Blackberry VerticalFieldManager with fixed size : Scroll issue - blackberry

Question

I'm trying to have a full screen UI with a fix header ( a manager with some fields) and a scrollable contents (a list of custom field). The idea is to emulate a kind of scrollable list.
For this I made a custom VerticalFieldManager that accept a maxHeight (the screen height - the header height).
I got the following problems:
The scroll arrows do not show up (ever)
On OS 4.7 (Storm), I can scroll lower that the last item, until having nothing on my screen but the header.
My code need to compile with the JDE 4.2.1 & 4.7 and to run on Pearl and Storm. (at worst I could have two version of this class)
I suspect the two problems are related. I probably do something wrong. I looked at a few example/forum and always found similar solution/code.
Do you guys can tell me what I did wrong?
/**
* custom class, so we can set a max height (to keep the header visible)
*/
class myVerticalFieldManager extends VerticalFieldManager{
private int maxHeight = 0;
myVerticalFieldManager(int _maxHeight){
super(
//this provoc an "empty scrollable zone" on Storm
// but if you don't put it, on other OS, the vertical manager does not scroll at all.
Manager.VERTICAL_SCROLL
| Manager.VERTICAL_SCROLLBAR
);
maxHeight = _maxHeight;
}
protected void sublayout(int width, int height){
super.sublayout(width, getPreferredHeight());
setExtent(width, getPreferredHeight());
}
public int getPreferredWidth() {
return Graphics.getScreenWidth();
}
/**
* allow the manager to use all the given height. (vs auto Height)
*/
public boolean forceMaxHeight = false;
public int getPreferredHeight() {
if (forceMaxHeight) return maxHeight;
int m = super.getPreferredHeight();
if (m > maxHeight) m = maxHeight;
return m;
}
////////////////////////////////////////////////////////////////////////////
protected boolean isUpArrowShown(){
//TODO: does not seem to work (4.2.1 emulator & 4.5 device). (called with good return value but the arrows are not painted)
int i = getFieldWithFocusIndex();
//Trace("isUpArrowShown " + i);
return i > 0;
// note: algo not correct, cause the up arrow will be visible event when no field are hidden.
// but not so bad, so the user "know" that he can go up.
}
protected boolean isDownArrowShown(){
int i = getFieldWithFocusIndex();
return i < getFieldCount();
}
////////////////////////////////////////////////////////////////////////////
// note : since 4.6 you can use
// http://www.blackberry.com/developers/docs/4.6.0api/net/rim/device/api/ui/decor/Background.html
public int myBackgroundColor = 0xffffff;
protected void paint(Graphics g){
g.setBackgroundColor(myBackgroundColor);
// Clears the entire graphic area to the current background
g.clear();
super.paint(g);
}
}
any helps is welcome.

Answer 1

so,
I came with this workaround for the "empty scrollable zone" problem on STORM
it's ugly and doesn't allow a custom ScrollChangeListener, but it's working on Pearl & Storm
implements ScrollChangeListener
//in constructor:
setScrollListener(null);
setScrollListener(this);
private boolean MY_CHANGING_SCROLL = false;
public void scrollChanged(Manager manager, int newHorizontalScroll, int newVerticalScroll){
if (!MY_CHANGING_SCROLL){
MY_CHANGING_SCROLL = true;
myCheckVerticalScroll();
MY_CHANGING_SCROLL = false;
}
}
protected int myMaxVerticalScrollPosition(){
int vh = getVirtualHeight();
int h = getHeight();
if (vh < h ) return 0; // no scroll
return vh - h; // don't scroll lower than limit.
}
protected void invCheckVerticalScroll() {
int i = getVerticalScroll();
int m = myMaxVerticalScrollPosition();
if ( i > m){
i = m;
setVerticalScroll(i);
}
}
I'm still looking for a solution to the scroll arrows problem...
If anybody got an idea...

Answer 2

You can use the method setBanner() instead of add for your header. Then you can add a default VerticalFieldManager to the screen and it will scroll normally but won't hide the header. Note that the MainScreen delegate manager is a VerticalScrollManager so you might not need a second vfm.
HorizontalFieldManager hfm = new HorizontalFieldManager();
setBanner(hfm)
add(new ButtonField("Hello 1");
add(new ButtonField("Hello 2");
...

Answer 3

Hey i did the same thing using a HorizontalFieldManager that contains an image and a title
header_img = Bitmap.getBitmapResource("header.png");
title = new LabelField("Welcome",LabelField.FIELD_RIGHT);
header_manager = new HorizontalFieldManager()
{
protected void paint(net.rim.device.api.ui.Graphics graphics)
{
int y = this.getVerticalScroll();
graphics.drawBitmap( 0, y, header_img.getWidth(), header_img.getHeight(), header_img, 0, 0 );
graphics.setColor(Color.LEMONCHIFFON);
super.paint( graphics );
}
protected void sublayout(int maxWidth, int maxHeight)
{
super.sublayout(Display.getWidth(), 240);
Field field = title;
layoutChild(field, title.getWidth(), title.getHeight());
setPositionChild(field, (Display.getWidth()/2) -10, 13);
setExtent(Display.getWidth(),55);
}
};
header_manager.add(title);