Improve your Android-Fu with Kotlin

POWER-UP YOUR
ANDROID-FU
WITH KOTLIN
@NICOLAS_FRANKEL

ME, MYSELF AND I
Blah, blah, blah…
@nicolas_frankel - Improve your Android-Fu with Kotlin
2

MY EXPERIENCE IN ANDROID
Back-end Java developer
Developing a To Do list
application with some
improvements
• Images
• Alarm
• Multiple lists
3

MY PERSONAL CONCLUSION
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
4

ORIGINAL STACK
Dagger 2
Butterknife
Retro-lambda
Streams
Green Robot’s EventBus
Picasso
5

OUTLINE
Kotlin - the language
Libraries
• Stdlib
• Kotlin extensions for Android
• Anko
6

KOTLIN
Language developed by
JetBrains
Open Source
Compiles to
• JVM bytecode
• JavaScript (experimental)
A "simpler Scala"
7

KOTLIN
Functional and object-oriented
Statically typed
Null safety
No checked exceptions
Named & optional arguments
Lambdas
Extension functions
Java compatibility
(And more...)
8

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!")
}
9

SETUP - BUILD.GRADLE
buildscript {
ext.kotlin_version = '1.0.0-beta-1038'
repositories {
jcenter()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-
plugin:$kotlin_version"
}
}
10

SETUP – APP/BUILD.GRADLE
apply plugin: 'kotlin-android'
android {
sourceSets {
main.java.srcDirs += 'src/main/kotlin'
test.java.srcDirs += 'src/test/kotlin'
}
}
11

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;
}
}
12

PAIN POINTS
Verbose!
• Not specific to Android
• Unfortunately Java
13

KOTLIN SOLUTION
class AddTaskEvent(task: Task, val list: List)
: AbstractTaskEvent(task)
14

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);
}
}
15

PAIN POINTS
Artificial grouping
Utility class
• No real Object, nor state
16

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)
}
17

SAMPLE CODE
SQLiteDatabase db = getReadableDatabase();
db.query(TASK_TABLE,
new String[] { T_ID_COL, T_NAME_COL },
null, null, null, null,
T_PRIO_COL);
18

PAIN POINTS
19
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)
}
20

KOTLIN SOLUTION – PART 2
readableDatabase.query(TASK_TABLE,
arrayOf(T_ID_COL, T_NAME_COL),
orderBy = T_PRIO_COL)
21

KOTLIN LIBRARY
Standard API for the
language
22

dependencies {
compile "org.jetbrains.kotlin:kotlin-
stdlib:$kotlin_version"
}
23

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;
}
}
24

PAIN POINTS
Utility class
Cannot return the object
directly
• Have to create a local variable
25

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
}
26

KOTLIN SOLUTION
fun getFrom(c: Context): DisplayMetrics {
val wm =
c.getSystemService(Context.WINDOW_SERVICE)
as WindowManager
return DisplayMetrics().apply {
wm.defaultDisplay.getMetrics(this)
}
}
27

KOTLIN EXTENSIONS FOR ANDROID
Plugin for the Kotlin
compiler
Provide a "synthetic
property" for each widget in a
layout
28

buildscript {
repositories {
jcenter()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-
android-extensions:$kotlin_version"
}
}
29

SAMPLE CODE
public class AddActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.task_add_item);
View imageView = findViewById(R.id.new_task_img);
imageView.setOnClickListener(...);
}
}
30

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(...);
}
}
31

PAIN POINTS
Do I have to detail???
32

KOTLINX SOLUTION
import
kotlinx.android.synthetic.task_add_item.new_task_img
class AddTaskActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.task_add_item)
new_task_img.setOnClickListener(...)
}
}
33

ANKO
Kotlin library for Android
Mostly about replacing XML
with fluent DSL for GUI
But a lot of other niceties
34

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_alarm_flip);
flip.setDisplayedChild(1);
}
}
35

PAIN POINTS
Cast
Setter (?)
36

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_alarm_flip)
flip.displayedChild = 1
}
}
37

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)
}
38

PAIN POINTS
Map-based API
Cast
39

ANKO SOLUTION
a.inputMethodManager.toggleSoftInput(
SHOW_FORCED, 0)
a.inputMethodManager.toggleSoftInput(
HIDE_IMPLICIT_ONLY, 0)
40

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();
41

PAIN POINTS
Verbose +++
Not structured
42

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()
43

FINAL STACK
Dagger 2
Butterknife  Kotlin ext.
Retro-lambda  Kotlin
Streams  Kotlin
Green Robot’s EventBus
Picasso
44

MIGRATION TACTICS
45
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
46
Null-able types

NULL-ABLE VS. NON NULL-ABLE TYPES
47
Different type whether
value can be null or not
• T: cannot be null
• T?: can be null

NULL-ABLE VS. NON NULL-ABLE TYPES
48
Parameter types should use
the right kind
• Know your API
• Or read the docs
• Or be conservative

MY EXPERIENCE
49
More expressive
Higher-level abstractions
One single class I couldn’t
migrate
• Dagger 2 module

Q&A
50
http://blog.frankel.ch/
@nicolas_frankel
http://frankel.in/

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