Implementation

Registration of content handlers, invocations

The classes needed for using the MIDlet suite are in the texthandler package.

The main class, TextHandler, implements a start screen that displays a list of the choices specified in the links.txt file. The list is implemented as a ChoiceList that has ChoiceBean items. A ChoiceBean instance has the attributes keyTitle (title of the choice) and choiceName (URL of the choice).

The TextHandler MIDlet is registered as a handler of plain text files at the startup by calling a custom register method:

    protected void startApp() throws MIDletStateChangeException {
        register();
    }

The startApp method creates a ContentHandlerServer instance using the Registry class. It also sets the RequestListener for any incoming Invocation:

    void register() {
        try {
            registry = Registry.getRegistry(TextHandler.CLASS_NAME);
            handler = registry.register(TextHandler.CLASS_NAME,
                    new String[]{TextHandler.CONTENT_TYPE}, null, null,
                    null, null, null);

            if (handler != null) {
                handler.setListener(this);
            }
            displayMessage(TextHandler.CONTENT_TYPE + " was successfully registered for this MIDlet!");
        }
        catch (Exception che) {
            //che.printStackTrace();
            displayMessage(TextHandler.CONTENT_TYPE + " registration failed!");
        }
    }

After the application startup and the registration, a ChoiceList form is displayed. The user can add URL links and execute them by selecting them on the list. When an URL is selected on the list, the doInvoke method of the TextHandler class is called. The doInvoke method creates an Invocation instance from the URL of the item (taken as a parameter) and invokes the correct handler for the content in the URL:

    void doInvoke(String url) {
        try {
            Invocation invoc = new Invocation(url);
            if (registry.invoke(invoc)) {
                destroyApp(true);
                notifyDestroyed();
            }
            else {
                //displayMessage("Could not link to " + url);
            }
        }
        catch (Exception ex) {
            //displayMessage("Could not link to " + url);
        }
    }

The TextHandler class itself functions as a RequestListener, so it has to implement the invocationRequestNotify method. It uses the ContentHandlerServer for handling incoming invocation requests. In this example, the current invocation is always finished before proceeding to the next one:

    public void invocationRequestNotify(ContentHandlerServer server) {

        if (invocation != null) {
            handler.finish(invocation, Invocation.OK);
        }
        invocation = handler.getRequest(false);
        if (invocation != null) {
            showCurrentInvocation(invocation);
        }
    }

When an invocation is completed., for example, when the user exits a handler showing content, the doFinish method is called (for example, in case of text content, from TextViewer). It finalizes the invocation process:

    void doFinish(int invocStatus) {
        if (invocStatus == Invocation.OK) {
            if (invocation != null) {
                invocation = null;
            }
        }
    }

When AudioHandler is executed, it registers itself as an audio content handler (similarly as TextHandler registers itself as a text content handler):

        try {
            registry = Registry.getRegistry(AudioHandler.CLASS_NAME);

            handler = registry.register(AudioHandler.CLASS_NAME,
                    new String[]{AudioHandler.CONTENT_TYPE_AUDIO}, null,
                    null, null, null, null);

            if (handler != null) {
                handler.setListener(this);
            }

        } catch (Exception che) {
            che.printStackTrace();
        }

So, if the requested content is audio, AudioHandler is invoked to handle it:

    public void invocationRequestNotify(ContentHandlerServer server) {

        if (invocation != null) {
            handler.finish(invocation, Invocation.OK);
        }

        invocation = handler.getRequest(false);

        if (invocation != null) {
            handleAudio(invocation);
        }
    }

If the content is an image, TextHandler invokes the native image content handler in the device (in the doInvoke method).