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J2ME
Introduction to J2ME
J2ME Development Kit
Understanding the Process of MIDlet
The MIDlet Lifecycle
User Interface Architecture
Alert
List
Text Box
Form
Images, Tickers and Gauges
Handling User Commands
Working with the Low-Level API
J2ME Gaming API
A Very Short Primer on Game Building
Building a J2ME Game
Defining Game Characteristics
TiledLayer
Sprites and LayerManager
Managing Layers
Sprites and Detecting Collisions
Mobile Media API
Using Mobile Media API (MMAPI)
Streaming media over the network

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Images, Tickers and Gauges


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Images, Tickers, and Gauges

Using images, tickers, and gauges as UI elements in MIDlets is quite straightforward.

A D V E R T I S E M E N T
A gauge, as you saw in the last section, is an item that can only be displayed on a form to indicate progress or to control a MIDlet feature (like volume). A ticker, on the other hand, can be attached to any UI element that extends the Displayable abstract class, and results in a running piece of text that is displayed across the screen whenever the element that is attached to it is shown on the screen. Finally, an image, can be used with various UI elements, including a form, as we saw in the last section.

Since the ticker can be used with all displayable elements, it provides a handy way to display information about the current element on the screen. The Displayable class provides the method setTicker(Ticker ticker), and the ticker can itself be created using its constructor Ticker(String msg), with the message that you want the ticker to display. By using setString(String MSG), you can change this message, and this change is effected immediately. For example, the form used in the previous section can have its own ticker displayed by setting form.setTicker(new Ticker("Welcome to Vandalay Industries!!!")). This will result in a ticker across the top of the screen (in the Toolkit's emulator), while the user is filling out the form. This is shown in Figure 6.

Figure 6
Figure 6. Setting a Ticker on a Displayable

In the previous section, we saw an example of a gauge in a non-interactive mode. It was there to represent the progress of a form being filled out by the MIDlet user. A non-interactive gauge can be used to represent the progress of a certain task; for example, when the device may be trying to make a network connection or reading a datastore, or when the user is filling out a form. In the previous section, we created a gauge by specifying four values. The label ("Step 1 of 3"), the interactive mode (false), the maximum value (3) and the initial value (1). However, when you don't know how long a particular activity is going to take, you can use the value of INDEFINITE for the maximum value.

A non-interactive gauge that has an INDEFINITE maximum value acquires special meaning. (You cannot create an interactive gauge with an INDEFINITE maximum value.) This type of gauge can be in one of four states, and this is reflected by the initial value (which is also the current value of the gauge). These states are CONTINUOUS_IDLE, INCREMENTAL_IDLE, CONTINUOUS_RUNNING, and INCREMENTAL_UPDATING. Each state represents the best effort of the device to let the user know the current activity of the MIDlet, and you can use them to represent these states yourself. Listing 5 shows an example of using these non-interactive gauges, along with an example of an interactive gauge. Remember that a Gauge is a UI Item, and therefore, can only be displayed as part of a Form.

Package com.j2me.part2;

import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.Gauge;
import javax.microedition.lcdui.Display;
import javax.microedition.midlet.MIDlet;

public class GaugeExample extends MIDlet {

	private Form form;
	private Gauge niIndefinate_CI;
	private Gauge niIndefinate_II;
	private Gauge niIndefinate_CR;
	private Gauge niIndefinate_IU;

	private Gauge interactive;

	public GaugeExample() {
		form = new Form("Gauge Examples");

		niIndefinate_CI =
		  new Gauge(
				"NI - Cont Idle",
				false,
				Gauge.INDEFINITE,
				Gauge.CONTINUOUS_IDLE);
		form.append(niIndefinate_CI);

		niIndefinate_II =
		  new Gauge(
				"NI - Inc Idle",
				false,
				Gauge.INDEFINITE,
				Gauge.INCREMENTAL_IDLE);
		form.append(niIndefinate_II);

		niIndefinate_CR =
		  new Gauge(
				"NI - Cont Run",
				false,
				Gauge.INDEFINITE,
				Gauge.CONTINUOUS_RUNNING);
		form.append(niIndefinate_CR);

		niIndefinate_IU =
		  new Gauge(
				"NI - Inc Upd",
				false,
				Gauge.INDEFINITE,
				Gauge.INCREMENTAL_UPDATING);
		form.append(niIndefinate_IU);

		interactive =
		  new Gauge(
				"Interactive ",
				true,
				10,
				0);
		form.append(interactive);

	}

	public void startApp() {
		Display display = Display.getDisplay(this);
		display.setCurrent(form);
	}

	public void pauseApp() {
	}

	public void destroyApp(boolean unconditional) {
	}
}

Listing 5. Using Gauges

Each mobile device will use its own set of images to represent these gauges. This will, in all probability, be different from the gauges that you will see if you run this listing in the Emulator supplied with the Toolkit.

You can also associate an image with an Alert and a Choice-based UI element. When an image is created, either by reading from a physical location or by making an image in-memory, it exists only in the off-screen memory. You should, therefore, be careful while using images, and restrict the size of images to the minimum possible to avoid filling the device's available memory.

The Image class provides several static methods to create or acquire images for use in MIDlets. An image that is created in-memory, by using the createImage(int width, int height) method, is mutable, which means that you can edit it. An image created this way initially has all of its pixels set to white, and you can acquire a graphics object on this image by using the method getGraphics() to modify the way it is rendered on screen. More about the Graphics object follows in the low-level API section.

To acquire an immutable image, you can use one of two methods: createImage(String imageName) or createImage(InputStream stream). The first method is used for looking up images from an associated packaged .jar file, while the second method is good for reading an image over a network. To create an immutable image from in-memory data, you can either use createImage(byte[] imageData, int imageOffset, int imageLength) or createImage(Image source). The first method allows you to form an image out of a byte array representation, while the second allows the creation of an image from an existing image.

Note that the MIDlet specification mandates support for the Portable Network Graphics format for images. Thus, all devices that support MIDlets will display a *.png image. These devices may support other formats, especially GIF and JPEG formats, but that is not a guarantee.

You have already seen an example of acquiring an image in the section on forms. In Listing 4, an image was wrapped up in an ImageItem class so that it could be displayed in a form. The image was kept in the res folder of the MIDlet for the Toolkit and the Emulator to find. The createImage(String imageName) method uses the Class.getResourceAsStream(String imageName) method to actually locate this image. The Toolkit takes care of packaging this image in the right folder when you create a ,jar file. In this case, this will be the top-level .jar folder. Make sure that whenever you reference images in your MIDlets that the images are kept in the right location. For example, if you want to keep all of your images for a MIDlet in an image folder in the final packaged .jar file, and not the top-level .jar folder, you will need to keep these images under an image folder under the res folder itself. To reference any of these images, you will need to ensure that you reference them via this images folder. For example: createImage("/images/duke.gif"); will reference the image duke.gif under the images folder.



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