Kotlin for Android apps programming

Barcamp Bangkhen 5

Transcript

1. Barcamp Bangkhen 5 @awkwin Kotlin for Android programming
2. Statically typed programming language targeting the JVM and JavaScript
3. Hello world fun main(args: Array<String>) { println("Hello World!") }
4. Break label Java: for(..){ boolean flag=false; for(...){ flag=true; break; } if(flag){ break; } } Kotlin: @loop for(..){ for(..){ break@loop; } }
5. No more getter & setter var stringRepresentation : String get() = this.toString() set(value) { setDataFromString(value) }
6. Data class! data class User(val name: String, val age: Int) - equals - toString → User(name=a, age=1) - component1() = user.name - copy()
7. Multiple inheritance with traits trait MyTrait { // must be stateless fun bar() // abstract fun foo() {} } class Child : MyTrait{ fun bar(){} } • Trait attributes can be overridden • Trait can access class' attribute without declaring it in trait itself
8. Default argument fun printSpace(text : String, spcBefore : Int = 0, spcAfter : Int = 0) { // ... }
9. Named argument fun printSpace(text : String, spcBefore : Int = 0, spcAfter : Int = 0) { // ... } printSpace("hello world", spcAfter=5)
10. Ruby-style function fun double(x : Int) : Int = x * 2 fun double(x : Int) = x * 2
11. Anonymous function (findViewById(R.id.button) as Button). setOnClickListener({ // argument is implicitly defined as it }) (findViewById(R.id.button) as Button). setOnClickListener { event -> // ... }
12. Null safety • var x : Int = 5 • Cannot be null • var y : Int? • Can be null
13. Null safety • x methods cannot return NPE • y methods can return NPE ◦ Does not compile!
14. Null safety • y?.length() ◦ Return y.length() or null if y is null • y?.length() ?: -1 ◦ Return y.length() or -1 if y is null • y!!.length() ◦ Return y.length() or NPE
15. Null safety • if(y != null){ print(y) } • Compiler keep track of null check • var z = y as? Double ◦ z = (Double) y if y instanceof Double ◦ z = null if !(y instanceof Double)
16. Null safety • if(y != null){ print(y) } • Compiler keep track of null check • var z = y as? Double ◦ z = (Double) y if y instanceof Double ◦ z = null if !(y instanceof Double)
17. Extension fun JSONArray.toArrayList() : ArrayList<Object> = Array<Object>(length(), {i -> get(i)}) val x = JSONArray(input) val y = x.toArrayList() Anonymous Function
18. Lazy delegation val txt : TextView by Delegates.lazy { findViewById(R.id.locationName) as TextView }
19. Android • Kotlin interoperates well with Java ◦ Java can call Kotlin stuff (may have magic name added) ◦ Kotlin can call Java stuff (non-primitive data type are platform type -- as safe as in Java) • Runtime size 884kB (ProGuard remove most of this) • Install as Android Studio plugin ◦ Automatically configure gradle
20. Sample App https://github. com/whs/IUPEat • See initial commit for Java version • Should be the language of choice for next KUSmartBus (if I get to write one)
21. Q&A