Kotlin in practice

About me
Colorgy Android Developer
Taiwan Android Developer
Study Group(Android 讀書會)
1 year experience

#1. Consider Android extension instead of
findViewById()

#1. Consider Android extension instead of
findViewById()
https://kotlinlang.org/docs/tutorials/android-plugin.html

#2. Avoid null and eliminate ‘!!’
private var adapter: CompanyItemAdapter? = null
...
adapter?.onItemClick = { ... }
private lateinit var adapter: CompanyItemAdapter
...
adapter.onItemClick = { ... }

#2. Avoid null and eliminate ‘!!’
private var repo: Repo? = null
...
if(repo == null){
repo = Repo()
}
repo!!.setId(id)
private var repo: Repo by lazy { repo = Repo() }

But !!
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
Val color = ContextCompat.getColor(context!!, R.color.white)
….
}
Can not be avoided

#3. Consider standard library to create
Collection
// old way of create a list
val items = ArrayList<Int>()
items.add(3)
items.add(4)
items.add(5)
// use standard library
val items = listOf(1, 2, 3)
val mutableItems = mutableListOf(1, 2, 3)
val map = mapOf(“key1” to 1, “key2” to 2)

Sometimes you don’t need them all

#5. Use apply{} to assign properties
// old way of building an object
val andre = Person()
andre.name = "andre"
andre.company = "Viacom"
andre.hobby = "losing in ping pong"
// after applying 'apply' (pun very much intended)
val andre = Person().apply {
name = "Andre"
company = "Viacom"
hobby = "losing in ping pong"
}
= Person()
andre.name = "andre

How about run, let, with, also?
the tldr; on Kotlin’s let, apply, also, with
and run functions

Why not create your own extension
function?
public inline fun <T, R> T?.whenNotNull(block: () -> R?): R? {
return this?.let { block() }
}
foo.whenNotNull{
//do something
}

#6. Consider lambda to implement callback
adapter.setOnItemClickListener { … }
interface OnItemClickListener{
fun onClick()
}

adapter.setOnItemClickListener { … }
interface OnItemClickListener{
fun onClick()
}
We don’t need it!!

adapter.onItemClick = { … }
class FooAdapter{
var onClick: (Item) -> Unit = {}
}

#7. Consider lambda to implement
strategy/template pattern
val typedArray = context.theme.obtainStyledAttributes(...)
try {
edgeLength = typedArray.getDimensionPixelSize(...)
charTextSize = typedArray.getDimensionPixelSize(...)
mainColor = typedArray.getColor(...)
textPadding = typedArray.getDimensionPixelSize(...)
}finally {
typedArray.recycle()
}

val typedArray = context.theme.obtainStyledAttributes(...)
try {
edgeLength = typedArray.getDimensionPixelSize(...)
charTextSize = typedArray.getDimensionPixelSize(...)
mainColor = typedArray.getColor(...)
textPadding = typedArray.getDimensionPixelSize(...)
}finally {
}
Don’t repeat yourself!!

use(typedArray) {
edgeLength = getDimensionPixelSize(...)
charTextSize = getDimensionPixelSize(...)
mainColor = getColor(...)
textPadding = getDimensionPixelSize(...)
}

fun use(typedArray: TypedArray, block : TypedArray.() -> Unit){
try {
typedArray.block()
}finally {
}
}

fun use(typedArray: TypedArray, block : TypedArray.() -> Unit){
try {
typedArray.block()
}finally {
}
}
Higher order function
Lambda with receiver

More about lambda design pattern(java)
https://www.youtube.com/watch?v=e4MT_Og
uDKg

#8. Consider extension to enhance
Framework Apis
● Dp to px
● OnPreDraw
● postDelayedSafely

Dp to px
private val padding= 16.toPx()

Dp to px
private val padding= 16.toPx()
fun Int.toPx(): Int = (this * Resources.getSystem().displayMetrics.density).toInt()

OnPreDraw
view.viewTreeObserver.addOnPreDrawListener(
object : ViewTreeObserver.OnPreDrawListener {
override fun onPreDraw(): Boolean {
viewTreeObserver.removeOnPreDrawListener(this)
actionToBeTriggered()
return true
}
})

OnPreDraw
view.doOnPreDraw {
actionToBeTriggered()
}

Post Delayed
Class FooFragment{
fun doSomething(){
view?.postDelayed({
txtLabel.text = "123"
}, 2000)
}
}

Post Delayed
Class FooFragment{
fun doSomething(){
view?.postDelayed({
}, 2000)
}
} potential NullPointer !!

Post Delayed
Class FooFragment{
fun doSomething(){
view?.postDelayed({
if(isAdded){
}
}, 2000)
}
}

Post Delayed
Class FooFragment{
fun doSomething(){
postDelayedSafely({
}, 2000)
}
}

Post Delayed
Class FooFragment{
fun doSomething(){
postDelayedSafely({
}, 2000)
}
}
fun Fragment.postDelaySafely(action: ()-> Unit, delayMillis: Long){
view?.postDelayed({
if(isAdded){
action()
}
}, delayMillis)
}

More on Android - ktx
https://github.com/android/android-ktx

Do not extend you own code!!
● You can control everything
● Extensions is hard to test

#9. Consider refied to replace ::class.java
inline fun <reified T> Gson.fromJson(jsonString: String?): T?{
val type = object : TypeToken<T>() {}.type
return fromJson<T>(jsonString, type)
}
val items: List<Item>? = gson.fromJson<List<Item>>(jsonString)

#10. DSL makes your life easier
toolbarSetup(toolbar) {
title("")
homeIcon(R.drawable.back_white)
backgroundColor(R.color.calendar_today_background)
onHomeClicked { pageNavigator.toLastPage() }
}

DSL? Things you need to know
1. Extensions function
2. Lambda with receiver
3. Operator overloading
4. Generic
5. Infix
6. Invoke operator

Resources
1. Kotlin Weekly
2. Official Documentation
3. Youtube
a. Google I/O
4. Medium
a. Effective Java in Kotlin
b. Coroutines and RxJava
5.

Kotlin in practice

More Related Content

Similar to Kotlin in practice (20)

More from 彥彬洪 (13)

Recently uploaded (20)