Pragmatic Kotlin :heart:

Practical Tips to migrate your Android App to Kotlin

By Ravindra Kumar @ravidsrk

About Me

• Ravindra Kumar @ravidsrk
• Android Developer @Fueled
• Speaker at Droidcon In, Jsfoo, TiConf, Devfest
• Creator of AndroidStarters.com and KotlinExtensions.com
• Open source contributor @ravidsrk
• Author of Android Testing Guide

Agenda

• Steps to Convert
• Common converter Issues
• Takeaways
• Eliminate all !! from your Kotlin code
• Kotlin Extensions
• Kotlin Android Extensions

Steps to Convert

Once you learn basics syntax of Kotlin

1. Convert files, one by one, via "⌥⇧⌘K", make sure tests still pass

Steps to Convert

Once you learn basics syntax of Kotlin

1. Convert files, one by one, via "⌥⇧⌘K", make sure tests still pass
2. Go over the Kotlin files and make them more idiomatic.

Steps to Convert

Once you learn basics syntax of Kotlin

1. Convert files, one by one, via "⌥⇧⌘K", make sure tests still pass
2. Go over the Kotlin files and make them more idiomatic.
3. Repeat step 2 until you convert all the files.

Steps to Convert

Once you learn basics syntax of Kotlin

1. Convert files, one by one, via "⌥⇧⌘K", make sure tests still pass
2. Go over the Kotlin files and make them more idiomatic.
3. Repeat step 2 until you convert all the files.
4. Ship it.

Common Converter Issues

• TypeCasting for the sake of Interoperability.
• Companion will add extra layer.
• If java method starting with getFoo(), converter looks for property with the name foo.
• Generics are hard to get it right on the first go.
• No argument captor.
• git diff If two developers are working on same java file and one guy converts it to Kotlin, it will be rework.

TypeCasting for the sake of Interoperability

Here is the Java class:

public class DemoFragment extends BaseFragment implements DemoView {
  
    @Override
    public void displayMessageFromApi(String apiMessage) {
      ...
    }
}

TypeCasting for the sake of Interoperability

// Kotlin class
class DemoResponse {
    @SerializedName("message") var message: String? = null
}

TypeCasting for the sake of Interoperability

// Kotlin class
class DemoResponse {
    @SerializedName("message") var message: String? = null
}

// Typecasting to String
mainView?.displayMessageFromApi(demoResponse.message) 

TypeCasting for the sake of Interoperability

// Kotlin class
class DemoResponse {
    @SerializedName("message") var message: String? = null
}

// Typecasting to String
mainView?.displayMessageFromApi(demoResponse.message) // Compiler Error

TypeCasting for the sake of Interoperability

// Kotlin class
class DemoResponse {
    @SerializedName("message") var message: String? = null
}

// Typecasting to String
mainView?.displayMessageFromApi(demoResponse.message as String)

Companion will add extra layer

Here is Java class:

public class DetailActivity extends BaseActivity implements DetailMvpView{
    public static final String EXTRA_POKEMON_NAME = "EXTRA_POKEMON_NAME";

    public static Intent getStartIntent(Context context, String pokemonName) {
        Intent intent = new Intent(context, DetailActivity.class);
        intent.putExtra(EXTRA_POKEMON_NAME, pokemonName);
        return intent;
    }
}

Companion will add extra layer

Converted Kotlin class:

class DetailActivity : BaseActivity(), DetailMvpView {
    companion object {
        val EXTRA_POKEMON_NAME = "EXTRA_POKEMON_NAME"

        fun getStartIntent(context: Context, pokemonName: String): Intent {
            val intent = Intent(context, DetailActivity::class.java)
            intent.putExtra(EXTRA_POKEMON_NAME, pokemonName)
            return intent
        }
    }
}

Companion will add extra layer

public class MainActivity extends BaseActivity implements MainMvpView {
  private void pokemonClicked(Pokemon pokemon) {
      startActivity(DetailActivity.Companion.getStartIntent(this, pokemon))    
  }
}

Add @JvmStatic

Converted Kotlin class:

class DetailActivity : BaseActivity(), DetailMvpView {
    companion object {
        val EXTRA_POKEMON_NAME = "EXTRA_POKEMON_NAME"

        @JvmStatic
        fun getStartIntent(context: Context, pokemonName: String): Intent {
            val intent = Intent(context, DetailActivity::class.java)
            intent.putExtra(EXTRA_POKEMON_NAME, pokemonName)
            return intent
        }
    }
}

Add @JvmStatic

public class MainActivity extends BaseActivity implements MainMvpView {
  private void pokemonClicked(Pokemon pokemon) {
      startActivity(DetailActivity.getStartIntent(this, pokemon))    
  }
}

Add @JvmStatic

public class MainActivity extends BaseActivity implements MainMvpView {
  private void pokemonClicked(Pokemon pokemon) {
      startActivity(DetailActivity.getStartIntent(this, pokemon))    
  }
}

Remember: *you do not need to stress about migrating the entire codebase.

Method names starting with get

Here is the Java class:

public interface DemoService {
    @GET("posts")
    Observable<PostResponse> getDemoResponse();

    @GET("categories")
    Observable<CategoryResponse> getDemoResponse2();
}

Method names starting with get

interface DemoService {
    @get:GET("posts")
    val demoResponse: Observable<PostResponse>
    
    @get:GET("categories")
    val demoResponse2: Observable<CategotyResponse>
}

Expecting methods demoResponse and demoResponse2, They are being interpreted as getter methods, this will cause lots of issues.

No ArgumentCaptor

If you are using Mockito’s ArgumentCaptor you will most probably get following error

java.lang.IllegalStateException: classCaptor.capture() must not be null

No ArgumentCaptor

If you are using Mockito’s ArgumentCaptor you will most probably get following error

java.lang.IllegalStateException: classCaptor.capture() must not be null

The return value of classCaptor.capture() is null, but the signature of SomeClass#someMethod(Class, Boolean) does not allow a null argument.

No ArgumentCaptor

If you are using Mockito’s ArgumentCaptor you will most probably get following error

java.lang.IllegalStateException: classCaptor.capture() must not be null

The return value of classCaptor.capture() is null, but the signature of SomeClass#someMethod(Class, Boolean) does not allow a null argument.

mockito-kotlin library provides supporting functions to solve this problem

Key Takeaways

• annotationProcessor must be replaced by kapt in build.gradle

Key Takeaways

• annotationProcessor must be replaced by kapt in build.gradle
• Configure tests to mock final classes

Key Takeaways

• annotationProcessor must be replaced by kapt in build.gradle
• Configure tests to mock final classes
• If you are using android data-binding, include:

kapt com.android.databinding:compiler:3.0.0

Key Takeaways

• annotationProcessor must be replaced by kapt in build.gradle
• Configure tests to mock final classes
• If you are using android data-binding, include:

kapt com.android.databinding:compiler:3.0.0

• @JvmField to rescue while using ButterKnife @InjectView and Espresso @Rule

Eliminate all !! from your Kotlin code

1. Use val instead of var

2. Use lateinit

3. Use let function

4. User Elivis operator

Use val instead of var

• Kotlin makes you think about immutability on the language level and that’s great.
• var and val mean "writable" and "read-only"
• If you use them as immutables, you don’t have to care about nullability.

Use lateinit

private var adapter: RecyclerAdapter<Droids>? = null

override fun onCreate(savedInstanceState: Bundle?) {
   super.onCreate(savedInstanceState)
   mAdapter = RecyclerAdapter(R.layout.item_droid)
}

fun updateTransactions() {
   adapter!!.notifyDataSetChanged()
}

Use lateinit

private lateinit var adapter: RecyclerAdapter<Droids>

override fun onCreate(savedInstanceState: Bundle?) {
   super.onCreate(savedInstanceState)
   mAdapter = RecyclerAdapter(R.layout.item_droid)
}

fun updateTransactions() {
   adapter?.notifyDataSetChanged()
}

Use let function

private var photoUrl: String? = null

fun uploadClicked() {
    if (photoUrl != null) {
        uploadPhoto(photoUrl!!)
    }
}

Use let function

private var photoUrl: String? = null

fun uploadClicked() {
    photoUrl?.let { uploadPhoto(it) }
}

User Elivis operator

Elvis operator is great when you have a fallback value for the null case. So you can replace this:

fun getUserName(): String {
   if (mUserName != null) {
       return mUserName!!
   } else {
       return "Anonymous"
   }
}

User Elivis operator

Elvis operator is great when you have a fallback value for the null case. So you can replace this:

fun getUserName(): String {
   return mUserName ?: "Anonymous"
}

Kotlin Extensions

Toast.makeText(context, "Hello #BlrKotlin :)", Toast.LENGTH_LONG).show()

Kotlin Extensions

Toast.makeText(context, "Hello #BlrKotlin :)", Toast.LENGTH_LONG).show()

/**
 * Extension method to show toast for Context.
 */
fun Context?.toast(@StringRes textId: Int, duration: Int = Toast.LENGTH_LONG) =
 this?.let { Toast.makeText(it, textId, duration).show() }

Kotlin Extensions

Toast.makeText(context, "Hello #BlrKotlin :)", Toast.LENGTH_LONG).show()

/**
 * Extension method to show toast for Context.
 */
fun Context?.toast(@StringRes textId: Int, duration: Int = Toast.LENGTH_LONG) =
 this?.let { Toast.makeText(it, textId, duration).show() }

Check http://kotlinextensions.com/

Kotlin Android Extensions

• Goodbye findViewById
• Using Parcelize annotation for Parcelable

Kotlin Android Extensions

• Goodbye findViewById
• Using Parcelize annotation for Parcelable

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

Goodbye findViewById

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/welcomeMessage"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="Hello World!"/>

</FrameLayout>

Goodbye findViewById

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    TextView welcomeMessageView = findViewById(R.id.welcomeMessage),

    welcomeMessageView.text = "Hello BlrKotlin!"
}

Goodbye findViewById

import kotlinx.android.synthetic.main.activity_main.*

Goodbye findViewById

import kotlinx.android.synthetic.main.activity_main.*

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    welcomeMessage.text = "Hello BlrKotlin!" 
}

Using Parcelize annotation for Parcelable

Here is the Java Class:

public class MyParcelable implements Parcelable {
     private int mData;
     public int describeContents() { return 0; }
     public void writeToParcel(Parcel out, int flags) { out.writeInt(mData); }
     public static final Parcelable.Creator<MyParcelable> CREATOR
             = new Parcelable.Creator<MyParcelable>() {
         
         public MyParcelable createFromParcel(Parcel in) { return new MyParcelable(in); }

         public MyParcelable[] newArray(int size) { return new MyParcelable[size];}
     };
    
     private MyParcelable(Parcel in) { mData = in.readInt(); }
 }

Using Parcelize annotation for Parcelable

Converted Kotlin Class:

data class MyParcelable(var data: Int): Parcelable {
    override fun describeContents() = 1
    override fun writeToParcel(dest: Parcel, flags: Int) { dest.writeInt(data)}

    companion object {
        @JvmField 
        val CREATOR = object : Parcelable.Creator<MyParcelable> {
            override fun createFromParcel(source: Parcel): MyParcelable {
                val data = source.readInt(); return MyParcelable(data)
            }
            override fun newArray(size: Int) = arrayOfNulls<MyParcelable>(size)
        }
    }
}

Using Parcelize annotation for Parcelable

To use @Parcelize we need to set experimental flag in build.gradle

androidExtensions {
    experimental = true
}

Using Parcelize annotation for Parcelable

To use @Parcelize we need to set experimental flag in build.gradle

androidExtensions {
    experimental = true
}

@Parcelize
class MyParcelable(val data: Int): Parcelable

Final tip

Final tip

Don’t try to learn the whole language at once

Questions?

Thank You

Android Testing Guide

• Everything to start writing tests for Android App.
• 75% discount on my upcoming book use BLRKOTLIN
• https://leanpub.com/android-testing/c/BLRKOTLIN