Android & Kotlin - The code awakens #02

Android & Kotlin
The code awakens #02

Omar Miatello
Member of GDG Milano (Italy)
Android Developer @ Satispay
Personal profile
google.com/+OmarMiatello
Google+ Community: Kotlin for Android
goo.gl/mUKF1w
Google Presentation
#01 goo.gl/0jHLmE
#02 goo.gl/h3uG8M
#03 goo.gl/hnwvqu
Google Photo
#01 goo.gl/photos/cKP9L6zqZcxDRGzQ8
#02 goo.gl/photos/sXdpkbihCi5xAAnx7
#03 goo.gl/photos/P6kGhLE8yrWYnhAW6

Previously on Android & Kotlin #01: goo.gl/0jHLmE
Properties
val a: Int = 1 // val = READ ONLY (getter)
var b: Int = 1 // var = READ/WRITE (getter/setter)
String templates
"My name is $name $surname"
Lambdas
view.setOnClickListener { Log.d("TAG", "Item clicked!") }
Delegated properties (example: lazy)
val item by lazy { MyItem() }

dummy/HeroAdapter.java
public class HeroAdapter extends RecyclerView.Adapter<HeroAdapter.ViewHolder> {
private final List<HeroItem> mValues;
private final HeroOnClickListener mListener;
public HeroAdapter(List<HeroItem> items, HeroOnClickListener listener) {
mValues = items;
mListener = listener;
}
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_hero, parent, false);
return new ViewHolder(view);
}
@Override
public void onBindViewHolder(final ViewHolder holder, int position) {
HeroItem item = mValues.get(position);
“Convert Java File to Kotlin File”
CTRL + ALT + SHIFT + K
(or CMD + ALT + SHIFT + K)

Kotlin vs Java - Part 2
Null-safety
Elvis Operator
Smart-cast
Collections

class MyKotlinClass {
val a: String = "ciao"
val b: String = null // Error at compile time
}
#5 Kotlin - Null Safety
http://kotlinlang.org/docs/reference/null-safety.html
vs

val c: String? = null
val d: String? = "ok"
}
vs

fun example(e: String, f: String?) {
e.length()
f.length() // Error at compile time
}
}
static class MyUtils {
void example(String e, String f) {
e.length(); // throw NullPointerException?
f.length(); // throw NullPointerException?
}
}
vs

e.length()
f?.length()
}
}
}
}
vs

e.length()
f?.length()
if (f != null) {
f.length()
}
}
}
if (e != null) {
e.length();
}
if (f != null) {
e.length();
}
}
}
vs

val map = mapOf(
"dog" to "woof",
"cat" to "meow",
"bird" to "tweet",
"mouse" to "squeek")
fun sound(animal: String): String? {
return map.get(animal)
}
class MyUtils {
static Map<String, String> map = // ...
static String sound(String animal) {
return map.get(animal);
}
}
#6 Kotlin - Elvis Operator
http://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand
vs

val map = mapOf(
"dog" to "woof",
"cat" to "meow",
"bird" to "tweet",
"mouse" to "squeek")
fun sound(animal: String): String? {
return map.get(animal)
}
fun example() {
val foxSay = sound("fox") ?: "No one knows"
}
class MyUtils {
static Map<String, String> map = // ...
static String sound(String animal) {
return map.get(animal);
}
static void example() {
String s = sound("fox");
String foxSay =
s != null ? s : "No one knows";
}
}
#6 Kotlin - Elvis Operator
http://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand
vs

class MyUtils {
static void example(View myView) {
if (myView instanceof ImageView) {
}
}
}
#7 Kotlin - Smart Cast
http://kotlinlang.org/docs/reference/typecasts.html#smart-casts
vs

class MyUtils {
ImageView imageView =
(ImageView) myView;
}
}
}
vs

class MyUtils {
(ImageView) myView;
imageView.setImageAlpha(10);
}
}
}
vs

class MyUtils {
(ImageView) myView;
} else if (myView instanceof TextView) {
TextView textView = (TextView) myView;
textView.setText("Ciao");
}
}
}
vs

fun example1(myView: View) {
if (myView is ImageView) {
myView.setImageAlpha(10)
} else if (myView is TextView) {
myView.setText("Ciao")
}
}
class MyUtils {
(ImageView) myView;
}
}
}
vs

if (myView is ImageView) {
myView.setImageAlpha(10)
} else if (myView is TextView) {
myView.setText("Ciao")
}
}
when (myView) {
is ImageView -> myView.imageAlpha = 10
is TextView -> myView.text = "Ciao"
}
}
class MyUtils {
(ImageView) myView;
}
}
}
vs

fun example() {
val os = listOf("Android", "iOS", null,
"Windows Phone")
}
class MyUtils {
List<String> os = Arrays.asList("Android",
"iOS", null, "Windows Phone");
}
}
#8 Kotlin - Using collections
http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections
vs

fun example() {
"Windows Phone")
os.filterNotNull()
}
class MyUtils {
List<String> osNotNull = new ArrayList<>();
for (String name : os) {
if (name != null) osNotNull.add(name);
}
}
}
vs

fun example() {
"Windows Phone")
os.filterNotNull().sortedBy { it.length() }
}
class MyUtils {
}
Collections.sort(osNotNull, new
Comparator<String>() { @Override
public int compare(String l, String r) {
return r.length() - l.length();
}
});
}
}
vs

fun example() {
"Windows Phone")
.map { it.toUpperCase() }
}
class MyUtils {
}
return l.length() - r.length();
}
});
for (String name : osNotNull) {
String value = name.toUpperCase();
}
}
}
vs

fun example() {
"Windows Phone")
.map { it.toUpperCase() }
.forEach { print(it) }
}
class MyUtils {
}
return l.length() - r.length();
}
});
for (String name : osNotNull) {
String value = name.toUpperCase();
print(value);
}
} }
vs

Android Example
From Java to Kotlin!
https://github.com/jacklt/KotlinExample/tree/ex1

dummy/Items.kt
data class HeroItem(val name: String, val gender: String, val power: Int)

dummy/HeroDummyContent.kt
object HeroDummyContent {
private val MALE = "Male"
private val FEMALE = "Female"
val ITEMS = listOf(
HeroItem("Hulk", MALE, 9),
HeroItem("Iron Man", MALE, 8),
HeroItem("Thor", MALE, 8),
HeroItem("Jessica Jones", FEMALE, 2),
HeroItem("Spiderman", MALE, 4)
)
}

dummy/HeroDummyContent.kt > object vs class
val ITEMS = listOf(
)
}

dummy/HeroDummyContent.kt > listOf(...)
val ITEMS = listOf(
)
}

dummy/HeroAdapter.kt
class HeroAdapter(val mValues: List<HeroItem>, val mListener: Function1<HeroItem, Unit>?) :
RecyclerView.Adapter<HeroAdapter.ViewHolder>() {
// ...
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
val hero = mValues[position]
holder.item = hero
holder.nameView.text = "${hero.name} (Power: ${hero.power})"
holder.genderView.text = hero.gender
holder.itemView.setOnClickListener {
mListener?.invoke(hero)
}
}
// ...
}

dummy/HeroAdapter.kt > String template
class HeroAdapter(val mValues: List<HeroItem>, val mListener: Function1<HeroItem, Unit>?) :
RecyclerView.Adapter<HeroAdapter.ViewHolder>() {
// ...
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
val hero = mValues[position]
holder.item = hero
holder.nameView.text = "${hero.name} (Power: ${hero.power})"
holder.genderView.text = hero.gender
holder.itemView.setOnClickListener {
mListener?.invoke(hero)
}
}
// ...
}

MainActivity.kt
class MainActivity : AppCompatActivity() {
private var currentHero: HeroItem? = null
private val adapter by lazy {
HeroAdapter(HeroDummyContent.ITEMS) {
// onItemClick ...
}
}
// ...
}

MainActivity.kt > adapter (lazy)
HeroAdapter(HeroDummyContent.ITEMS) { // onClick
val previousHero = currentHero
if (previousHero != null) { // fight
val conclusion = if (previousHero.power == it.power) {
"It's a draw"
} else if (previousHero.power > it.power) {
"${previousHero.name} wins!"
} else {
"${it.name} wins!"
}
Snackbar.make(fab, "${previousHero.name} vs ${it.name}!n$conclusion", Snackbar.LENGTH_LONG).show()
currentHero = null
} else { // assign current hero
Snackbar.make(fab, "Choose: ${it.name}", Snackbar.LENGTH_SHORT).show()
currentHero = it
}
}

MainActivity.kt > adapter (lazy) > Smart cast
"It's a draw"
} else {
"${it.name} wins!"
}
currentHero = null
currentHero = it
}
}

MainActivity.kt > adapter (lazy) > return if expression
"It's a draw"
} else {
"${it.name} wins!"
}
currentHero = null
currentHero = it
}
}

Questions?
Developers playground - #EX2
- Aggiungere le proprietà a HeroItem: hair (String?), eyes (String?)
- Mostrarle (se presenti) nella lista
- Se vengono scelti 2 eroi di sesso opposto, viene creato un nuovo eroe
- Si possono usare le proprietà dei genitori, oppure dei valori di default se
mancano
Start with: https://github.com/jacklt/KotlinExample/tree/ex1
Solution: https://github.com/jacklt/KotlinExample/tree/ex2

THANKS!
Omar Miatello, Member of GDG Milano (Italy)
Android Developer @ Satispay

Android & Kotlin - The code awakens #02

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