Migrating FOSDEM Companion to Kotlin Christophe Beyls - @BladeCoder FOSDEM 2020

FOSDEM Companion ● Most popular FOSDEM Schedule app for Android ● Source code available on GitHub https://github.com/cbeyls/fosdem-companion-android ● Available from: 3500-4000 active users no usage data

1. History / Changelog 2014 - 2020

History / Changelog 1/2 2015 Material Design Lightning talk at FOSDEM 2016 Vector Drawables 2010 Original fosdem-android app release 2014 First release of FOSDEM Companion on Google Play Store and F-Droid. Uses Appcompat, fragments and loaders everywhere [minSDK 7]

2/2 2019 AndroidX, new database layer (Room), Material components library, Preferences library [minSDK 16] 2020 Dark theme, OkHttp, ViewPager2, fully rewritten in Kotlin. [minSDK 17] 2017 Animated Vector Drawables, RecyclerView New feature: Bookmarks export 2018 Architecture components (ViewModel & LiveData) New feature: Room status [minSDK 15]

Code rewrite process Fragments Activities Custom widgets Remote data source & Parsers Database Layer Adapters ViewModels Services & ContentProviders Utility classes Model & Entities

Android Studio code refactoring help

2. Cleaning up the code Examples of Kotlin features

public static String remove(String str, final char remove) { if (TextUtils.isEmpty(str) || str.indexOf(remove) == -1) { return str; } final char[] chars = str.toCharArray(); int pos = 0; for (int i = 0; i < chars.length; i++) { if (chars[i] != remove) { chars[pos++] = chars[i]; } } return new String(chars, 0, pos); } StringUtils.java

Extensions functions + Kotlin standard library fun String.remove(remove: Char): String { return if (remove !in this) { this } else { filterNot { it == remove } } }

public class DownloadScheduleResult { private static final DownloadScheduleResult RESULT_ERROR = new DownloadScheduleResult(0); private static final DownloadScheduleResult RESULT_UP_TO_DATE = new DownloadScheduleResult(0); private final int eventsCount; private DownloadScheduleResult(int eventsCount) { this.eventsCount = eventsCount; } public static DownloadScheduleResult success(int eventsCount) { return new DownloadScheduleResult(eventsCount); } public static DownloadScheduleResult error() { return RESULT_ERROR; } public static DownloadScheduleResult upToDate() { return RESULT_UP_TO_DATE; } public boolean isSuccess() { return this != RESULT_ERROR && this != RESULT_UP_TO_DATE; } public boolean isError() { return this == RESULT_ERROR; } public boolean isUpToDate() { return this == RESULT_UP_TO_DATE; } public int getEventsCount() { return eventsCount; } }

Sealed classes sealed class DownloadScheduleResult { class Success(val eventsCount: Int) : DownloadScheduleResult() object Error : DownloadScheduleResult() object UpToDate : DownloadScheduleResult() }

@Query("SELECT e.id, e.start_time, e.end_time, e.room_name, e.slug, et.title, et.subtitle, e.abstract, e.description" + ", GROUP_CONCAT(p.name, ', ') AS persons, e.day_index, d.date AS day_date, e.track_id, t.name AS track_name, t.type AS track_type" + ", b.event_id IS NOT NULL AS is_bookmarked" + " FROM events e" + " JOIN events_titles et ON e.id = et.`rowid`" + " JOIN days d ON e.day_index = d.`index`" + " JOIN tracks t ON e.track_id = t.id" + " LEFT JOIN events_persons ep ON e.id = ep.event_id" + " LEFT JOIN persons p ON ep.person_id = p.`rowid`" + " LEFT JOIN bookmarks b ON e.id = b.event_id" + " WHERE e.day_index = :day AND e.track_id = :track" + " GROUP BY e.id" + " ORDER BY e.start_time ASC") public abstract LiveData> getEvents(Day day, Track track); ScheduleDao.java

Multiline strings @Query("""SELECT e.id, e.start_time, e.end_time, e.room_name, e.slug, et.title, et.subtitle, e.abstract, e.description, GROUP_CONCAT(p.name, ', ') AS persons, e.day_index, d.date AS day_date, e.track_id, t.name AS track_name, t.type AS track_type, b.event_id IS NOT NULL AS is_bookmarked FROM events e JOIN events_titles et ON e.id = et.`rowid` JOIN days d ON e.day_index = d.`index` JOIN tracks t ON e.track_id = t.id LEFT JOIN events_persons ep ON e.id = ep.event_id LEFT JOIN persons p ON ep.person_id = p.`rowid` LEFT JOIN bookmarks b ON e.id = b.event_id WHERE e.day_index = :day AND e.track_id = :track GROUP BY e.id ORDER BY e.start_time ASC""") abstract fun getEvents(day: Day, track: Track): LiveData>

3. Simplifying model classes “The best code is no code”

public class Link implements Parcelable { public static final String TABLE_NAME = "links"; @PrimaryKey(autoGenerate = true) private long id; @ColumnInfo(name = "event_id") private long eventId; @NonNull private String url; private String description; public Link() { } public long getId() { return id; } public void setId(long id) { this.id = id; } public long getEventId() { return eventId; } public void setEventId(long eventId) { this.eventId = eventId; }

@NonNull public String getUrl() { return url; } public void setUrl(@NonNull String url) { this.url = url; } public String getDescription() { return description; } public void setDescription(String description) { this.description = description; } @Override public int hashCode() { return url.hashCode(); } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; Link other = (Link) obj; return url.equals(other.url); }

@Override public int describeContents() { return 0; } @Override public void writeToParcel(Parcel out, int flags) { out.writeLong(id); out.writeLong(eventId); out.writeString(url); out.writeString(description); } public static final Parcelable.Creator CREATOR = new Parcelable.Creator() { public Link createFromParcel(Parcel in) { return new Link(in); } public Link[] newArray(int size) { return new Link[size]; } }; Link(Parcel in) { id = in.readLong(); eventId = in.readLong(); url = in.readString(); description = in.readString(); } }

Data class + Parcelize = @Parcelize data class Link( @PrimaryKey(autoGenerate = true) val id: Long = 0L, @ColumnInfo(name = "event_id") val eventId: Long, val url: String, val description: String? ) : Parcelable { companion object { const val TABLE_NAME = "links" } }

https://medium.com/@BladeCoder More about Parcelize

4. Null safety

void bind(@NonNull Event event, boolean isBookmarked) { Context context = itemView.getContext(); this.event = event; time.setText(timeDateFormat.format(event.getStartTime())); title.setText(event.getTitle()); Drawable bookmarkDrawable = isBookmarked ? AppCompatResources.getDrawable(context, R.drawable.ic_bookmark_24dp) : null; TextViewCompat.setCompoundDrawablesRelativeWithIntrinsicBounds(title, null, null, bookmarkDrawable, null); ... NullPointerException

Null safety fun bind(event: Event, isBookmarked: Boolean) { val context = itemView.context this.event = event time.text = event.startTime?.let { timeDateFormat.format(it) } title.text = event.title val bookmarkDrawable = if (isBookmarked) AppCompatResources.getDrawable(context, R.drawable.ic_bookmark_24dp) else null TextViewCompat.setCompoundDrawablesRelativeWithIntrinsicBounds(title, null, null, bookmarkDrawable, null) ...

Prefer non-null and read-only fields When a field is non-null: ● You don’t have to check for nullability ... ● … but you have to assign an initial value: ○ Immediately val a = x() ○ Later: ■ lateinit var a: MyType (☠ nullable in disguise) ■ val a: MyType by lazy { x() }

A good way to read Fragment arguments class EventDetailsFragment : Fragment() { private val viewModel: EventDetailsViewModel by viewModels() private var holder: ViewHolder? = null val event by lazy(LazyThreadSafetyMode.NONE) { requireArguments().getParcelable(ARG_EVENT) } ... }

5. Android KTX “A set of Kotlin extensions that are included with Android Jetpack and other Android libraries.”

KTX Delegated properties for ViewModel class EventDetailsFragment : Fragment() { private val viewModel: EventDetailsViewModel by viewModels() private var holder: ViewHolder? = null val event by lazy(LazyThreadSafetyMode.NONE) { requireArguments().getParcelable(ARG_EVENT)!! }

// Auto refresh during the day passed as argument private final LiveData scheduler = Transformations.switchMap(dayTrackLiveData, dayTrack -> { final long dayStart = dayTrack.first.getDate().getTime(); return LiveDataFactory.scheduler(dayStart, dayStart + DateUtils.DAY_IN_MILLIS); }); private final LiveData currentTime = Transformations.switchMap(scheduler, isOn -> { if (isOn) { return Transformations.map( LiveDataFactory.interval(REFRESH_TIME_INTERVAL, TimeUnit.MILLISECONDS), count -> System.currentTimeMillis() ); } return new MutableLiveData<>(-1L); }); Then: LiveData transformations methods

Now: KTX extension functions for LiveData val currentTime: LiveData = dayTrackLiveData .switchMap { (day, _) -> // Auto refresh during the day passed as argument val dayStart = day.date.time LiveDataFactory.scheduler(dayStart, dayStart + DateUtils.DAY_IN_MILLIS) } .switchMap { isOn -> if (isOn) { LiveDataFactory.interval(REFRESH_TIME_INTERVAL, TimeUnit.MILLISECONDS) .map { System.currentTimeMillis() } } else { MutableLiveData(-1L) } }

Now: KTX extension functions for LiveData val currentTime: LiveData = dayTrackLiveData .switchMap { (day, _) -> // Auto refresh during the day passed as argument val dayStart = day.date.time LiveDataFactory.scheduler(dayStart, dayStart + DateUtils.DAY_IN_MILLIS) } .switchMap { isOn -> if (isOn) { LiveDataFactory.interval(REFRESH_TIME_INTERVAL, TimeUnit.MILLISECONDS) .map { System.currentTimeMillis() } } else { MutableLiveData(-1L) } }

final SpannableStringBuilder sb = new SpannableStringBuilder(); int length = 0; for (Person person : persons) { if (length != 0) { sb.append(", "); } final String name = person.getName(); sb.append(name); length = sb.length(); sb.setSpan(new PersonClickableSpan(person), length - name.length(), length, Spannable.SPAN_INCLUSIVE_EXCLUSIVE); } final CharSequence clickablePersonsSummary = sb.toString(); Then: SpannableStringBuilder hell

Now: KTX functions for Spannable val clickablePersonsSummary = buildSpannedString { for (person in persons) { if (length != 0) { append(", ") } inSpans(PersonClickableSpan(person)) { append(person.name) } } }

Now: KTX functions for Spannable val clickablePersonsSummary = buildSpannedString { for (person in persons) { if (length != 0) { append(", ") } inSpans(PersonClickableSpan(person)) { append(person.name) } } }

6. Coroutines

Easy and safe thread switching @MainThread private suspend fun downloadScheduleInternal(context: Context) { _downloadScheduleProgress.value = -1 val res = withContext(Dispatchers.IO) { ... } _downloadScheduleProgress.value = 100 _downloadScheduleResult.value = SingleEvent(res) }

Easy and safe thread switching @MainThread private suspend fun downloadScheduleInternal(context: Context) { _downloadScheduleProgress.value = -1 val res = withContext(Dispatchers.IO) { ... } _downloadScheduleProgress.value = 100 _downloadScheduleResult.value = SingleEvent(res) } background thread

Context preservation @MainThread private suspend fun downloadScheduleInternal(context: Context) { _downloadScheduleProgress.value = -1 val res = withContext(Dispatchers.IO) { ... } _downloadScheduleProgress.value = 100 _downloadScheduleResult.value = SingleEvent(res) } main thread

public abstract class IterableAbstractPullParser extends AbstractPullParser> { private class ParserIterator implements Iterator { // Removed for brevity. Calls parseHeader(), parseNext() and parseFooter() } @Override protected Iterable parse(final XmlPullParser parser) throws Exception { return () -> new ParserIterator(parser); } protected abstract boolean parseHeader(XmlPullParser parser) throws Exception; protected abstract T parseNext(XmlPullParser parser) throws Exception; protected void parseFooter(XmlPullParser parser) throws Exception { while (!isEndDocument()) { parser.next(); } } } Then: complex streaming parser

public abstract class IterableAbstractPullParser extends AbstractPullParser> { private class ParserIterator implements Iterator { // Removed for brevity. Calls parseHeader(), parseNext() and parseFooter() } @Override protected Iterable parse(final XmlPullParser parser) throws Exception { return () -> new ParserIterator(parser); } protected abstract boolean parseHeader(XmlPullParser parser) throws Exception; protected abstract T parseNext(XmlPullParser parser) throws Exception; protected void parseFooter(XmlPullParser parser) throws Exception { while (!isEndDocument()) { parser.next(); } } } Then: complex streaming parser

Now: Kotlin Sequence builder override fun parse(source: XmlPullParser): Sequence { return sequence { while (!parser.isEndDocument) { if (parser.isStartTag("schedule")) { while (!parser.isNextEndTag("schedule")) { if (parser.isStartTag) { when (parser.name) { "event" -> yield(parseEvent(parser)) else -> parser.skipToEndTag() } } } } parser.next() } } }

Now: Kotlin Sequence builder override fun parse(parser: XmlPullParser): Sequence { return sequence { while (!parser.isEndDocument) { if (parser.isStartTag("schedule")) { while (!parser.isNextEndTag("schedule")) { if (parser.isStartTag) { when (parser.name) { "event" -> yield(parseEvent(parser)) else -> parser.skipToEndTag() } } } } parser.next() } } }

The case of the delayed drawer close navigationView.setNavigationItemSelectedListener( menuItem -> { pendingNavigationMenuItem = menuItem; drawerLayout.closeDrawer(navigationView); return true; }); Callback #1

drawerLayout.addDrawerListener(new DrawerLayout.SimpleDrawerListener() { @Override public void onDrawerClosed(View drawerView) { if (pendingNavigationMenuItem != null) { handleNavigationMenuItem(pendingNavigationMenuItem); pendingNavigationMenuItem = null; } } }); The case of the delayed drawer close Callback #2

@Override protected void onSaveInstanceState(@NonNull Bundle outState) { // Ensure no fragment transaction occurs after onSaveInstanceState() if (pendingNavigationMenuItem != null) { pendingNavigationMenuItem = null; if (currentSection != null) { navigationView.setCheckedItem(currentSection.getMenuItemId()); } } super.onSaveInstanceState(outState); } The case of the delayed drawer close Callback #3

https://medium.com/androiddevelopers/suspending-over-views-19de9ebd7020 Using coroutines to simplify UI logic

Suspending over DrawerLayout suspend fun DrawerLayout.awaitCloseDrawer(drawerView: View) = suspendCancellableCoroutine { continuation -> val listener = object : DrawerLayout.SimpleDrawerListener() { ... } continuation.invokeOnCancellation { removeDrawerListener(listener) } addDrawerListener(listener) closeDrawer(drawerView) }

suspend fun DrawerLayout.awaitCloseDrawer(drawerView: View) = suspendCancellableCoroutine { continuation -> continuation.invokeOnCancellation { removeDrawerListener(listener) } addDrawerListener(listener) closeDrawer(drawerView) } val listener = object : DrawerLayout.SimpleDrawerListener() { ... } Suspending over DrawerLayout

Suspending over DrawerLayout val listener = object : DrawerLayout.SimpleDrawerListener() { override fun onDrawerClosed(drawerView: View) { removeDrawerListener(this) continuation.resume(Unit) } override fun onDrawerStateChanged(newState: Int) { if (newState == DrawerLayout.STATE_DRAGGING) { continuation.cancel() } } }

Now: single callback launching a coroutine navigationView.setNavigationItemSelectedListener { menuItem: MenuItem -> lifecycleScope.launchWhenStarted { try { drawerLayout.awaitCloseDrawer(navigationView) handleNavigationMenuItem(menuItem) } catch (e: CancellationException) { // reset the menu to the current selection navigationView.setCheckedItem(currentSection.menuItemId) } } true }

Now: single callback launching a coroutine navigationView.setNavigationItemSelectedListener { menuItem: MenuItem -> lifecycleScope.launchWhenStarted { try { drawerLayout.awaitCloseDrawer(navigationView) handleNavigationMenuItem(menuItem) } catch (e: CancellationException) { // reset the menu to the current selection navigationView.setCheckedItem(currentSection.menuItemId) } } true } KTX extensions

Now: single callback launching a coroutine navigationView.setNavigationItemSelectedListener { menuItem: MenuItem -> lifecycleScope.launchWhenStarted { try { drawerLayout.awaitCloseDrawer(navigationView) handleNavigationMenuItem(menuItem) } catch (e: CancellationException) { // reset the menu to the current selection navigationView.setCheckedItem(currentSection.menuItemId) } } true }

Conclusion: a few metrics

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