Vertical scrollbar with jump points - setVerticalScroll locking UI - blackberry

Question

I have a question about the BlackBerry VerticalScrollField and scrolling which seems to lock or make the UI unstable. The following code is a BlackBerry screen with worlds as content on the left (in a scroll field) and a jumpbar off to the right that allows clicking into the content.
When a jump letter is clicked the setVerticalScroll method is called, it performs the scroll but has the unfortunate side effect of rendering the UI unstable or unusable. The scroll call is done on the UI thread so its not clear what the source of the error is. The app is being tested in a 6.0 simulator.
I've included the class which can be copied into BB Eclipse for hacking/testing.
The section that kicks of the scrolling can be found towards the bottom with the following code:
UiApplication.getUiApplication().invokeLater(new Runnable(){
public void run() {
scroller.setVerticalScroll(y, true);
}});
Here's the full class:
package test;
import java.util.Vector;
import net.rim.device.api.system.ApplicationManager;
import net.rim.device.api.ui.Field;
import net.rim.device.api.ui.Font;
import net.rim.device.api.ui.Graphics;
import net.rim.device.api.ui.TouchEvent;
import net.rim.device.api.ui.UiApplication;
import net.rim.device.api.ui.component.LabelField;
import net.rim.device.api.ui.component.Status;
import net.rim.device.api.ui.container.HorizontalFieldManager;
import net.rim.device.api.ui.container.MainScreen;
import net.rim.device.api.ui.container.VerticalFieldManager;
public class Startup extends UiApplication {
private int[] jump;
static final String[] words = new String[]{
"auto", "apple", "bear", "car", "farm", "ferret", "gold",
"green", "garden", "hedge", "happy", "igloo", "infrared",
"jelly", "kangaroo", "lemon", "lion", "marble", "moon",
"nine", "opera", "orange", "people", "puppy", "pear",
"quince", "race", "run", "sunset", "token", "willow", "zebra"
};
private final static String[] alphabet = new String[]{"A","B","C","D","E",
"F","G","H","I","J","K","L","M","N","O","P","Q","R",
"S","T","U","V","W","X","Y","Z","#"};
private VerticalFieldManager scroller;
public Startup() {
UiApplication.getUiApplication().invokeLater(new Runnable() {
public void run() {
UiApplication.getUiApplication().pushScreen(new ScrollScreen());
}
});
}
public static void main(String[] args) {
ApplicationManager app = ApplicationManager.getApplicationManager();
while (app.inStartup()) {
try { Thread.sleep(200); } catch (Throwable e) {}
}
Startup startup = new Startup();
startup.enterEventDispatcher();
}
/**
* Screen with content in a scrollbar left and a letters on the right that
* can be used to jump into the content.
*/
class ScrollScreen extends MainScreen {
public ScrollScreen() {
super(NO_HORIZONTAL_SCROLL | NO_VERTICAL_SCROLL);
HorizontalFieldManager hfm = new HorizontalFieldManager(USE_ALL_HEIGHT | NO_VERTICAL_SCROLL | NO_HORIZONTAL_SCROLL){
protected void sublayout(int maxWidth, int maxHeight) {
Field scroll = getField(0);
Field alpha = getField(1);
layoutChild(alpha, maxWidth, maxHeight);
layoutChild(scroll, maxWidth-alpha.getWidth(), maxHeight);
setPositionChild(scroll, 0, 0);
setPositionChild(alpha, maxWidth-alpha.getWidth(), 0);
setExtent(maxWidth, maxHeight);
}
};
hfm.add(createScrollContent());
hfm.add(createAlphabetJumpBar());
add(hfm);
}
private Field createScrollContent() {
Vector vocabulary = new Vector();
for (int ii=0; ii<alphabet.length; ii++)
vocabulary.addElement(alphabet[ii]);
scroller = new VerticalFieldManager(VERTICAL_SCROLL | USE_ALL_WIDTH) {
protected void sublayout(int maxWidth, int maxHeight) {
// Record the jump offsets
int y = 0;
for (int ii=0; ii<getFieldCount(); ii++) {
Field field = getField(ii);
layoutChild(field, maxWidth, maxHeight);
setPositionChild(field, 0, y);
if (field instanceof WordField) {
WordField object = (WordField)field;;
char character = object.getWord().toLowerCase().charAt(0);
int offset = ((int)character)-(int)alphabet[0].toLowerCase().charAt(0);
if (offset < 0 || offset > jump.length)
offset = jump.length-1;
while (offset >= 0 && offset < jump.length && jump[offset] == 0) {
jump[offset] = y;
offset--;
}
}
y += field.getHeight();
}
int offset = jump.length-1;
do {
jump[offset] = y;
offset--;
} while (offset >= 0 && jump[offset] == 0);
setExtent(maxWidth, maxHeight);
setVirtualExtent(maxWidth, y+10);
}
};
jump = new int[alphabet.length];
Font largeFont = Font.getDefault().derive(Font.PLAIN, 46);
for (int ii=0; ii<words.length; ii++) {
WordField wordField = new WordField(words[ii]);
wordField.setFont(largeFont);
scroller.add(wordField);
}
return scroller;
}
private Field createAlphabetJumpBar() {
VerticalFieldManager vfm = new VerticalFieldManager() {
protected void sublayout(int maxWidth, int maxHeight) {
int y = 0;
int width = 0;
double allowedAlphaHeight = (double)maxHeight / (double)getFieldCount();
for (int ii=0; ii<getFieldCount(); ii++) {
WordField field = (WordField)getField(ii);
layoutChild(field, maxWidth, (int)allowedAlphaHeight);
setPositionChild(field, 0, y);
y += field.getHeight();
double paddedY = Math.floor(allowedAlphaHeight*(ii+1));
if (y < paddedY) y = (int)paddedY;
width = Math.max(width, field.getWidth());
}
setExtent(width, maxHeight);
}
};
for (int ii=0; ii<alphabet.length; ii++) {
vfm.add(new AlphaField(alphabet[ii]){
protected boolean touchEvent(TouchEvent message) {
if (message.getEvent() == TouchEvent.UP) {
int startOffset = (int)alphabet[0].charAt(0);
int offset = ((int)getWord().charAt(0)) - startOffset;
final int y = offset == 0 ? 0 : jump[offset - 1];
UiApplication.getUiApplication().invokeLater(new Runnable(){
public void run() {
scroller.setVerticalScroll(y, true);
}});
}
return true;
}
});
}
return vfm;
}
class WordField extends LabelField {
private final String word;
public WordField(String word) {
super(word);
this.word = word;
}
public String getWord() { return word; }
}
Font alphaFont = null;
class AlphaField extends WordField {
public AlphaField(String word) {
super(word);
}
protected void layout(int width, int height) {
if (alphaFont == null)
alphaFont = Font.getDefault().derive(Font.PLAIN, height);
setExtent(alphaFont.getAdvance(getWord()), alphaFont.getHeight());
}
protected void paint(Graphics graphics) {
graphics.setFont(alphaFont);
graphics.drawText(getWord(), 0, 0);
}
}
/**
* For debugging.
* #see net.rim.device.api.ui.Screen#keyChar(char, int, int)
*/
protected boolean keyChar(char c, int status, int time) {
if ('o' == c) { // shows the jump offsets into the scroll field
UiApplication.getUiApplication().invokeLater(new Runnable(){
public void run() {
StringBuffer buf = new StringBuffer();
for (int ii=0; ii<jump.length; ii++) {
buf.append(alphabet[ii]+"="+jump[ii]);
if (ii<jump.length-1)
buf.append(",");
}
Status.show("offsets="+buf.toString());
}});
}
return super.keyChar(c, status, time);
}
}
}

Answer 1

You're using UiApplication.invokeLater in a few places where you're already on the UI event thread, so those are redundant - the debug code in keyChar and the setVerticalScroll call from the touchEvent handler. The Runnable is executed synchronously when you do an invokeLater from the UI thread, with no delay specified.
Are you sure you want to set the scroll explicitly? One option would be to set the focus on the WordField you are interested in, by calling setFocus(), then the OS will do the scrolling events to move that field on screen for you.
If you really need to explicitly set the vertical scroll, your problem may be that the touch event is already causing scroll, so setting it again causes problems. You can get around this by specifying a one millisecond delay for your invokeLater(...). This means your Runnable will be added to the event queue, instead of executing synchronously. That way the scroll won't be changed in the middle of another event call-stack.

Answer 2

Finally tracked down the issue - if the touchEvent for the alphabet label field returns a true then it locks up the main scroll field, if however return super.touchEvent(message) is called the scrolling happens and the scroll field can still be scrolled up and down by clicking on the screen.
This may be a bug in the BlackBerry OS or just the simulator. The Field.touchEvent() documentation for 6.0 recommends returning true if the method consumes the event; however doing so (at least in the above code) causes another UI field to loose the ability to detect touch events which would cause it to scroll.