Kotlin After 9 Months

KOTLIN AFTER 9 MONTHS
André Luiz Cardoso
Thiago “Fred" Porciúncula

Kotlin | What is it?
- JetBrains
- JVM
- Concise, Safe, Versatile, Interoperable

Gains | Synthetic properties
closeButton = (Button) findViewById(R.id.close_button); 
closeButton.setOnClickListener(new View.OnClickListener() { 
@Override 
public void onClick(View v) { 
presenter.close(); 
} 
});

closeButton = findViewById(R.id.close_button); 
closeButton.setOnClickListener(new View.OnClickListener() { 
@Override 
public void onClick(View v) { 
presenter.close(); 
} 
});

closeButton.setOnClickListener { presenter.close() }

Gains | Lambdas
moviesApi.loadMovies(genre) 
.map { it.movies }
.map { it.rating > 4 } 
.subscribe({ goodMovies -> 
goodMovies?.let { view.showMovies(it) } 
}, { 
Log.e(TAG, "Error loading movies from $genre", it) 
})

Gains | Collections
movies.filter { it.isFavorite }
movies.sortedWith(compareBy(
Movie::year, Movie::title, Movie::rating))
val movies = listOf("ET", “Scream")
val rating = listOf(5, 1)
print(movies.zip(rating))
//[(ET, 5), (Tomates Assassinos, 1)]

Gains | Collections
var horrorMoviesTitles = movies
.asSequence()
.filter { it.genre == "horror" }
.map { it.title }
.toList()

Gains | Null safety
public String getUrl(Movie movie) { 
if (movie == null || movie.getPosters() == null) { 
return fallbackUrl; 
} 
 
for (Poster poster : movie.getPosters()) { 
if (poster.type.equals("P") && poster.getUrl() != null) { 
return poster.getUrl(); 
} 
} 
 
return fallbackUrl; 
}

Gains | Null safety
val url =
movie?.posters?.find { it.type == "P" }?.url ?: fallbackUrl

Gains | Extension functions
val Context.isLandscape: Boolean get() =
resources.configuration.orientation == Configuration.ORIENTATION_LANDSCAPE
fun View.fadeIn() { 
visibility = View.VISIBLE 
ViewCompat.animate(this) 
.alpha(1F) 
.start( 
}
var View.visible: Boolean 
get() = visibility == View.VISIBLE 
set(value) { 
visibility = if (value) View.VISIBLE else View.GONE 
}

Gains | Interoperability
Glide 
.with(this) 
.load(url) 
.diskCacheStrategy(DiskCacheStrategy.SOURCE) 
.error(R.drawable.default_poster) 
.into(posterImageView)

Gains | Interoperability
// MovieExtensions.kt 
fun Movie.isNew(currentDate: Date): Boolean { 
return launchDate?.let { it.daysBefore(currentDate) < 7 } ?: false 
}
// SomeFile.java
boolean isNew = MovieExtensions.isNew(movie);

Gains | Data classes
data class Movie(
val title: String,  
val year: Int,
val rating: Float
)
val et = Movie(title = "ET", year = 1982, rating = 5.0f)
val (title, year) = et
moviesSet.add(et)

Gains | Immutability
val movies = mutableListOf<Movie>()
movies.add(et)
val immutableMovies = listOf<Movie>()
immutableMovies.add(et)
var movie = et
movie = et.copy(title = "Alien")

Gains | Sealed classes
sealed class MovieViewState {
data class Loaded(val movies: List<Movie>): MovieViewState()
class Error: MovieViewState()
class Loading: MovieViewState()
}

Gains | Sealed classes
fun render(viewState: MovieViewState) {
return when(viewState) {
is MovieViewState.Loaded -> print(viewState.movies)
}
}
//'when' expression must be exhaustive, add necessary 'is Error', 'is
Loading' branches or 'else' branch instead

Gains | Multiple classes per file
// MoviesInjection.kt
@ActivityScope 
@Subcomponent(modules = arrayOf(MovieModule::class)) 
interface MovieComponent { 
fun inject(activity: MovieActivity) 
} 
 
@Module 
class MovieModule(private val view: Movie.MovieView) { 
 
@Provides @ActivityScope 
fun provideMoviePresenter(movieLoader: MovieLoader): Movie.Presenter { 
return MoviePresenter(view, movieLoader) 
} 
}

Gains | Multiple classes per file
// MoviesMvp.kt
interface MoviesView {
fun showMovies(movies: List<Movie>)
}
interface MoviesPresenter {
fun loadMovies()
}

Pains | More methods
trouble writing output:
Too many field references: 131000; max is 65536.
You may try using --multi-dex option.

Pains | Crest of the wave
moviesApi.loadMovies(genre) 
.map { it.movies }

Pains | Abuse
fun Context.checkSelfPermissionCompat(permission: String): Int { 
return ContextCompat.checkSelfPermission(this, permission) 
}

Tips | Starting with Java works

Tips | Avoid writing Kotlin as Java
if (movieTitle == "Guardians of the galaxy") { 
movie.setGenre(comedy) 
} else if (movieRating > 4) { 
movie.setGenre(good) 
} else { 
movie.setGenre(others) 
}
if (movie != null) { 
presenter.list(movie.getGenre()) 
}

Tips | Avoid writing Kotlin as Java
movie.genre = when { 
movieTitle == "Guardians of the galaxy" -> comedy
movieRating > 4 -> good
else -> others 
}
movie?.let { 
presenter.list(it.genre) 
}

Tips | JVM annotations
class MovieTextView @JvmOverloads constructor(
context: Context,
attrs: AttributeSet? = null,
defStyleAttr: Int = 0): TextView(context, attrs, defStyleAttr)
data class Movie(val title: String) : Parcelable {
// ...
companion object {
@JvmField val CREATOR = object: Parcelable.Creator<Movie> {
// ...
}
}
}

Tips | JVM annotations
@file:JvmName(“ExtensionsUtils")
companion object { 
@JvmStatic fun newInstance(tourCode: String): Fragment { 
// ... 
} 
}

Tips | Adopt a code standard
class MoviesPresenter( 
movies: List<Movie>, 
private val moviesApi: MoviesApi, 
override val view: Movies.View, 
router: Router, 
private val user: User, 
override val shareManager: ShareManager 
): BaseMoviesPresenter(movies, router, user), Movies.Presenter { … }

@file:JvmName(“ImageFormatUtil”)
…
ImageFormatUtil.isGif(file);
vs
@file:JvmName(“ImageFormatUtil”)
…
ImageFormatUtilKt.isGif(file);

titleTextView.text = movie.title
vs
title_text_view.text = movie.title

import kotlinx.android.synthetic.main.movie_activity.title_text_view
as titleTextView

Tips | Utils turn into extensions
fun Date?.format(pattern: String): String { 
return this?.let { SimpleDateFormat(pattern, Locale.US).format(it) } ?: "" 
}
public class DateUtils { 
public String format(Date date, String pattern) { 
if (date == null) { 
return ""; 
} 
return new SimpleDateFormat(pattern, Locale.US).format(date); 
} 
}

Tips | Watch out for nulls coming from Java
val movie = Movie() // Classe Java
 
movie.genre = null 
movie.genre.toString() // NPE

Tips | Null annotations
// Java 
public static void prettyPrint(@NonNull String message) { 
System.out.print(message); 
} 
 
public @Nullable Rating getRating() { … }
// Kotlin 
prettyPrint(null) 
val x: Rating = Test.getRating()

// Java 
public static void prettyPrint(@NonNull String message) { 
System.out.print(message); 
} 
 
public @Nullable Rating getRating() { … }
// Kotlin 
prettyPrint("oi") 
val x: Rating? = Test.getRating()

@javax.annotation.ParametersAreNonnullByDefault
package com.arctouch.movies;

Tips | Named parameters
val view = LayoutInflater.from(parent.context).inflate( 
resource = R.layout.listitem_diagram_showcase_category, 
root = parent, 
attachToRoot = false)

Tips | Builders
val movie = Movie( 
title = "Back to the Future 4", 
year = 2030, 
rating = 5f 
) 
 
val movie = Movie().apply { 
title = "Back to the Future 4" 
year = 2030 
rating = 5f 
}

Tips | Interface default implementations
// Kotlin
interface MovieHandler {
fun handleMovie() {
print("default handling")
}
}
// Java
class JavaMovieHandler implements MovieHandler {
}

Tips | Annotation use-site target
data class SomeDataClass( 
@field:SomeAnnotation 
@get:[Annotation1 Annotation2] 
@set:YetAnotherAnnotation 
@param:LastAnnotation 
var someProperty: SomeType,
…

Tips | Dagger
MoviesComponent.java:38: error:
io.reactivex.Observable<error.NonExistentClass> cannot be provided
without an @Provides- or @Produces-annotated method.

Tips | Java 8 lambda
val map = mapOf("Java" to "Kotlin")
map.forEach { key, value -> print("Java 8") }
map.forEach { (key, value) -> print("Kotlin") }

Tips | Kotlin Standard Library in Java
goodMovies = CollectionsKt.minus(movies, badMovies);
List<Movie> shortMovies =
CollectionsKt.filter(movies, movie -> movie.length() > 100); 
shortMoviesTitles =
CollectionsKt.map(shortMovies, Movie::getTitle);

Tips | Unit leaking to Java
// Kotlin 
val movieLoadedSubject: Subject<Unit> = PublishSubject.create<Unit>() 
 
movieLoadedSubject.subscribe { 
// no arg 
} 
// Java
movieLoadedSubject.subscribe(unit -> { 
// can't ignore Unit arg :( 
});

HM… SHOULD WE
REALLY USE IT?

Kotlin After 9 Months

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