Creating the Screenlet Class

The Screenlet class is the central hub of a Screenlet. It contains attributes for configuring the Screenlet’s behavior, a reference to the Screenlet’s View, methods for invoking Interactor operations, and more. When using a Screenlet, app developers primarily interact with its Screenlet class. In other words, if a Screenlet were to become self-aware, it would happen in its Screenlet class (though we’re reasonably confident this won’t happen).

To make all this possible, your Screenlet class must implement the Interactor’s listener interface and extend Screens’s BaseScreenlet class with the View Model interface and Interactor class as type arguments. Your Screenlet class should also contain instance variables and accompanying getters and setters for the listener and any other attributes that the app developer needs to access. For constructors, you can call BaseScreenlet’s constructors.

For example, Add Bookmark Screenlet’s Screenlet class extends BaseScreenlet<AddBookmarkViewModel, AddBookmarkInteractor> and implements AddBookmarkListener. It also contains instance variables for AddBookmarkListener and the bookmark’s folder ID, and getters and setters for these variables. Also note the constructors call BaseScreenlet’s constructors:

public class AddBookmarkScreenlet extends 
    BaseScreenlet<AddBookmarkViewModel, AddBookmarkInteractor>
    implements AddBookmarkListener {

    private long folderId;
    private AddBookmarkListener listener;

    public AddBookmarkScreenlet(Context context) {

    public AddBookmarkScreenlet(Context context, AttributeSet attributes) {
        super(context, attributes);

    public AddBookmarkScreenlet(Context context, AttributeSet attributes, int defaultStyle) {
        super(context, attributes, defaultStyle);

    public long getFolderId() {
        return folderId;

    public void setFolderId(long folderId) {
        this.folderId = folderId;

    public AddBookmarkListener getListener() {
        return listener;

    public void setListener(AddBookmarkListener listener) {
        this.listener = listener;


Next, implement the Screenlet’s listener methods. This lets the Screenlet class receive the server call’s results and thus act as the listener. These methods should communicate the server call’s results to the View (via the View Model) and any other listener instances (via the Screenlet class’s listener instance). For example, here are Add Bookmark Screenlet’s listener method implementations:

public void onAddBookmarkSuccess() {

    if (listener != null) {

public void onAddBookmarkFailure(Exception e) {
    getViewModel().showFailedOperation(null, e);

    if (listener != null) {

These methods are called when the server call succeeds or fails, respectively. They first use getViewModel() to get a View Model instance and then call the BaseViewModel methods showFinishOperation and showFailedOperation to send the server call’s results to the View. The showFinishOperation call sends null because a successful server call to add a bookmark doesn’t return any objects. If a successful server call in your Screenlet returns any objects you need to display, then you should send them in this showFinishOperation call. The showFailedOperation call sends the Exception that results from a failed server call to the View. This lets you display an informative error to the user. The onAddBookmarkSuccess and onAddBookmarkFailure implementations then call the listener instance’s method of the same name. This sends the server call’s results to any other classes that implement the listener interface, such as the activity or fragment that uses the Screenlet.

Next, you must implement BaseScreenlet’s abstract methods:

  • createScreenletView: Reads the app developer’s Screenlet attribute settings, and inflates the View. You’ll use an Android TypedArray to retrieve the attribute settings. You should set the attribute values to the appropriate variables, and set any default values you need to display via a View Model reference.

    For example, Add Bookmark Screenlet’s createScreenletView method gets the app developer’s attribute settings via a TypedArray. This includes the layoutId, defaultTitle, and folderId attributes. The layoutId is used to inflate a View reference (view), which is then cast to a View Model instance (viewModel). The View Model instance’s setTitle method is then called with defaultTitle to set the bookmark’s default title. The method concludes by returning the View reference.

      protected View createScreenletView(Context context, AttributeSet attributes) {
          TypedArray typedArray = context.getTheme()
              .obtainStyledAttributes(attributes, R.styleable.AddBookmarkScreenlet, 0, 0);
          int layoutId = typedArray.getResourceId(R.styleable.AddBookmarkScreenlet_layoutId, 0);
          View view = LayoutInflater.from(context).inflate(layoutId, null);
          String defaultTitle = typedArray.getString(R.styleable.AddBookmarkScreenlet_defaultTitle);
          folderId = castToLong(typedArray.getString(R.styleable.AddBookmarkScreenlet_folderId));
          AddBookmarkViewModel viewModel = (AddBookmarkViewModel) view;
          return view;
  • createInteractor: Instantiates the Screenlet’s Interactor. For example, Add Bookmark Screenlet’s createInteractor method calls the AddBookmarkInteractor constructor to create a new instance of this Interactor:

      protected AddBookmarkInteractor createInteractor(String actionName) {
          return new AddBookmarkInteractor(getScreenletId());
  • onUserAction: Retrieves any data the user has entered in the View, and starts the Screenlet’s server operation via an Interactor instance. If your Screenlet doesn’t take user input, this method only needs to do the latter.

    The example Add Bookmark Screenlet takes user input (the bookmark’s URL and title), so its onUserAction method must retrieve this data. This method does so via a View Model instance it retrieves with the getViewModel() method. The onUserAction method starts the server operation by calling the Interactor’s start method with the user input. Note that the Interactor inherits the start method from the BaseInteractor class. Invoking the start method causes the Interactor’s execute method to run in a background thread:

      protected void onUserAction(String userActionName, AddBookmarkInteractor interactor, Object... args) {
          AddBookmarkViewModel viewModel = getViewModel();
          String url = viewModel.getURL();
          String title = viewModel.getTitle();
          interactor.start(url, title, folderId);

Nice! Your Screenlet is finished! You can now use it the same way you would any other. If you created your Screenlet in its own project, you can also package and distribute it via the Screens project, JCenter, or Maven Central.

To finish the Add Bookmark Screenlet example, the following section shows you how to use this Screenlet. It also shows how you can set default attribute values in an app’s server_context.xml file. Although you may not need to do this when using your Screenlets, it might come in handy on your way to becoming a master of Screenlets.

Creating the UI

Creating the Interactor

Defining the Attributes

Packaging Your Screenlets

« Defining the AttributesUsing Your Screenlet »
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