko doesn’t abstract those.
▸ Handling of cursors and parsing data
via lambdas
database.use {
createTable("Customer", ifNotExists = true,
"id" to INTEGER + PRIMARY_KEY + UNIQUE,
"name" to TEXT,
"photo" to BLOB)
}
61. APP
STEP 10 - CLICK ON LIST
▸ Adapter/RecyclerView click handler changes
▸ when-statement
62. APP
STEP 11 - EXCEPTIONS
▸ Exception handler in Anko
63. APP
STEP 12 - ANKO DSL
▸ Anko layout for main view
▸ Removing layout XML and replacing it with Anko DSL
64. APP
STEP 13 - ANKO COMPONENTS
▸ Refactoring into Anko component
65. APP
STEP 14
▸ Mini refactoring for better testing
▸ jUnit tests in Kotlin
67. APP
IDEAS
▸ Add second Activity or Fragment to display details
▸ Use Kotson instead of Gson
▸ Change RecyclerView to Anko Layout DSL
▸ Use Butterknife instead of Kotterknife
▸ Write some proper tests
▸ Try Android integration tests