Geecon - Improve your Android-fu with Kotlin

Power-up your Android-Fu
with Kotlin
@nicolas_frankel
Nicolas Fränkel, 12 May 2016

Me, Myself and I
Blah, blah, blah…
@nicolas_frankel #kotlin #android

My experience in Android
Back-end Java developer
Developing a To Do list application with
some improvements
Images
Alarm
Multiple lists

My personal view
Developing Android app is a pain
Backend Java is very mature compared to
Android
High-level libraries required to cope up with
low-level APIs

Starting stack
Dagger 2
Butterknife
Retro-lambda
Streams
Green Robot’s EventBus
Picasso

Outline
Kotlin - the language
Libraries
Stdlib
Kotlin extensions for Android
Anko

Kotlin
Language developed by JetBrains
Open Source
Compiles to
JVM bytecode
JavaScript (experimental)
A "simpler Scala"

Kotlin
Functional and object-oriented
Statically typed
Null safety
No checked exceptions
Named & optional arguments
Lambdas
Extension functions
Java compatibility
(And more...)

Hello world!
package hello // optional semicolons
// namespace-level functions
// types on the right
// no special syntax for arrays
// optional return type
fun main(args: Array<String>) {
println("Hello, world!")
}

Setup - build.gradle
buildscript {
ext.kotlin_version = '1.0.0-beta-1038'
repositories {
jcenter()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-
plugin:$kotlin_version"
}
}

Setup – app/build.gradle
apply plugin: 'kotlin-android'
android {
sourceSets {
main.java.srcDirs += 'src/main/kotlin'
test.java.srcDirs += 'src/test/kotlin'
}
}

Sample code
public class AddTaskEvent extends AbstractTaskEvent {
private final List list;
public AddTaskEvent(Task task, List list) {
super(task);
this.list = list;
}
public List getList() {
return list;
}
} @nicolas_frankel #kotlin #android

Pain points
Verbose!
Not specific to Android
Unfortunately Java

Kotlin solution
class AddTaskEvent(task: Task, val list: List)
: AbstractTaskEvent(task)

Sample code
public class KeyboardDisplay {
public void show(Activity a) {
InputMethodManager imm =
(InputMethodManager)
a.getSystemService(INPUT_METHOD_SERVICE);
imm.toggleSoftInput(SHOW_FORCED, 0);
}
public void hide(Activity a) {
InputMethodManager imm =
(InputMethodManager)
a.getSystemService(INPUT_METHOD_SERVICE);
imm.toggleSoftInput(HIDE_IMPLICIT_ONLY, 0);
}

Pain points
Artificial grouping
Utility class
No state so not a real Object

Kotlin solution
fun show(a: Activity) {
val imm =
a.getSystemService(INPUT_METHOD_SERVICE)
as InputMethodManager
imm.toggleSoftInput(SHOW_FORCED, 0)
}
fun hide(a: Activity) {
val imm =
imm.toggleSoftInput(HIDE_IMPLICIT_ONLY, 0)
}

Sample code
• SQLiteDatabase db = getReadableDatabase();
db.query(TASK_TABLE,
new String[] { T_ID_COL, T_NAME_COL },
null, null, null, null,
T_PRIO_COL);

Pain points
Really, pass all those null values?
Create local variable for ease of use

Kotlin solution – part 1
fun SQLiteDatabase.query(table:String,
columns:Array<String>,
selection:String? = null,
selectionArgs:Array<String>? = null,
groupBy:String? = null,
having:String? = null,
orderBy:String? = null): Cursor {
return query(table, columns, selection,
selectionArgs, groupBy, having, orderBy)
}

Kotlin solution – part 2
readableDatabase.query(TASK_TABLE,
arrayOf(T_ID_COL, T_NAME_COL),
orderBy = T_PRIO_COL)

One-use private method

Kotlin solution
Nested methods

Kotlin Library
Standard API for the language

dependencies {
compile "org.jetbrains.kotlin:kotlin-
stdlib:$kotlin_version"
}

Sample code
public class DisplayMetricsGetter {
public DisplayMetrics getFrom(Context c) {
WindowManager wm = (WindowManager)
c.getSystemService(WINDOW_SERVICE);
DisplayMetrics metrics = new DisplayMetrics();
wm.getDefaultDisplay().getMetrics(metrics);
return metrics;
}
}

Pain points
Utility class
Cannot return the object directly
Have to create a local variable

Kotlin solution
fun getFrom(c: Context): DisplayMetrics {
val wm =
c.getSystemService(Context.WINDOW_SERVICE)
as WindowManager
val metrics = DisplayMetrics()
wm.defaultDisplay.getMetrics(metrics)
return metrics
}

Kotlin solution
fun getFrom(c: Context): DisplayMetrics {
val wm =
c.getSystemService(Context.WINDOW_SERVICE)
as WindowManager
return DisplayMetrics().apply {
wm.defaultDisplay.getMetrics(this)
}
}

Kotlin extensions for Android
Plugin for the Kotlin compiler
Provide a "synthetic property" for each
widget in a layout

buildscript {
repositories {
jcenter()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-
android-extensions:$kotlin_version"
}
}

Sample Code
public class AActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstState) {
super.onCreate(savedInstaState);
setContentView(R.layout.task_add_item);
View imageView =
findViewById(R.id.new_task_img);
imageView.setOnClickListener(...);
}
}

Sample Code – With Butterknife
public class AddActivity extends AppCompatActivity {
@Bind(R.id.new_task_img)
View imageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.task_add_item);
ButterKnife.bind(this);
imageView.setOnClickListener(...);
}

Pain points
Do I have to detail???

Kotlinx solution
import
kotlinx.android.synthetic.task_add_item.new_task_i
mg
class AddTaskActivity : AppCompatActivity() {
override fun onCreate(savedInstState: Bundle?) {
super.onCreate(savedInstState)
setContentView(R.layout.task_add_item)
new_task_img.setOnClickListener(...)
}

Anko
Kotlin library for Android
Meant to replace XML with fluent DSL for
GUI
But a lot of other niceties

Code sample
public class AddAlarmClickListener implements
View.OnClickListener {
@Override
public void onClick(View view) {
View rootView = view.getRootView();
ViewFlipper flip = (ViewFlipper)
rootView.findViewById(R.id.new_task_flip);
flip.setDisplayedChild(1);
}
}

Pain points
Cast
Setter (?)

Anko solution
import org.jetbrains.anko.*
class AddAlarmClickListener:
View.OnClickListener {
override fun onClick(view: View) {
val parent:View = view.rootView
val flip =
parent.find<ViewFlipper>(R.id.new_task_flip)
flip.displayedChild = 1
}
}

Code sample
fun show(a: Activity) {
val imm =
imm.toggleSoftInput(SHOW_FORCED, 0)
}
fun hide(a: Activity) {
val imm =
imm.toggleSoftInput(HIDE_IMPLICIT_ONLY, 0)
}

Pain points
Map-based API
Cast

Anko solution
a.inputMethodManager.toggleSoftInput(
SHOW_FORCED, 0)
a.inputMethodManager.toggleSoftInput(
HIDE_IMPLICIT_ONLY, 0)

Code sample
AlertDialog.Builder builder = new AlertDialog.Builder(activity);
builder.setMessage("New list name");
EditText editText = new EditText(activity);
editText.setId(
android.support.v7.appcompat.R.id.select_dialog_listview);
String listName = global.getList().getName();
editText.setText(listName, NORMAL);
editText.selectAll();
builder.setView(editText);
builder.setPositiveButton("OK", editListNameListener);
builder.setNegativeButton("Cancel", (dialog, which) -> {
dialog.cancel();
});
AlertDialog dialog = builder.create();
dialog.show();

Pain points
Verbose +++
Not structured

Anko solution
view.context.alert('New list name') {
builder.setPositiveButton('OK', editListNameListener)
negativeButton('Cancel') {
cancel()
}
customView {
editText(global.list.name) {
id = an.sup.v7.appcompat.R.id.select_dialog_listview
}.selectAll()
}
}.show()

Final stack
Dagger 2
Butterknife  Kotlin ext.
Retro-lambda  Kotlin
Streams  Kotlin
Green Robot’s EventBus
Picasso

Migration tactics
Start with standard classes
Then with Android-dependent classes
Use Android Studio provided migration tool
Take a class
Migrate to Kotlin extension
Migrate to Anko
After each single change, test!
Repeat

Pitfall
Null-able types

Null-able vs. non null-able types
Different type whether value can be null or
not
T: cannot be null
T?: can be null

Null-able vs. non null-able types
Parameter types should use the right kind
Know your API
Or read the docs
Or be conservative

My experience
More expressive
Higher-level abstractions
One single class I couldn’t migrate
Dagger 2 module

Q&A
http://blog.frankel.ch/
@nicolas_frankel
http://frankel.in/

Geecon - Improve your Android-fu with Kotlin

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