Blackberry - How to implement ListField Smooth Scrolling?

Question

Overview
I'm using a listfield class to display a set of information vertically. Each row of that listfield takes up 2/5th's of the screen height.

As such, when scrolling to the next item (especially when displaying an item partially obscured by the constraints of the screen height), the whole scroll/focus action is very jumpy.

I would like to fix this jumpiness by implementing smooth scrolling between scroll/focus actions. Is this possible with the ListField class?

Example
Below is a screenshot displaying the issue at hand.

_{(source: perkmobile.com)}

Once the user scrolls down to ListFieldTHREE row, this row is "scrolled" into view in a very jumpy manner, no smooth scrolling. I know making the row height smaller will mitigate this issue, but I don't wan to go that way.

Main Question
How do I do smooth scrolling in a ListField?

Answer 1

Assuming you want the behavior that the user scrolls down 1 'click' of the trackball, and the next item is then highlighted but instead of an immediate scroll jump you get a smooth scroll to make the new item visible (like in Google's Gmail app for BlackBerry), you'll have to roll your own component.

The basic idea is to subclass VerticalFieldManager, then on a scroll (key off the moveFocus method) you have a separate Thread update a vertical position variable, and invalidate the manager multiple times.

The thread is necessary because if you think about it you're driving an animation off of a user event - the smooth scroll is really an animation on the BlackBerry, as it lasts longer than the event that triggered it.

I've been a bit vague on details, and this isn't a really easy thing to do, so hopefully this helps a bit.

Answer 2

There isn't an official API way of doing this, as far as I know, but it can probably be fudged through a clever use of NullField(Field.FOCUSABLE), which is how many custom BlackBerry UIs implement forced focus behavior.

One approach would be to derive each "list item" from a class that interlaces focusable NullFields with the visible contents of the list item itself -- this would essentially force the scrolling system to "jump" at smaller intervals rather than the large intervals dictated by the natural divisions between the list items, and would have the side benefit of not modifying the visible positioning of the contents of the list item.

Answer 3

unless you want to override the how the listfield paints or create your own wrapper, you will always have this issue, this is because each line is always visible when scrolling. Try using labelfield instead.