Writing a Scala API for Android

Today we look in more detail at the scala-android.jar library we mentioned at the end of our article "Translating Java code to Scala". At this stage of its development that library provides basic functionality but should support more advanced features in a next future as does the scala-swing.jar library (part of the standard Scala software distribution).

A closer look at scala-android.jar

The scala-android.jar library exists in two slightly different versions: the android-8 version is actually a superset of android-7 version and provides a few API additions (mostly constants) introduced in release 2.2 of the Android platform.

android-examples> tree scala-framework/android/src/
scala-framework/android/src/
|-- android-7
|   `-- scala
|       `-- android
|           |-- app
|           |   |-- Activity$.java
|           |   `-- Activity.scala
|           |-- maps
|           |   |-- Overlay$.java
|           |   `-- Overlay.scala
|           |-- package.scala
|           `-- provider
|               |-- Contacts.java
|               `-- ContactsContract.java
`-- android-8
    `-- <same hierarchy as in "android-7">

12 directories, 14 files

• The Activity class declares convience methods such as findButton, findTextView, etc. in addition to the findViewById method and implicit conversions for coding common callback methods such as onClick, onCheckedChanged, etc. in a concise way (see example 2 below).
• The Overlay class declares one single compatibility method, drawAt, which provides a bridge to the corresponding (yet unaccessible) Java method of the Google Maps API (see article "Translating Java code to Scala").
• The deprecated class Contacts is provided for convenience only and both Java and Scala programmers are strongly encouraged to use the ContactsContract class instead.
• Finally, the package object android — defined in the file android/package.scala by convention — declares convenience methods for helping the programmer to write more concise code.

Example 1

Wrapper objects such as scala.android.provider.ContactsContract give direct access to inherited Java constants defined in Java static inner interfaces. For instance, in the following Scala code we access the constant Phone.TYPE directly as in Java although it is actually defined in the static inner interface CommonColumns (see the blog entry "Scala on Android Gotcha: HelloGridView Error" for more details).

// ContactAdder.java
import android.provider.ContactsContract.CommonDataKinds.Phone;
//..
ops.add(ContentProviderOperation.newInsert(ContactsContract.Data.CONTENT_URI)
        .withValueBackReference(ContactsContract.Data.RAW_CONTACT_ID, 0)
        .withValue(ContactsContract.Data.MIMETYPE,
                   Phone.CONTENT_ITEM_TYPE)
        .withValue(Phone.NUMBER, phone)
        .withValue(Phone.TYPE, phoneType)
        .build());

// ContactAdder.scala
import scala.android.provider.ContactsContract.CommonDataKinds.Phone
//..
ops.add(ContentProviderOperation.newInsert(ContactsContract.Data.CONTENT_URI)
        .withValueBackReference(ContactsContract.Data.RAW_CONTACT_ID, 0)
        .withValue(ContactsContract.Data.MIMETYPE,
                   Phone.CONTENT_ITEM_TYPE)
        .withValue(Phone.NUMBER, phone)
        .withValue(Phone.TYPE, phoneType)
        .build())

Example 2

Boilerplate code can be reduced using Scala implicit conversions. Thus passing event handlers to methods setOnItemSelectedListener and setOnClickListener can be expressed in a more concise way by the Scala programmer (see the StackOverflow blog entry "What is in scala-android.jar ?" for an example).

// ContactAdder.java
import android.app.Activity;
//..
mAccountSpinner.setOnItemSelectedListener(new OnItemSelectedListener() {
    public void onItemSelected(AdapterView<?> parent, View view,
                               int position, long i) {
        updateAccountSelection();
    }
    public void onNothingSelected(AdapterView<?> parent) {
        // We don't need to worry about nothing being selected,
        // since Spinners don't allow this.
    }
});
mContactSaveButton.setOnClickListener(new View.OnClickListener() {
    public void onClick(View v) {
        onSaveButtonClicked();
    }
});
private void onSaveButtonClicked() { /* .. */ }

// ContactAdder.scala
import scala.android.app.Activity
//..
mAccountSpinner setOnItemSelectedListener {
  updateAccountSelection()
}
mContactSaveButton setOnClickListener {
  onSaveButtonClicked()
}

Note: In case the Scala programmer needs to provide some code for the onNothingSelected event he can simply write:

mAccountSpinner setOnItemSelectedListener {
  updateAccountSelection()
} orNothing {
  // onNothingSelected code
}

Example 3

to be done

About the Author

Stéphane Micheloud is a senior software engineer. He holds a Ph.D in computer science from EPFL and a M.Sc in computer science from ETHZ. At EPFL he worked on distributed programming and advanced compiler techniques and participated for over six years to the Scala project. Previously he was professor in computer science at HES-SO // Valais in Sierre, Switzerland.

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Other Articles

• Targeting Scala to the Android platform (May 2010).
• Shrinking the Scala library code (May 2010).
• Ant scripting in the Android SDK (June 2010).
• Tweaking the Android emulator (June 2010).
• Translating Java code to Scala (July 2010).

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Last updated: July 28, 2010.

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