Kotlin, a Year on

Mike Gouline

gouline.net | @mikestruct

What has changed?

• New features
• Improved interoperability
• Better documentation
• Example code
• Complete tutorials
• Community exposure
• Books published
• Libraries written
• Developer articles

Annotation processing (kapt)

• Dagger 2 works!
• Effortless
• Latest Kotlin plugin for Gradle
• Usual Dagger 2 dependencies
• Fine print
• Dagger builder must be used in Java
• Related
• Late-init properties

Anko

• Programmatic layouts
• Why would you?
• DSL instead of XML
• Compact and readable
• IDE integration with previews
• Why wouldn’t you?
• Nothing wrong with XML layouts
• No support for screen-based variants

Anko: Example

verticalLayout {

padding = dip(16)

textView("Username:") {

textSize = 18f

}.layoutParams { verticalMargin = dip(4) }

​

val login = editText()

​

button("Sign up") {

textSize = 20f

onClick { login(login.text) }

}.layoutParams { topMargin = dip(8) }

}

​

Anko: Example

Android extensions

• Butter Knife not supported; who needs it?
• No need for (explicit) instance variables
• How?
• Import synthetic layout
• import kotlinx.android.synthetic.<layout>.*
• Just use views by ID
• e.g. this.txt_status.setText(“OK”);
• Synthetic calls are replaced by functional calls

Android extensions: Example

<TextView

android:id="@+id/hello"

android:layout_width="fill_parent"

android:layout_height="wrap_content"/>

​

import kotlinx.android.synthetic.main.*

public class MyActivity : Activity() {

override fun onCreate(savedInstanceState: Bundle?) {

super.onCreate(savedInstanceState)

setContentView(R.layout.main)

this.hello.setText("Hi!")

}

}

​

Nullability

• Remember nullable types? e.g. String?
• Annoyance
• Objects not created in your Kotlin class must be nullable, even if guaranteed to never be null
• Solutions
• @NonNull and @Nullable annotations
• Now honoured in Java interop
• Late-init properties
• Injected values no longer have to be nullable
• e.g. @Inject lateinit val foo: Foo

Sealed classes

sealed class Pet(val name: String) {

class Dog(name: String): Pet(name)

class Cat(name: String): Pet(name)

}

​

fun Pet.saySomething(): String {

return when (this) {

is Dog -> "woof"

is Cat -> "meow"

}

}

​

• Algebraic Data Types (ADTs)

Miscellaneous

• JUnit support for Android
• Secondary constructors
• In addition to the class signature
• Annotation syntax
• [Inject] became @Inject
• Companion objects
• Replacing “class objects” for static members
• Lazy evaluations
• Delayed evaluation of variable definition
• Filling properties from a map
• Creating properties (getter/setter) from a map

Thank you!

Resources: https://gouline.net/talks

Documentation: http://kotlinlang.org/docs/reference/

gouline.net | @mikestruct