User Guide

Using the List component 763
■ Lists don’t measure text.
This restriction has the most potential ramifications. Because a list must scale to
thousands of records, any one of which could contain an unusually long string, it
shouldn’t grow to fit the largest string of text within it, or add a horizontal scroll bar in
“auto” mode. Also, measuring thousands of strings would be too intensive. The
compromise is the
maxHPosition property, which, when vScrollPolicy is set to "on",
gives the list extra buffer space for scrolling.
If you know you’re likely to deal with long strings, turn
hScrollPolicy to "on", and add
a 200-pixel
maxHPosition value to your List or Tree component. A user is more or less
guaranteed to be able to scroll to see everything. The DataGrid component, however, does
support
"auto" as an hScrollPolicy value, because it measures columns (which are the
same width per item), not text.
The fact that lists don’t measure text also explains why lists have uniform row heights.
Sizing individual rows to fit text would require intensive measuring. For example, if you
wanted to accurately show the scroll bars on a list with nonuniform row height, you’d
need to premeasure every row.
■ Lists perform worse as a function of their visible rows.
Although lists can display 5000 records, they can’t render 5000 records at once. The more
visible rows (specified by the
rowCount property) you have on the Stage, the more work
the list must to do to render. Limiting the number of visible rows, if at all possible, is the
best solution.
■ Lists aren’t tables.
For example, DataGrid components, which extend the List class, are intended to provide
an interface for many records. They’re not designed to display complete information;
they’re designed to display enough information so that users can drill down to see more.
The message view in Microsoft Outlook is a prime example. You don't read the entire e-
mail in the grid; the mail would be difficult to read and the client would perform terribly.
Outlook displays enough information so that a user can drill into the post to see
the details.