Kotlin for Android Developers - Victor Kropp - Codemotion Rome 2018

Kotlin for Android developers
Victor Kropp, JetBrains
@kropp
ROME - APRIL 13/14 2018

Kotlin on
JVM + Android
JS
In development: Kotlin/Native
iOS/macOS/Windows/Linux

Links
Kotlin
https://kotlinlang.org
Kotlin Koans
https://try.kotl.in
Kotlin in Action book
by Dmitry Jemerov & Svetlana Isakova

Sample Java App
package kropp.name.myapp;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}

Converting to Kotlin
package kropp.name.myapp;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
@Override
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}

package kropp.name.myapp
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
@Override
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
}
}

class MainActivity extends AppCompatActivity {
@Override
}
}

class MainActivity : AppCompatActivity {
@Override
}
}

override fun onCreate(Bundle savedInstanceState) {
}
}

override fun onCreate(savedInstanceState: Bundle) {
}
}

class MainActivity : AppCompatActivity() {
}
}

Sample Kotlin App
class MainActivity : AppCompatActivity() {
}
}

Data classes
data class Person(
var firstName: String,
var lastName: String,
var age: Int
)

Java equivalent
package kotlindemo;
import java.util.Objects;
public class Person {
private String firstName;
private String lastName;
private int age;
public Person(String firstName, String lastName, int age) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person person = (Person) o;
return age == person.age &&
Objects.equals(firstName, person.firstName) &&
Objects.equals(lastName, person.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName, age);
}
}

Properties
class Person {
var firstName: String = ""
}

Properties
return firstName;
}
}
public Person() { this.firstName = ""; }
}

Make val not var
var mutable: String
val immutable: String

Properties
class Person(
var firstName: String
)

Properties
return firstName;
}
}
public Person(String firstName) {
}
}

Null safety
val canBeNull: String?
val notNull: String

Null safety
fun nullability(str: String?) {
val dot = str.indexOf(".")
}

Null safety
} Only safe (?.) or non-null asserted (!!.) calls are allowed
on a nullable receiver of type String?

Null safety
val dot = str!!.indexOf(".")
} Non-null asserted call
May throw NullPointerException
Usually a bad style,
use only when you know what you are doing

Null safety
val dot = str?.indexOf(".")
} Safe call
The result will be null if str is null

Null safety
val dot = str?.indexOf(".") ?: 0
} Elvis operator
The result will be 0 if str?.indexOf() returns null

Null safety
val dot = str?.indexOf(".") ?: throw Exception()
}

Type casts
fun cast(obj: Any) {
if (obj is String) {
val dot = obj.indexOf(".")
}
} Smart cast
obj is String inside ‘then’ branch

Type casts
val str = obj as String
}

Type casts
val str = obj as? String
}
Safe cast
str is null if obj is not a String

Extension functions
fun Int.days(): Period = …
fun Period.ago(): Date = …
3.days().ago()
2.months().later()

Extension properties
val Int.days: Period
get() = …
val Period.ago: Date
get() = …
3.days.ago
2.months.later

Lambda expressions
val list = listOf<Int>()
list.filter({ it > 0 })

Lambda expressions
list.filter { it > 0 }

Lambda expressions
list.filter { it > 0 }.map { it*2 }

inline functions
inline fun <T> Iterable<T>.filter(predicate: (T) -> Boolean):
List<T> {
val result = mutableListOf<T>()
for (it in this) {
if (predicate(it)) {
result.add(it)
}
}
return result
}

Anko is a Kotlin library which makes Android application
development faster and easier.
https://github.com/Kotlin/anko/
Anko

Anko
Anko Commons: a lightweight library with helpers for:
Intents, Dialogs and toasts, Logging, Resources and
dimensions
Anko Layouts: a fast and type-safe way to write dynamic Android
layouts
Anko SQLite: a query DSL and parser collection for Android
SQLite
Anko Coroutines: utilities based on the kotlinx.coroutines library

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

A set of Kotlin extensions for Android app development
https://github.com/android/android-ktx
Android KTX

Android KTX
sharedPreferences.edit()
.putBoolean("key", value)
.apply()

Android KTX
sharedPreferences.edit {
putBoolean("key", value)
}

Android KTX
val spannedString = buildSpannedString {
bold { "Hello" }
italic { "KTX!" }
}

Kotlin Android Extensions
apply plugin: 'kotlin-android-extensions'

View binding
findViewById<TextView>(R.id.label)

View binding
// Using R.layout.activity_main from the 'main' source set
import kotlinx.android.synthetic.main.activity_main.*

View binding
label.text = "Hello!"

Parcelable
import android.os.Parcelable
import kotlinx.android.parcel.Parcelize
@Parcelize
data class Person(
var firstName: String,
var lastName: String,
var age: Int
) : Parcelable

Coroutines (Kotlin 1.1)
Asynchronous programming made easy
Write asynchronous code in synchronous style

Coroutines (Kotlin 1.1)
Asynchronous programming made easy
Write asynchronous code in synchronous style
val team = api.team().await()
val lead = api.profile(team.lead.id).await()

Multi-platform projects
Share code between different platforms
JVM + Android
JS
In development:
iOS/macOS/Windows/Linux Kotlin/Native

Thank you!
Victor Kropp
@kropp
victor.kropp.name

Questions?
Victor Kropp
@kropp
victor.kropp.name

Kotlin for Android Developers - Victor Kropp - Codemotion Rome 2018

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