One of the Reader 9 enhancements was a new API call to populate list boxes: field.setItems(). The motivation for the new API is to provide better performance for populating lists.
Prior to Reader 9, the standard way to populate a list box is to call:
field.addItem(displayValue [, boundValue])
for each item in the list. The new API looks like:
field.setItems(itemListString [, numColumns])
The first parameter is a comma-separated list of values, the second parameter is an integer telling the field how many columns are in the data (defaults to one). The second parameter is designed for future extensibility if we choose to some day implement a multi-column list box.
A call to populate a listbox with currencies might look like:
"US Dollar,Canadian Dollar,Euro,United Kingdom Pounds");
Or if there were a bound value, it would look like:
Currency.setItems("US Dollar,USD,Canadian Dollar,CAD,Euro,EUR,United Kingdom Pounds,GBP", 2);
Prior to Reader 9, this second variation would have been coded as:
Currency.addItem("US Dollar", "USD");
Currency.addItem("Canadian Dollar", "CAD");
Currency.addItem("United Kingdom Pounds", "GBP");
There is a 3rd method for populating listboxes: binding them to data. Designer allows you to point your field at a location in your instance data where list box contents will be stored. While this method has very good performance, it has the disadvantages that a) your data is not always in the correct format for binding, b) the listbox gets populated from data only during the initial data load.
If you are using listboxes only casually you probably will not notice the difference in performance between the two methods. But if you are using listboxes intensively, the new method is a life-saver.
I have attached a form where I compare the old performance to the new. On my laptop, I populate a listbox with 500 items in 125 milliseconds using addItem() calls, and in 16 milliseconds using setItems(). Neither of these numbers may seem significant, but we have customers with forms containing many list boxes with many, many entries where the difference in performance is critical.
If you are designing a form to use this new API, be sure and set your target version (in Form Properties/Default) to "Acrobat and Adobe Reader 9.0 or later". Unless you do this, calls to setItems() will not work — even though you might open the form in Reader 9.