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Agera (Swedish for “to act”) is a super lightweight Android library that helps prepare data for consumption by the Android application components (such as Activities), or objects therein (such as Views), that have life-cycles in one form or another. It introduces a flavor of functional reactive programming, facilitates clear separation of the when, where and what factors of a data processing flow, and enables describing such a complex and asynchronous flow with a single expression, in near natural language.

Below is an sample demonstrating some of the features of Agera. This wiki together with the javadoc, explains how each part of Agera works.

public class AgeraActivity extends Activity
    implements Receiver<Bitmap>, Updatable {
  private static final ExecutorService NETWORK_EXECUTOR =
      newSingleThreadExecutor();
  private static final ExecutorService DECODE_EXECUTOR =
      newSingleThreadExecutor();
  private static final String BACKGROUND_BASE_URL =
      "http://www.gravatar.com/avatar/4df6f4fe5976df17deeea19443d4429d?s=";

  private Repository<Result<Bitmap>> background;
  private ImageView backgroundView;

  @Override
  protected void onCreate(final Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Set the content view
    setContentView(R.layout.activity_main);

    // Find the background view
    backgroundView = (ImageView) findViewById(R.id.background);

    // Create a repository containing the result of a bitmap request. Initially
    // absent, but configured to fetch the bitmap over the network based on
    // display size.
     background = repositoryWithInitialValue(Result.<Bitmap>absent())
        .observe() // Optionally refresh the bitmap on events. In this case never
        .onUpdatesPerLoop() // Refresh per Looper thread loop. In this case never
        .getFrom(new Supplier<HttpRequest>() {
          @NonNull
          @Override
          public HttpRequest get() {
            DisplayMetrics displayMetrics = getResources().getDisplayMetrics();
            int size = Math.max(displayMetrics.heightPixels,
                displayMetrics.widthPixels);
            return httpGetRequest(BACKGROUND_BASE_URL + size)
                .compile();
          }
        }) // Supply an HttpRequest based on the display size
        .goTo(NETWORK_EXECUTOR) // Change execution to the network executor
        .attemptTransform(httpFunction())
        .orSkip() // Make the actual http request, skip on failure
        .goTo(DECODE_EXECUTOR) // Change execution to the decode executor
        .thenTransform(new Function<HttpResponse, Result<Bitmap>>() {
          @NonNull
          @Override
          public Result<Bitmap> apply(@NonNull HttpResponse response) {
            byte[] body = response.getBody();
            return absentIfNull(decodeByteArray(body, 0, body.length));
          }
        }) // Decode the response to the result of a bitmap, absent on failure
        .onDeactivation(SEND_INTERRUPT) // Interrupt thread on deactivation
        .compile(); // Create the repository
  }

  @Override
  protected void onResume() {
    super.onResume();
    // Start listening to the repository, triggering the flow
    background.addUpdatable(this);
  }

  @Override
  protected void onPause() {
    super.onPause();
    // Stop listening to the repository, deactivating it
    background.removeUpdatable(this);
  }

  @Override
  public void update() {
    // Called as the repository is updated
    // If containing a valid bitmap, send to accept below
    background.get().ifSucceededSendTo(this);
  }

  @Override
  public void accept(@NonNull Bitmap background) {
    // Set the background bitmap to the background view
    backgroundView.setImageBitmap(background);
  }
}