Kotlin on Android by Svetlana Isakova

Transcript

1. Svetlana Isakova You can do better with Kotlin
2. - modern - pragmatic - Android-friendly - pragmatic - Android-friendly - modern Kotlin Programming Language
3. Pragmatic
4. From
5. has good tooling
6. - completion - navigation - refactorings - inspections …
7. can be easily mixed with Java code
8. *.java *.class *.dex compiled to Java bytecode *.kt
9. Kotlin code Java code you can have Java & Kotlin code in one project
10. Android-friendly
11. Android Studio is based on IntelliJ Idea
12. just another jar for your app kotlin-runtime-1.0.0.jar rxjava-1.1.1.jar 843 KB 979 KB
13. No Kotlin SDK …just JDK + extensions small runtime jar easy Java interop
14. Modern - concise - safe - expressive
15. concise
16. public class Person {
 private final String name;
 private final int age;
 
 public Person(String name, int age) {
 this.name = name;
 this.age = age;
 }
 
 public String getName() {
 return name;
 }
 
 public int getAge() {
 return age;
 }
 }
17. - equals - hashCode - toString 
 data 
 class Person(val name: String, val age: Int)
18. safe
19. Billion Dollar Mistake
20. Modern approach: to make NPE compile-time error, not run-time error
21. Nullable types in Kotlin val s1: String = "always not null” 
 val s2: String? = null s1.length ✓ ✗ s2.length
22. val s: String? if (s != null) {
 s.length
 } smart cast if (s == null) fail()
 s.length Dealing with Nullable Types the result has type Int? s!! throws NPE if s is null s?.length ?: 0
23. Nullable Types Under the Hood No performance overhead @Nullable, @NotNull annotations
24. expressive
25. you can avoid any repetition you can make the code look nicer you can create API looking like DSL expressive
26. Extension Functions Lambdas Lambdas with receiver
27. fun String.lastChar() = this.get(this.length - 1) fun String.lastChar() = get(length - 1) this can be omitted Extension Functions ✳
28. import com.example.util.lastChar import com.example.util.* Extension Functions val c: Char = "abc".lastChar() fun String.lastChar() = get(length - 1) ✳
29. fun String.lastChar() = get(length - 1) Calling Extension Functions from Java code StringExtensions.kt char c = StringExtensionsKt.lastChar("abc"); JavaClass.java import static StringExtensionsKt.lastChar; char c = lastChar("abc");
30. Extensions for Android Toast.makeText(this, "Thank you!”, Toast.LENGTH_SHORT).show() Activity extension function on Activity toast("Thank you!") this.toast("Thank you!")
31. val intent = Intent(this, NewActivity::class.java)
 intent.putExtra("ANSWER", 42)
 startActivity(intent) startActivity<NewActivity>("ANSWER" to 42) this.startActivity<NewActivity>(“ANSWER" to 42) Extensions for Android
32. val employees: List<Employee> employees.filter { it.city == City.PRAGUE }.map(Employee::age).average() employees.filter { it.city == City.MUNICH }.map(Employee::age).average() Avoiding Duplication (I) employees.averageAgeFor(City.PRAGUE) employees.averageAgeFor(City.MUNICH) add an extension function averageAgeFor
33. employees.filter { it.city == City.PRAGUE } Lambdas ✳
34. Working with collections with Lambdas & Member References { employee -> employee.age } { employee: Employee -> employee.city == City.PRAGUE } employees.filter { it.city == City.PRAGUE }.map(Employee::age).average()
35. button.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View v) {
 Toast.makeText(MainActivity.this, "Thank you!", Toast.LENGTH_SHORT).show();
 }
 }); Lambdas vs anonymous classes button.setOnClickListener {
 toast("Thank you!")
 }
36. val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 } toString()
 } The with function with is a function val sb = StringBuilder()
 sb.appendln("Alphabet: ")
 for (c in 'a'..'z') {
 sb.append(c)
 }
 sb.toString()

37. lambda is its second argument val sb = StringBuilder()
 with (sb) {
 this.appendln(“Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 } this.toString()
 } val sb = StringBuilder()
 with (sb, {
 this.appendln(“Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 } this.toString()
 }) lambda is its second argument Lambda with receiver with is a function this is an implicit receiver in the lambda val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 } toString()
 } this can be omitted
38. buildString {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 }
 } Make it even more simple val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 } toString()
 }
39. buildString {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 }
 } Lambda with receiver buildString {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 }
 } lambda with implicit this this can be omitted ✳
40. val db: SQLiteDatabase = … db.beginTransaction()
 try {
 db.delete("users", "first_name = ?", arrayOf("Jake"))
 db.setTransactionSuccessful()
 } finally {
 db.endTransaction()
 } db.inTransaction {
 delete("users", "first_name = ?", arrayOf("Jake"))
 } Avoiding Duplication (II)
41. Under the Hood No performance overhead Lambdas can be inlined
42. db.beginTransaction()
 try {
 db.delete("users", "first_name = ?", arrayOf("Jake"))
 db.setTransactionSuccessful()
 } finally {
 db.endTransaction()
 } db.inTransaction {
 delete("users", "first_name = ?", arrayOf("Jake"))
 } Inline functions is declared as inline function generated bytecode is similar to:
43. ANKO DSL for dynamic layouts
44. Alerts fun Activity.showAreYouSureAlert(process: () -> Unit) {
 alert(title = "Are you sure?",
 message = "Are you really sure?") {
 positiveButton("Yes") { process() }
 negativeButton("No") { }
 }.show()
 } Are you sure? Are you really sure? No Yes
45. customView {
 verticalLayout {
 val email = editText {
 hint = "Email"
 }
 
 val password = editText {
 hint = "Password"
 transformationMethod = PasswordTransformationMethod.getInstance()
 }
 
 positiveButton("Log In") {
 logIn(email.text, password.text)
 }
 }
 } Password Log In Email Custom layouts
46. Kotlin Android Extensions BONUS (old plugin name)
47. Kotlin Android Extensions <Button
 android:id="@+id/click_me_button"
 />
 class KotlinActivity : Activity() {
 override fun onCreate(savedInstanceState: Bundle?) {
 
 click_me_button.setOnClickListener { }
 }
 } import kotlinx.android.synthetic.activity_buttons.click_me_button
48. class KotlinActivity : Activity() {
 
 
 
 
 
 override fun onCreate(savedInstanceState: Bundle?) {
 Button click_me_button.setOnClickListener { }
 }
 } How it works var cachedView: HashMap<Int, View>?
 
 fun getCachedViewById(id: Int): View this code is generated by compiler var click_me_button = 
 getCachedViewById(R.id.click_me_button) as Button
49. kotlinlang.org
50. try.kotlinlang.org
51. Kotlin Koans
52. 39% discount code: 39jemerov
53. Have a nice Kotlin!