Scala on Android

@jberkel

/me

• From Java to Ruby and back
• Work on the official SoundCloud Android app
• <3 Android open source
• Would love to see more people use Scala on Android

Quick Android recap

• From 1.0 to 4.0 in < 4 years
• 700k activations every day
• 10 billion app downloads (Dec 2011)

Basics

• at the core: Linux + DalvikVM (not JVM!)
• Officially supported: Java + C/C++ (NDK)

What's this Dalvik thing?

• Dalvik bytecode != JVM bytecode
• optimised for smaller devices
• one VM per process (forked)
• used to be very slow, JIT since 2.2

Not just Java

Why Scala on Android?

Android is Java framework design, ca. 2001

public class MyActivity extends android.app.Activity {

  @Override public void onCreate(Bundle b) {
    super.onCreate(b);
    BatteryHelper.initialize(this);
  }
}

Traits to split concerns

trait BatteryAware extends Activity {
  override def onCreate(b: Bundle) {
    // register battery handler
  }
  override def onPause() { ... }
}

class MyActivity extends Activity
  with BatteryAware
  with Logger {
}

Android & APIs

actually not so bad.

But sometimes awkward

// database lookup
Cursor c = resolver.query(...);
List<MyModel> list = new ArrayList<MyModel>();
// iterate over rows and create objects
while (c != null && c.moveToNext()) {
    list.add(MyModel.fromCursor(c));
}
if (c != null) c.close(); // release resource
return list;

ugh.

Let's make a nice functional Cursor

class NiceCursor(c: Cursor) extends Iterable[Cursor] {
  def iterator = new Iterator[Cursor] {
    def hasNext = c.getCount > 0 && !c.isLast
    def next() = { c.moveToNext(); c }
  }
}
implicit def NiceCursor(c: Cursor) = new NiceCursor(c)

and a better query interface

def query[T](uri: Uri)(fun: Cursor => T) = {
    val cursor = resolver.query(uri)
    try {
      fun(cursor)
    } finally {
      cursor.close()
    }
}
// that's better
val list = query(...)(_.map(MyModel.fromCursor(_)))

shorter callbacks

...pimp my GpsStatusListeners

locationManager.addGpsStatusListener(
  new GpsStatusListener() {
    public void onGpsStatusChanged(int evt) {
      System.out.printn(evt)
    }
  }
}
// vs.
locationManager.addGpsStatusListener(println(_))

Testing on Android

...sucks.

• slow
• many devices & versions
• verbose test code

Scala for testing

ScalaMock

• Uses a compiler plugin to generate mocks
• Currently not able to mock Android core classes

github.com/paulbutcher/scalamock

robolectric / robospecs

• Testable reimplementation of the SDK (don't ask)
• Classloading trickery allow test execution on development machine
• Works with specs2 and ScalaTest

github.com/pivotal/robolectric github.com/jbrechtel/robospecs

ScalaTest Example

lazy val track = ...

it should "insert an read back a track" in {
  val uri = provider.insert(Content.TRACKS.uri,
    track.buildContentValues())

  val read = query(uri, 1)(_.map(new Track(_))).head
  read.id should equal(track.id)
  read.title should equal(track.title)
  // etc.
}

Problems with Scala

many runtime assumptions based on JVM:

• object allocation is cheap
• efficient and fast GC

However, Android developer guidelines say:

Avoid Creating Unnecessary Objects

But performance is improving (parallel GC, JIT etc)

Problems #2

Scala programs need a runtime lib

$ ls -lh scala-library.jar
rw-r--r--  jan  wheel  8.5M 24 May  2011 scala-library.jar

$ jar tfv scala-library.jar | wc -l
  5480

Solution

• use proguard / treeshaker before dexing
• preinstall Scala libs on the phone

Proguard is slow, long dev cycles

Only run it before releasing

Problems #3

Scala / Java / Android interop problems

issues.scala-lang.org/browse/SI-4620

But Android is "open" (sometimes...)

android-review.googlesource.com/#/c/30900/

Parcelables

public class Foo {
  public static final Parcelable.Creator<Foo> CREATOR =
    new Parcelable.Creator<Foo>() {
      public Foo createFromParcel(Parcel in) {
          return new Foo(in);
      }

      public Foo[] newArray(int size) {
          return new Foo[size];
      }
  };
 }

You cannot do this in Scala!

When to use Scala

• Suitable for most types of apps
• But not good for realtime (i.e. games)
• Lack of experience / libraries
• If unsure, start with Scala tests

Tools

• sbt-android-plugin
• Intellij IDEA CE (fsc + Scala / Android facets)
• positronic lib

• github.com/jberkel/android-plugin
• www.jetbrains.com/idea/download/
• github.com/rst/positronic_net

How to get started

Demo time...

Thanks!

need help? ask scala-on-android

groups.google.com/forum/#!forum/scala-on-android jberkel.github.com/presentation-scala-on-android