bhanditz / devfestahm2017

Slides and references used for "Pragmatic Kotlin for Android Developers" talk at #DevfestAhm

Pragmatic Kotlin ❤️

Practical Tips to migrate your Android App to Kotlin

By Ravindra Kumar @ravidsrk

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About Me

• Ravindra Kumar @ravidsrk
• Android Developer @Fueled
• Proud Kannadiga from Bengaluru
• Speaker at Droidcon In, Jsfoo, TiConf
• Creator of AndroidStarters
• Open source contributor @ravidsrk
• Author of Android Testing Guide

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Agenda

• Talk Intro
• About Me
• Agenda - In Progress
• Steps to Convert
• Common converter Issues
• Takeaways
• Eliminate all !! from your Kotlin code

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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.

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Common Converter Issues

• TypeCasting for the sake of Interoperability.
• Companion will add extra layer.
• If java method starting with getX(), converter looks for property with the name X.
• 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.

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TypeCasting for the sake of Interoperability

Kotlin is not Interoperable right away, but you need to do a lot of work around to make it Interoperable

Here is the Java class:

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

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

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

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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;
    }
}

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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
        }
    }
}

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

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

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

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Method names starting with get

Here is the Java class:

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

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

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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.

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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

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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

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Eliminate all !! from your Kotlin code

1. Use val instead of var

2. Use lateinit

3. Use let function

4. User Elivis operator

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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.

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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()
}

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private lateinit var adapter: RecyclerAdapter<Droids>

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

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

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Use let function

private var photoUrl: String? = null

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

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private var photoUrl: String? = null

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

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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"
   }
}

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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"
}

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Final tip

Don’t try to learn the whole language at once

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Questions?

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Thank You

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• Everything to start writing tests for Android App.
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• https://leanpub.com/android-testing/c/DEVFESTAHM