Progress Indicator & Threads In LWUIT

LWUIT doesn't ship with a pre-existing progress indicator, mostly because making something generic enough for all the common cases is not as simple as it might seem in the beginning. Especially when considering how easy it is to write your own progress indicator...
This is a simple example of how to create a custom component in LWUIT in this specific case a progress indicator that supports both drawing itself (as a filled round rectangle and as a couple of images overlayed one on top of the other. The progress indicator component is fully themeable and customizable and will accept all L&F settings seamlessly.
The screenshots show both an image based indicator (its ugliness is just a testament to my bad drawing skills) and an indicator drawn in graphics primitives (fill/drawRoundRect). These are the images used to draw the image progress:

As part of this I also wanted to create something else familiar to Swing developers, the SwingWorker. Generally I prefer the foxtrot approach implemented in LWUIT as invokeAndBlock in Display, however lots of people like the SwingWorker approach so I used it here as part of the explanations.

First lets create the Progress indicator component:

/**
* Simple progress indicator component that fills out the progress made.
* Progress is assumed to always be horizontal in this widget
*
* @author Shai Almog
*/
public class Progress extends Component {
private byte percent;
private Image unfilled;
private Image filled;

/**
 * The default constructor uses internal rendering to draw the progress
*/
public Progress() {
   setFocusable(false);
}

/**
* Allows indicating the progress using a filled/unfilled images.
* The unfilled image is always drawn and the filled image is drawn on top with
* clipping to indicate the amount of progress made.
*
* @param unfilled an image containing the progress bar without any of its
* content being filled (with the progress color)
* @param filled an image identicall to unfilled in every way except that progress
* is completed in this bar.
*/
public Progress(Image unfilled, Image filled) {
   this();
   this.unfilled = unfilled;
   this.filled = filled;
}

/**
* Indicate to LWUIT the component name for theming in this case "Progress"
*/
public String getUIID() {
   return "Progress";
}

/**
* Indicates the percent of progress made
*/
public byte getProgress() {
   return percent;
}

/**
* Indicates the percent of progress made, this method is thread safe and
* can be invoked from any thread although discression should still be kept
* so one thread doesn't regress progress made by another thread...
*/
public void setProgress(byte percent) {
   this.percent = percent;
   repaint();
}

/**
* Return the size we would generally like for the component
*/
protected Dimension calcPreferredSize() {
   if(filled != null) {
       return new Dimension(filled.getWidth(), filled.getHeight());
   } else {
       // we don't really need to be in the font height but this provides
       // a generally good indication for size expectations
       return new Dimension(Display.getInstance().getDisplayWidth(),
           Font.getDefaultFont().getHeight());
   }
}

/**
* Paint the progress indicator
*/
public void paint(Graphics g) {
   int width = (int)((((float)percent) / 100.0f) * getWidth());
   if(filled != null) {
       if(filled.getWidth() != getWidth()) {
           filled = filled.scaled(getWidth(), getHeight());
           unfilled = unfilled.scaled(getWidth(), getHeight());
       }
  
       // draw based on two user supplied images
       g.drawImage(unfilled, getX(), getY());
       g.clipRect(getX(), getY(), width, getHeight());
       g.drawImage(filled, getX(), getY());
   } else {
       // draw based on simple graphics primitives
       Style s = getStyle();
       g.setColor(s.getBgColor());
       int curve = getHeight() / 2 - 1;
       g.fillRoundRect(getX(), getY(), getWidth() - 1, getHeight() - 1, curve, curve);
       g.setColor(s.getFgColor());
       g.drawRoundRect(getX(), getY(), getWidth() - 1, getHeight() - 1, curve, curve);
       g.clipRect(getX(), getY(), width - 1, getHeight() - 1);
       g.setColor(s.getBgSelectionColor());
       g.fillRoundRect(getX(), getY(), getWidth() - 1, getHeight() - 1, curve, curve);
   }
}
}

This code seems to me to be simple but obviously I'm not objective, if something is not clear or you think it might not be clear to others please let me know in the comments.

BackgroundTask is my equivalent to SwingWorker, its much simpler than SwingWorker:

/**
* A tool allowing to respond to an event in the background possibly with
* progress indication inspired by Swings "SwingWorker" tool. This class
* should be used from event dispatching code to prevent the UI from blocking.
* State can be stored in this class the separate thread and it can be used by
* the finish method which will be invoked after running.
*
* @author Shai Almog
*/
public abstract class BackgroundTask {
/**
* Start this task
*/
public final void start() {
   if(Display.getInstance().isEdt()) {
       taskStarted();
   } else {
       Display.getInstance().callSeriallyAndWait(new Runnable() {
           public void run() {
               taskStarted();
           }
       });
   }
   new Thread(new Runnable() {
       public void run() {
           if(Display.getInstance().isEdt()) {
               taskFinished();
           } else {
               performTask();
               Display.getInstance().callSerially(this);
           }
       }
   }).start();
}

/**
* Invoked on the LWUIT EDT before spawning the background thread, this allows
* the developer to perform initialization easily.
*/
public void taskStarted() {
}

/**
* Invoked on a separate thread in the background, this task should not alter
* UI except to indicate progress.
*/
public abstract void performTask();

/**
* Invoked on the LWUIT EDT after the background thread completed its
* execution.
*/
public void taskFinished() {
}
}

And this is the code to display these two progress bars:

Form progressForm = new Form("Progress");
progressForm.setLayout(new BoxLayout(BoxLayout.Y_AXIS));
Progress p1 = new Progress();
progressForm.addComponent(new Label("Drawn"));
progressForm.addComponent(p1);
Progress p2 = new Progress(Image.createImage("/unfilled.png"), Image.createImage("/filled.png"));
p2.getStyle().setBgTransparency(0);
progressForm.addComponent(new Label("Image Based"));
progressForm.addComponent(p2);
progressForm.show();

class ProgressCommand extends Command {
private Progress p;
public ProgressCommand(String name, Progress p) {
   super(name);
   this.p = p;
}
public void actionPerformed(ActionEvent ev) {
   new BackgroundTask() {
       public void performTask() {
           for(byte b = 0 ; b <= 100 ; b++) {
               try {
                   p.setProgress(b);
                   Thread.sleep(100);
               } catch (InterruptedException ex) {
                   ex.printStackTrace();
               }
           }
       }
   }.start();
}
}

progressForm.addCommand(new ProgressCommand("Drawn", p1));
progressForm.addCommand(new ProgressCommand("Images", p2));