Practical tips for building apps with kotlin

Practical tips for building apps with kotlin Adit Lal | @aditlal

Agenda What is Kotlin? Perfect for Android Migration guide Best Practices Summary - QnA Building with kotlin

100 % interoperable with Java Developed by JetBrains Background

What is Kotlin? Statically typed programming language for the JVM, Android and the browser

OOP language with functional aspects Modern and powerful language Safe concise & readable code First class tool support What is Kotlin?

Kotlin • Null safety • Conciseness • Lambdas • Higher order functions • Mutability protection • Static typing + Smart Casts We will cover :

Kotlin Kotlin works on same byte code

Kotlin - Syntax • Types follow variable/function names • Functions start with fun keyword • Default constructor in class signature • Semicolons not required

Kotlin - Syntax private fun sum(first: Int , second: Int): Int { return first + second } Access modiﬁer Keyword Function name Param name Param type Return type

Kotlin - Syntax // Omit access modifier fun sum(a: Int , b: Int): Int { return a + b } // Inline return fun sum(a: Int , b: Int): Int = a + b // Omit return type fun sum(a: Int , b: Int) = a + b

Kotlin - Syntax val name: String ="John" // ﬁnal var name: String ="John" //Mutable ex: name= "Johnny" Two types of variables val name ="John" // Types are auto-inferred

Kotlin - Syntax //java String sName = person.getSecretName() //kotlin person.secretName = “Batman” var fName = person.secretName

Kotlin - Null Safety NULLS ARE PART OF THE TYPE SYSTEM

Kotlin - Null Safety Types default to non-nullable Mark nullable with the addition of a ? var a: String = "abc" a = null var b: String? = "abc" b = null var l = a.length var l = b.length // compilation error // OK // error: variable 'b' can be null

Kotlin - Null Safety Helpers Safe accessor ? Elvis operator ?:

val toDisplay: String = serverResponse Kotlin - Null // if response is null, we'll display a default message val toDisplay: String = response ?: defaultMessage

Kotlin - Null // cascading null safety val maybeName: String? = maybeNullValue?.someProperty?.name val maybeNullValue: MyValue? = /*..*/

Kotlin - Null // safe operation maybeNullValue?.let { //executed if not null } val maybeNullValue: MyValue? = /*..*/

Casting if (obj instanceOf MyType) { value = ((MyType)obj).getXValue(); } else { // handle else }

Kotlin - Smart casting if (obj is MyType) { value = obj.x } else { // handle else }

Class // Wont Compileclass Student : Person() class Person

open class Person Class //It works nowclass Student : Person()

Higher order functions Functions that can take functions as arguments and also return function as the output

fun <T> forEach(list: List<T>, funcToCall: (T) -> Unit) { for (elm in list) { funcToCall(elm) } } val list = listOf("Alice", "Bob", "Claire") forEach(list, ::println) forEach(list, { str -> println(str) }) Higher order functions

Kotlin - Features … and More • Visibility Modiﬁers • Companion Objects • Nested, Sealed Classes • Generics • Coroutines • Operator overloading • Exceptions • Annotations • Reﬂection • and more

Kotlin - Features … and Even More • Inﬁx extension methods • Interfaces • Interface Delegation • Property Delegation • Destructuring • Safe Singletons • Init blocks • Enums • Multiline Strings • Tail recursion

Perfect for • Versatile language • Complete interop with Java • Compact runtime • Enhance development speed with less code

“But I’m comfortable and experienced with Java.” Common Concerns

“I don’t want to convert my entire codebase to a new language.” Common Concerns

•Compiled to byte code (like Java) •No impact on performance •Some Kotlin code faster •Lambdas that can be inlined •Built-in operations faster than DIY implementations Kotlin

How to migrate Start in small steps - Convert a Java / Android app (side project) Try the kotlin plugin converter Understand how code is converted from Java to Kotlin Experiment with kotlin features and tweak code

•Add Gradle dependencies (plugin, runtime, etc.) •Start writing .kt ﬁles instead of .java ones •No need to migrate everything at once •Kotlin classes can co-exist with Java ones • IntelliJ has a Java-to-Kotlin converter •Not perfect but good start • Works with pasted code Try kotlin

class HomeActivity : AppCompatActivity() { override fun onCreate(b: Bundle?) { super.onCreate(b) setContentView(R.layout.activity_main) val view = findViewById<View>(R.id.view) view.alpha = 1f } } Android & Kotlin

• View binding (like Butter Knife) • No instance variables required • How? Android extensions apply plugin: 'kotlin-android-extensions' • Import synthetic layout import kotlinx.android.synthetic.main..* • Use view by ID E.g. toolbar.title = "Home" • Under the hood: synthetic calls replaced by functions

Android extensions class HomeActivity : AppCompatActivity() { override fun onCreate(b: Bundle?) { super.onCreate(b) setContentView(R.layout.activity_main) val view = findViewById<View>(R.id.view) view.alpha = 1f } }

Android extensions import kotlinx.android.synthetic.activity_main.* class HomeActivity : AppCompatActivity() { override fun onCreate(b: Bundle?) { super.onCreate(b) setContentView(R.layout.activity_main) view.alpha = 1f } }

view.setOnClickListener{ toast(“Hello") } Concise view.setOnClickListener(object : View.OnClickListener { override fun onClick(v: View?) { toast("Hello") } }) view.setOnClickListener{ view -> doSomethingWithView() }

Concise fun Context.inflate( res:Int, parent:ViewGroup? = null) : View { return LayoutInflater.from(this) .inflate(res, parent, false) } activity.inﬂate(R.layout.my_layout, container) //Fragment

public class Person { private String name; private String surname; private String id; /* Setters and getters - 20 lines */ @Override public boolean equals(Object o) { //another few lines here } @Override public int hashCode() { //some more lines } @Override public String toString() { //some more } } Data classes

Kotlin - data class Person( val name: String, var surname: String = "", val id: String ) Data classes

Kotlin - Data classes equals() & hashCode() toString() copy() function

fun evaluateObject(obj: Any): String { return when(obj){ is String -> "Ahoy !!" is Int -> "Lucky no is $obj" else -> "Object is in space" } } Kotlin When expression

// Java if(name.toLowerCase().contains(str)) { ... } Android

// Kotlin if (name.contains(str, true)) { ... } Android

for (i in 1..100) { ... } for (i in 0 until 100) { ... } for (i in 2..10 step 2) { ... } for (i in 10 downTo 1) { ... } if (x in 1..10) { ... } Android Kotlin tricks

fun ImageView.loadUrl(url: String) { Glide.with(context).load(url).into(this) } ImageLoadingExtensions.kt Android //Then ivAvatar.loadUrl("some_fancy_url")

Utils fun View.visible() { visibility = View.VISIBLE } fun View.gone() { visibility = View.GONE } //call view.visible()

Utils fun View.setHeight(height: Int) { val params = layoutParams params.height = height layoutParams = params } //call tvTotalPrice.setHeight(50)

fun Activity.showOnUiThread(init: Activity.() -> Unit) : Activity { if (!isFinishing) { runOnUiThread { init() } } return this } Utils //call showOnUiThread(updateUIWithNewData())

fun Activity.hideKeyboard(): Boolean { val view = currentFocus view?.let { val imm = getSystemService(INPUT_METHOD_SERVICE) as InputMethodManager return imm.hideSoftInputFromWindow(view.windowToken, HIDE_NOT_ALWAYS) } return false } Utils

Adopting kotlin into android code

class MyActivity : AppCompactActivity(){ // wont be init here lateinit var obj: CustomObject override fun onCreate(...){ obj = intent.getExtras().get("key") // this will init the variable } } lateinit Modiﬁer

When to use lateinit If you’re variable is mutable If you’re sending a object from another component/ screen — ex Intent extras lateinit Modiﬁer

val myUtil by lazy { SomeUtil(parameter1, parameter2) } Lazy Initialisation

val hashMap = HashMap<String, String>() hashMap.put("keyA", "valueA") hashMap.put("keyB", "valueB") hashMap.put("keyC", "valueC") //change to val map = mapOf( "keyA" to "valueA", "keyB" to "valueB", "keyC" to "valueC" ) Kotlin

fun listItemsToDisplay(): List<ListItem> { return listOf(users) + friends() + recentSearches() } Kotlin

fun parseResponse(response: Response?) = when (response?.code()) { null -> throw HTTPException(“Oh no”) 200, 201 -> parse(response.body()) in 400..499 -> throw HTTPException("Invalid request") in 500..599 -> throw HTTPException("Server error") else -> throw HTTPException("Error! Code {response.code()}”) } Kotlin

Convert .java Automatic migration might not always result the best code //Java private static final String MY_EXTRA= "extra_sauce"; //Kotlin class SomeClass : AppCompatActivity() { companion object { } //Other stuff here } private val MY_EXTRA = “extra_sauce”

Kotlin Do this instead //Kotlin class SomeClass : AppCompatActivity() { //Other stuff here } private val MY_EXTRA = “extra_sauce”

Kotlin Do this instead //Kotlin class SomeClass : AppCompatActivity() { //Other stuff here } private const val MY_EXTRA = “extra_sauce”

Kotlin class SomeClass : AppCompatActivity() { var someString = “” var someValue : Payment = /*..*/ override fun onCreate(b: Bundle?) { super.onCreate(b) if (someValue != null) someString = someValue!!.getCost() } }

Kotlin class SomeClass : AppCompatActivity() { var someString = “” var someValue : Payment override fun onCreate(b: Bundle?) { super.onCreate(b) if (someValue != null) someString = someValue!!.getCost() } }

Kotlin class SomeClass : AppCompatActivity() { var someString = “” var someValue : Payment override fun onCreate(b: Bundle?) { super.onCreate(b) someValue?.let{ someValue!!.getCost() } } }

run val password: Password = PasswordGenerator().run { seed = "someString" hash = {s -> someHash(s)} hashRepetitions = 1000 generate() }

val generator = PasswordGenerator().apply { seed = "someString" hash = {s -> someHash(s)} hashRepetitions = 1000 } val pasword = generator.generate() apply

class MyFragment : Fragment(){ companion object { @JvmStatic fun newInstance(b:Boolean): MyFragment { val fragment = MyFragment() val args = Bundle() args.putBoolean("isValid",b) fragment.arguments = args return fragment } Fragments

Kotlin fun handleView() { for (item in list) if (item is TextView) item.text = getString(R.string.play) else doRandomOperationOn(item) }

Kotlin fun handleView() { list.forEach { item -> when (item) { is TextView -> item.text = getString(R.string.play) else -> doRandomOperationOn(item) } } }

class MyRecyclerAdapter( val items: List, val listener: (Item) -> Unit ){ …. } Kotlin recycler.adapter = MyRecyclerAdapter(items) { navigateToDetail(it) }

fun View.fadeOut(duration: Long = 500): ViewPropertyAnimator { return animate() .alpha(0.0f) .setDuration(duration) } Optional params icon.fadeOut() // fade out with default time (500) icon.fadeOut(1000) // fade out with custom time

Kotlin inline fun SharedPreferences.edit( func: SharedPreferences.Editor.()-> Unit) { val editor = edit() editor.func() editor.apply() } //Then preferences.edit { putString("foo", "bar") putString("fizz", "buzz") remove("username") }

CODE ❤ Kotlin Java to Kotlin http://bit.ly/2glBYh8 http://bit.ly/KotlinIsAwesome Koans - http://bit.ly/KotlinKoans

Thank you https://medium.com/@aditlal @aditlal http://bit.ly/devfestKotlin

Practical tips for building apps with kotlin

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