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fun() Talk Exploring Kotlin Functions @n8ebel

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♥ == Kotlin + Functions @n8ebel #androidsummit 2

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♥ == Kotlin + Functions 4 Free functions 4 Higher-order functions 4 Rich stdlib @n8ebel #androidsummit 3

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♥ == Kotlin + Functions 4 arrayOf(), listOf() 4 measureTimeMillis { doSomething() } 4 orEmpty() 4 forEach(), groupBy() @n8ebel #androidsummit 4

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Let's Have Some fun() Functions in Kotlin: - Easy to get started - Flexible & Convenient - Enable Freedom to Reimagine @n8ebel #androidsummit 5

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Easy to Get Started @n8ebel #androidsummit 6

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Easy to Get Started 4 Can use the IDE conversion tool 4 Can try online (https://try.kotlinlang.org) 4 Easy to transfer existing knowledge @n8ebel #androidsummit 7

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From Java to Kotlin @n8ebel #androidsummit 8

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Hello Java Method class Greeter { void helloFunctions() { System.out.println("Yay, Functions"); } } 4 return type, name, method body @n8ebel #androidsummit 9

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Java Method Converted to Kotlin class Greeter { fun helloFunctions() { println("Yay, Functions") } } 4 adds the fun keyword 4 no explicit return type @n8ebel #androidsummit 10

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Further Simplification class Greeter { fun helloFunctions() = println("Yay, Functions") } @n8ebel #androidsummit 11

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Further Simplification fun helloFunctions() = println("Yay, Functions") @n8ebel #androidsummit 12

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Flexible & Convenient @n8ebel #androidsummit 13

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Flexible & Convenient 4 Parameter & type flexibility 4 Variations in scoping 4 Variety of modifiers @n8ebel #androidsummit 14

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Parameter & Type Freedom 4 Default parameters 4 Named arguments 4 Variable arguments 4 Inferred return types 4 Generic functions @n8ebel #androidsummit 15

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Parameter Basics @n8ebel #androidsummit 16

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Parameter Basics fun helloFunctions(excitingThing:String) { println("Yay, " + excitingThing) } helloFunctions("functions") // outputs "Yay, functions" @n8ebel #androidsummit 17

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Parameter Basics fun helloFunctions(exclamation:String, excitingThing:String) { println(exclamation + ", " + excitingThing) } helloFunctions("Yay", "functions") // outputs "Yay, functions" @n8ebel #androidsummit 18

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Default Parameters @n8ebel #androidsummit 19

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Default Parameters fun helloFunctions(exclamation:String, excitingThing:String = "functions") { println(exclamation + ", " + excitingThing) } // outputs "Yay, functions" helloFunctions("Yay", "functions") // outputs "Yay, functions" helloFunctions("Yay") @n8ebel #androidsummit 20

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Default Parameters 4 function parameters can have default values 4 when an argument is omitted, the default value is used @n8ebel #androidsummit 21

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Default Parameters 4 allows us the flexibility of overloads without the verbosity of writing them 4 help document the function contract by indicating what "sensible defaults" might be @n8ebel #androidsummit 22

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Default Parameters & Java 4 Java doesn't have default parameter values 4 must specify all parameter values when calling from Java @n8ebel #androidsummit 23

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Default Parameters & Java public class Main { // compiles GreetingFunctionsKt.helloFunctions("Yay", "functions"); // does not compile GreetingFunctionsKt.helloFunctions("Yay"); } @n8ebel #androidsummit 24

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Default Parameters & Java 4 can use @JvmOverloads to generate overloads for each parameter 4 generated overloads will use the specified default values @n8ebel #androidsummit 25

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Default Parameters & @JvmOverloads @JvmOverloads fun helloFunctions(exclamation:String, excitingThing:String = "functions") { println(exclamation + ", " + excitingThing) } @n8ebel #androidsummit 26

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@n8ebel #androidsummit 27

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Default Parameters & @JvmOverloads With the generated Java methods now available, both Java invocations now compile public class Main { GreetingFunctionsKt.helloFunctions("Yay", "functions"); GreetingFunctionsKt.helloFunctions("Yay"); } @n8ebel #androidsummit 28

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Named Arguments @n8ebel #androidsummit 29

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Named Arguments helloFunctions("functions", "functions") 4 How do we know which value is correct? @n8ebel #androidsummit 30

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Named Arguments helloFunctions(exclamation = "yay!", excitingThing = "functions") 4 Much easier to understand with named arguments @n8ebel #androidsummit 31

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Named Arguments 4 Improve readability of function invocations 4 Modify order of passed parameters by using named arguments 4 Refactor parameter count/order without breaking code @n8ebel #androidsummit 32

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Named Arguments fun helloFunctions(exclamation:String, excitingThing:String = "functions") { println(exclamation + ", " + excitingThing) } helloFunctions("Hooray", "functions") helloFunctions("Hooray") helloFunctions(excitingThing = "functions", exclamation = "Hooray") // all output "Hooray, functions" @n8ebel #androidsummit 33

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Named Argument Limitations 4 there are limitations to how named & positioned arguments are used 4 once an argument name is specified, all subsequent arguments must be named as well @n8ebel #androidsummit 34

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Named Argument Limitations helloFunctions("hooray", "Android Summit") helloFunctions("hooray", excitingThing = "Android Summit") // both output "hooray, Android Summit" helloFunctions(excitingThing = "Android Summit", "hooray") // error: Mixing named and positioned arguments not allowed @n8ebel #androidsummit 35

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Named Arguments & Java 4 Named arguments are not supported from Java 4 Arguments must be passed in the order they are defined @n8ebel #androidsummit 36

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Variable Arguments @n8ebel #androidsummit 37

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Variable Number of Arguments We can define a parameter to accept a variable number of arguments T - use the vararg keyword - vararg param is then treated as an array of type T - default value must now be an array @n8ebel #androidsummit 38

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Variable Number of Arguments fun helloFunctions(exclamation:String, vararg excitingThings:String) { for(excitingThing in excitingThings) { println(exclamation + ", " + excitingThing) } } helloFunctions("yay!", "Android Summit", "Kotlin", "Android") // outputs: // yay!, Android Summit // yay!, Kotlin // yay!, Android @n8ebel #androidsummit 39

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Variable Number of Arguments Typically, a vararg parameter will be the last one Can be used in any order if: - other params are called using named argument syntax - last param is a function passed outside the parentheses @n8ebel #androidsummit 40

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Variable Number of Arguments This works helloFunctions("yay!", "Android Summit", "Kotlin", "Android") helloFunctions("Android Summit", "Kotlin", "Android", exclamation = "yay!") // both output: // yay!, Android Summit // yay!, Kotlin // yay!, Android @n8ebel #androidsummit 41

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Variable Number of Arguments This works helloFunctions("Android Summit", "Kotlin", "Android") // output: // "Android Summit, Kotlin" // "Android Summit, Android" @n8ebel #androidsummit 42

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Variable Number of Arguments This won't compile helloFunctions("Android Summit", exclamation = "yay!", "Kotlin", "Android") // error: "no matching function" @n8ebel #androidsummit 43

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Variable Number of Arguments Use "spread" operator to pass an existing array of values val thingsToBeExcitedAbout = arrayOf("Android Summit", "Kotlin", "Android") helloFunctions("yay!", *thingsToBeExcitedAbout) // output: // yay!, Android Summit // yay!, Kotlin // yay!, Android @n8ebel #androidsummit 44

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Variable Number of Arguments helloFunctions("yay!", "coffee", *thingsToBeExcitedAbout) helloFunctions("yay!", *thingsToBeExcitedAbout, "coffee") 4 "Spreading" can be used alone, or with other passed varargs as well 4 input array to the vararg parameter is handled in order @n8ebel #androidsummit 45

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Return Types @n8ebel #androidsummit 46

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Return Types What is the return type? fun helloFunctions(exclamation:String, excitingThing:String="functions") { println(exclamation + ", " + excitingThing) } 4 If a function does not return any useful value, i ts return type is Unit @n8ebel #androidsummit 47

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Return Types These are equivalent fun helloFunctions(exclamation:String, excitingThing:String="functions") : Unit { println(exclamation + ", " + excitingThing) } fun helloFunctions(exclamation:String, excitingThing:String="functions") { println(exclamation + ", " + excitingThing) } @n8ebel #androidsummit 48

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Return Types: Non-Unit Type Functions with a block body require explicit return type & call for non-Unit functions fun helloFunctions(exclamation:String, excitingThing:String="functions") : String { return exclamation + ", " + excitingThing } @n8ebel #androidsummit 49

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Return Types: Non-Unit Type Can infer return type for single-expression functions fun helloFunctions(exclamation:String, excitingThing:String="functions") = exclamation + ", " + excitingThing @n8ebel #androidsummit 50

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Generic Functions @n8ebel #androidsummit 51

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Generic Functions 4 Like classes, functions may have generic type parameters 4 many stdlib functions are built with generics @n8ebel #androidsummit 52

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@n8ebel #androidsummit 53

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Variations In Scope @n8ebel #androidsummit 54

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Variations In Scope 4 Top-level 4 Member functions 4 Local 4 CompanionObject 4 Extension functions @n8ebel #androidsummit 55

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Top-Level functions @n8ebel #androidsummit 56

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Top-Level functions 4 Not associated with a class or object 4 Defined within a Kotlin file 4 Belong to their declared file's package 4 Import to use within other packages @n8ebel #androidsummit 57

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Top-Level Function Patterns 4 Replace stateless classes filled with static methods 4 Swap your "Util" or "Helper" classes with functions @n8ebel #androidsummit 58

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Top-Level Function Considerations 4 Not truly removing classes 4 Generated as a public static method on a class using a special convention 4 Kt.java @n8ebel #androidsummit 59

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Top-Level Function Considerations // inside Logging.kt package logging fun log(error:Throwable) {...} @n8ebel #androidsummit 60

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Top-Level Function Considerations Generated Code public class LoggingKt { public static void log(Throwable error) {...} } @n8ebel #androidsummit 61

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Top-Level Function Considerations // call from Kotlin log(Throwable("oops")) // call from Java LoggingKt.log(new Throwable("oops")) @n8ebel #androidsummit 62

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Top-Level Function Considerations Can override the generated class/file name 4 Add @file:JvmName() to function's file 4 Must be before the declared package @n8ebel #androidsummit 63

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Top-Level Function Considerations // inside Logging.kt @file:JvmName("LoggingFunctions") package logging fun log(error:Throwable) {...} @n8ebel #androidsummit 64

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Top-Level Function Considerations // call from Kotlin log(Throwable("oops")) // call from Java LoggingFunctions.log(new Throwable("oops")) @n8ebel #androidsummit 65

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Member Functions @n8ebel #androidsummit 66

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Member Functions 4 Function associated with a class or object 4 Have access to private members of the class or object @n8ebel #androidsummit 67

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Member Functions class Speaker() { fun giveTalk() { ... } } // create instance of class Speaker and call giveTalk() Speaker().giveTalk() @n8ebel #androidsummit 68

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Member Function Considerations 4 Default arguments can't be changed in overridden methods 4 If overriding a method, you must omit the default values @n8ebel #androidsummit 69

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Local Functions @n8ebel #androidsummit 70

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Local Functions Functions inside of functions 4 Create a function that is scoped to another function 4 Useful if your function is only ever called from another function @n8ebel #androidsummit 71

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Local Functions 4 Declare like any other function, but within an existing function 4 Have access to all params and variables of the enclosing function @n8ebel #androidsummit 72

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Local Functions fun outerFunction(name:String) { fun innerFunction() { println(name) } ... innerFunction() // will print the passed name } @n8ebel #androidsummit 73

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Local Functions Why would you want this? 4 Clean code 4 Avoids code duplication 4 Avoid deep chains of function calls @n8ebel #androidsummit 74

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Local Functions fun parseAccount(response:AccountResponse) : Account { ... val hobbies = response.getField("hobbies").map{ val key = it.getField("key") Hobby(key) } val favoriteFoods = response.getField("favorite_foods").map{ val key = it.getField("key") Food(key) } } @n8ebel #androidsummit 75

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Local Functions fun parseAccount(response:AccountResponse) : Account { fun parseKey(entity:ResponseEntity) = entity.getField("key") ... val hobbies = response.getField("hobbies").map{ val key = parseKey(it) Hobby(key) } val favoriteFoods = response.getField("favorite_foods").map{ val key = parseKey(it) Food(key) } } @n8ebel #androidsummit 76

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Local Function Considerations 4 Local function or private method? 4 Is the logic going to be needed outside the current function? 4 Does the logic need to be tested in isolation? 4 Is the enclosing function still readable? @n8ebel #androidsummit 77

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Companion Objects @n8ebel #androidsummit 78

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Companion Objects 4 No static method/functions in Kotlin 4 Recommended to use top-level functions instead 4 What if you need access to private members of an object? @n8ebel #androidsummit 79

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Companion Objects 4 Want to create a factory method? 4 Define a member function on a companion object to gain access to private members/constructors @n8ebel #androidsummit 80

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Companion Objects: Use from Kotlin class Course private constructor(val key:String) // won't work // can't access the private constructor fun createCourse(key:String) : Course { return Course(key) } @n8ebel #androidsummit 81

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Companion Objects: Use from Kotlin class Course private constructor(val key:String) { companion object { fun createCourse(key:String) : Course { return Course(key) } } } @n8ebel #androidsummit 82

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Companion Objects: Use from Kotlin class Course private constructor(val key:String) { companion object { fun createCourse(key:String) : Course { return Course(key) } } } // can then call the factory method Course.createCourse("somekey") @n8ebel #androidsummit 83

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Companion Objects: Use From Java class AuthFragment : Fragment() { ... companion object { fun newInstance(username: String) = ... } } @n8ebel #androidsummit 84

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Companion Objects: Use From Java @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // explicitly reference `Companion` from Java AuthFragment.Companion.newInstance("username") } @n8ebel #androidsummit 85

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Companion Objects: Renaming Usage from Java is verbose AuthFragment.Companion.newInstance("username") @n8ebel #androidsummit 86

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Companion Objects: Renaming class AuthFragment : Fragment() { ... companion object Factory { fun newInstance(username: String) = ... } } AuthFragment.Factory.newInstance("username") @n8ebel #androidsummit 87

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Companion Objects: @JvmStatic class AuthFragment : Fragment() { ... companion object { @JvmStatic fun newInstance(username: String) = ... } } @n8ebel #androidsummit 88

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Companion Objects: @JvmStatic @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // no need to reference `Companion` AuthFragment.newInstance("username") } @n8ebel #androidsummit 89

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Variations @n8ebel #androidsummit 90

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Variations 4 infix 4 extension 4 higher-order 4 inline @n8ebel #androidsummit 91

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infix Functions @n8ebel #androidsummit 92

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infix Functions 4 infix keyword enables usage of infix notation 4 What is infix notation? 4 Can omit the dot & parentheses for a function call 4 "key" to "value" @n8ebel #androidsummit 93

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infix Functions 4 Must be a member function or extension function 4 Must take a single, non-varargs, parameter with no default value @n8ebel #androidsummit 94

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infix Functions class ConferenceAttendee { infix fun addInterest(name:String){...} } // call the function without dot or parentheses val attendee = ConferenceAttendee() attendee.addInterest("Kotlin") attendee addInterest "Kotlin" @n8ebel #androidsummit 95

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infix Functions 4 Provides a very clean, human-readable syntax 4 Core building block of custom DSLs @n8ebel #androidsummit 96

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infix Functions: stdlib for (i in 0 until 10 step 2) { println(i) // prints 0,2,4,6,8 } "key" to someValue // creates a Pair var result: Boolean result = true or false // perform logical operations result = true and false @n8ebel #androidsummit 97

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infix Functions: Kotlin Test "hello" should haveSubstring("ell") "hello" shouldNot haveSubstring("olleh") https://github.com/kotlintest/kotlintest @n8ebel #androidsummit 98

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Extension Functions @n8ebel #androidsummit 99

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Extension Functions 4 Extend the functionality of an existing class 4 Defined outside the class 4 Used as if they were a member of a class @n8ebel #androidsummit 100

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Why Extension Functions? 4 Clean-up or extend classes & apis you don't control 4 Remove helper classes & simplify top-level functions @n8ebel #androidsummit 101

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Extension Functions // add a new function to the View class fun View.isVisible() = visibility == View.VISIBLE yourView.isVisible() @n8ebel #androidsummit 102

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Extension Functions fun showToast( context: Context, msg:String, duration: Int = Toast.LENGTH_SHORT) { Toast.makeText(context, msg, duration).show() } showToast(context, "Toast!") @n8ebel #androidsummit 103

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Extension Functions fun Context.showToast( msg: CharSequence, duration: Int = Toast.LENGTH_SHORT) { Toast.makeText(this, msg, duration).show() } context.showToast("Toast!") @n8ebel #androidsummit 104

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Extension Function Considerations 4 How are these generated under the hood? 4 How are these called from Java? @n8ebel #androidsummit 105

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Extension Function Considerations 4 Generated as static methods that accept the receiver object as it's first argument 4 Default behavior is to use Kt. @n8ebel #androidsummit 106

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@n8ebel #androidsummit 107

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Extension Function Considerations // ContextExtensions.kt fun Context.showToast(...) { ... } // when called from Java ContextExtensionsKt.showToast(context, "Toast!"); @n8ebel #androidsummit 108

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Higher-Order Functions @n8ebel #androidsummit 109

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Higher-Order Functions 4 Functions that take, or return, other functions 4 Can be lambda or function reference 4 Many examples in the Kotlin standard library apply, also, run @n8ebel #androidsummit 110

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Higher-Order Functions 4 Enable interesting patterns & conventions 4 Support functional programming 4 Can cleanup setup/teardown patterns such as shared prefs @n8ebel #androidsummit 111

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Higher-Order Functions fun getScoreCalculator(level:Level) { return when (level) { Level.EASY -> { state:ScoreState -> state.score * 10 } Level.HARD -> { state:ScoreState -> state.score * 5 * state.accuracy } } } @n8ebel #androidsummit 112

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Higher-Order Functions val predicate = { number:Int -> number > 5 } listOf(2,4,6,8).filter(predicate) @n8ebel #androidsummit 113

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Higher-Order Functions fun filterTheList(value:Int) = value > 5 listOf(2,4,6,8).filter(::filterTheList) @n8ebel #androidsummit 114

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Higher-Order Functions If the last parameter of a function is a function, you can omit the parentheses listOf(2,4,6,8).filter{ number -> number > 5 } @n8ebel #androidsummit 115

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Higher-Order Functions public inline fun synchronized(lock: Any, block: () -> R): R { monitorEnter(lock) try { return block() } finally { monitorExit(lock) } } // call from Kotlin synchronized(database) { database.prePopulate() } @n8ebel #androidsummit 116

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Higher-Order Function Performance "Using higher-order functions imposes certain runtime penalties" 4 Extra class created when using lambda 4 If lambda captures variables, extra object created on each call @n8ebel #androidsummit 117

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inline Functions @n8ebel #androidsummit 118

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inline Functions 4 Helps solve higher-order function performance hits 4 Body of the inlined function is substituted for invocations of the function @n8ebel #androidsummit 119

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inline Functions inline fun synchronized(lock: Lock, action: () -> T): T { lock.lock() try { return action() } finally { lock.unlock() } } // call from Kotlin synchronized(Lock()) {...} @n8ebel #androidsummit 120

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inline Functions // sample usage fun inlineExample(l:Lock) { println("before") synchronized(l) { println("action") } println("after") } @n8ebel #androidsummit 121

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inline Functions With inline the generated code is equivalent to this // resulting code fun inlineExample(l:Lock) { println("before") lock.lock() try { println("action") } finally { lock.unlock() } println("after") } @n8ebel #androidsummit 122

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Android Reimagined @n8ebel #androidsummit 123

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Fewer Helper Classes 4 ContextHelper, ViewUtils 4 Replace with 4 top-level functions 4 extension functions @n8ebel #androidsummit 124

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Less Boilerplate fun doTheThingSafely(theThing:() -> Unit) { try { theThing() } catch(error:Throwable) { // handle error } } doTheThingSafely { unsafeFunction() } @n8ebel #androidsummit 125

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Upgrade Our Apis Can use extensions, default params, etc to cleanup/ simplify/extend common apis 4 Now seeing community supported examples of this 4 Android KTX: https://github.com/android/android- ktx 4 Anko: https://github.com/Kotlin/anko @n8ebel #androidsummit 126

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Android KTX sharedPreferences.edit() .putString("key", "without KTX") .putBoolean("isLessBoilerplate", false) .apply() sharedPreferences.edit { putString("key", "with KTX") putBoolean("isLessBoilerplate", true) } @n8ebel #androidsummit 127

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Android KTX // without KTX supportFragmentManager.beginTransaction() .replace(R.id.fragmentContainer, SomeFragment()) .addToBackStack("name") .commit() // with KTX supportFragmentManager.transaction { replace(R.id.fagmentContainer, SomeFragment()) addToBackStack("name") } @n8ebel #androidsummit 128

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Cleaner Syntax fun log(msg:String) {...} inline fun runOnBackgroundThread(action:() -> Unit) { ... } 4 More fluent syntax 4 Simplify test mocking 4 Avoids extra classes @n8ebel #androidsummit 129

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Useful Collection Functions 4 orEmpty() 4 arrayOf(), listOf(), mapOf() 4 associateBy(), groupBy() 4 map(), mapIndexed() 4 reduce(), sum(), filter(), partition() @n8ebel #androidsummit 130

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Useful Collection Functions // easily calculate sum from collection val totalDownloadSize = downloadRecords.sumBy { it.downloadSize } // sort all DownloadRecords based on download time val sortedDownloadEntities = downloadRecords.sortedByDescending { record -> record.downloadTime } @n8ebel #androidsummit 131

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Useful Collection Functions // return map with all DownloadRecords mapped to their parent enrollmentKey val entitiesGroupedByEnrollmentKey = downloadRecords.groupBy { record -> record.enrollmentKey } // ensure we have a non-null collection and map all records to view models val viewModels = inputRecords.orEmpty().map { record -> createViewModel(record) } @n8ebel #androidsummit 132

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Useful String Functions 4 isEmpty(), isNotEmpty() 4 substringBefore(), substringAfter() 4 orEmpty() @n8ebel #androidsummit 133

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Useful String Functions if(!url.isNullOrEmpty()) { loadData(url) } viewModel.body = model.description.orEmpty() @n8ebel #androidsummit 134

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Kotlin functions provide flexibility & freedom in how you build your apps @n8ebel #androidsummit 135

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Go, and Have fun() 4 Easy to get started with functions 4 Flexibility gives convenience 4 Enables us to reimagine how we build our apps @n8ebel #androidsummit 136

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Let's Continue the Discussion with("n8ebel").apply { Twitter YouTube .com Medium Facebook GitHub } @n8ebel #androidsummit 137

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Thanks For Coming @n8ebel #androidsummit 138