A slide of a session in Thailand Mobile Conference 2018

1. Building a DRYer Android App with
Kotlin
by Boonya Kitpitak

2. DRY
➔ Don’t Repeat Yourself
➔ Reducing Repetition
WET
➔ Waste Everyone Time
➔ We Enjoy Typing
➔ Write Everything Twice
(or more)

4. Building a DRYer Android App with Kotlin
Avoid repeating yourself by using Higher
Order Function and Extension function.

5. Outline
➔ Higher order function
➔ Extension function
➔ Higher order function + Extension function
➔ Conclusion

6. Higher Order Function
“A higher-order function is a function that takes functions
as parameters, or returns a function.”

7. fun higherOrderFunction(block: () -> Unit) {
…
block()
…
}
Parameter Return value

8. Example : Suppose I want to measure the execution time of a function
val startTime = System.currentTimeMillis()
printSpeakerList()
val endTime = System.currentTimeMillis()
println("Executed in ${endTime - startTime}ms")
*https://hackernoon.com/a-practical-use-for-higher-order-functions-in-kotlin-7ff89cebf6fd

9. What if I want to measure the execution time of other lines of code ?
val startTime = System.currentTimeMillis()
printSpeakerList()
val endTime = System.currentTimeMillis()
println("Executed in ${endTime - startTime}ms")
val startTime = System.currentTimeMillis()
printAudienceList()
val endTime = System.currentTimeMillis()
println("Executed in ${endTime - startTime}ms")
val startTime = System.currentTimeMillis()
printOrganizerList()
val endTime = System.currentTimeMillis()
println("Executed in ${endTime - startTime}ms")

10. Make it DRY: Define a higher order function that accept function
val startTime = System.currentTimeMillis()
val endTime = System.currentTimeMillis()
println("Executed in ${endTime - startTime}ms")
fun printExecutionTime( block: () -> Unit ) {
}
block()
//Operation your want to measure

11. printExecutionTime ({
printOrganizerList()
})
Using our higher order function
printExecutionTime ({
printAudienceList()
})
printExecutionTime ({
printSpeakerList()
})
fun printExecutionTime(block: () -> Unit) {
val startTime = System.currentTimeMillis()
block()
val endTime = System.currentTimeMillis()
println("Executed in ${endTime - startTime}ms")
}
Higher Order Function
Lambda

12. Higher Order Function != Lambda
printExecutionTime ({
printAudienceList()
})

13. Setting Status bar color : Only available after Lollipop
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
window.setStatusBarColor(Color.BLACK)
}
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
window.setStatusBarColor(Color.BLACK)
}
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
window.setStatusBarColor(Color.BLACK) }
* https://www.androidauthority.com/android-m-dark-status-bar-icons-613241/

14. blocks of code that are only executed if the version is Lollipop or newer:
supportsLollipop ({
window.setStatusBarColor(Color.BLACK)
})
inline fun supportsLollipop(block: () -> Unit) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
block()
}
}
supportsLollipop {
window.setStatusBarColor(Color.BLACK)
}

15. Also work when checking Build Variant
inline fun debug(block:() -> Unit) {
if (BuildConfig.DEBUG) {
block()
}
}
debug {
// do something
}

16. Extension Function
“Ability to extend a class with new functionality without
having to inherit from the class or use any type of design
pattern.”

17. Extension function example: String to uri
val uri1 = Uri.parse(“https://stackoverflow.com”)
val uri2 = Uri.parse(“https://youtube.com”)
/**
* Creates a Uri from the given encoded URI string.
*
* @see Uri.parse
*/
fun String.toUri(): Uri = Uri.parse(this)
val uri1 = “https://stackoverflow.com”.toUri()
val uri2 = “https://youtube.com”.toUri()

18. Extension function example: Create toast in Fragment
Toast.makeText(context, "Hello MobileConf 2018", Toast.LENGTH_LONG).show()
Toast.makeText(context, "Hello MobileConf 2018", Toast.LENGTH_SHORT).show()
fun Fragment.longToast(text: String) {
Toast.makeText(context, text, Toast.LENGTH_LONG).show()
}
fun Fragment.shortToast(text: String) {
Toast.makeText(context, text, Toast.LENGTH_SHORT).show()
}
longToast("Hello MobileConf 2018")
shortToast( "Hello MobileConf 2018")

19. Extension Function + Higher Order Function
“ Let’s make it even shorter ”

20. Extension function example: Edit SharePreference
sharedPreferences.edit()
.putString(“event_name”, “MobileConf2018”)
.apply()
sharedPreferences.edit {
putString(“event_name”, “MobileConf2018”)
}
inline fun SharedPreferences.edit(
action: SharedPreferences.Editor.() -> Unit
) {
val editor = edit()
action(editor)
editor.apply()
}

21. Let’s make Retrofit network call DRYer
val call = api.getSpeakerList()
call.enqueue(object : Callback<List<Speaker>> {
override fun onResponse (call:Call<List<Speaker>>?, response:Response<List<Speaker>>?) {
response?.let {
if (response.isSuccessful) {
processSpeakerList(it)
} else {
handleError()
}
}
}
override fun onFailure(call: Call<List<Speaker>>?, t: Throwable?) {
handleError()
logErrorToFabric()
logErrorToFirebase()
}
})

22. inline fun <T> retrofit2.Call <T>.enqueue(crossinline successHandler : (T?) -> Unit,
crossinline failureHandler: () -> Unit) =
this.enqueue(object : Callback<T> {
override fun onResponse(call: Call<T>?, response: Response<T>?) {
response?.let {
if (response.isSuccessful) {
successHandler(it.body())
} else {
failureHandler()
}
}
}
override fun onFailure(call: Call<T>?, t: Throwable?) {
failureHandler()
logErrorToFabric()
logErrorToFirebase()
}
}
})
}
Declaring extension function

23. val call = api.getSpeakerList()
call.enqueue(object : Callback<List<Speaker>> {
override fun onResponse(call:Call<List<Speaker>>?, response:Response<List<Speaker>>?){
response?.let {
if (response.isSuccessful) {
processSpeakerList(it)
} else {
handleError()
}
}
}
override fun onFailure(call: Call<List<Speaker>>?, t: Throwable?) {
handleError()
logErrorToFabric()
logErrorToFirebase()
}
})
call.enqueue({
processSpeakerList(it)
}, {
handleError()
})

24. Where should we declare extension function ?
Extension function should be declare as top-level function ( function that
locate outside of the class )

25. Inline function ?
An inline function will be substituted by its code during compilation, instead of
doing the real call to a function. It will reduce memory allocations and runtime
overhead in some situations.
inline fun debug(block:() -> Unit) {
if (BuildConfig.DEBUG) {
block()
}
}
inline fun supportsLollipop(block: () -> Unit) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
block()
}
}

26. Conclusion
- Higher order function is function that accept or return function
- Extension function can add more functionality to existing class
- They both help make our code DRYer

27. Q/A

28. Boonya Kitpitak (Ben)
Android Developer from Oozou
ben@oozou.com
medium.com/@boonya.kitpitak

29. Thank you.