Hello Ian,
I might have a solution for you. It is based on the preOpen event and can be scaled to an arbitrary number of list elements and drop down lists.
You can download an example from my Acrobat.com workspace (click on the download button on the top left so save the file locally, as Acrobat.com won't display Designer/XFA documents in the browser).
The preOpen event is executed each time a user opens the list to make a selection and allows you to filter or add elements just before the list is shown. Because all subforms use the same code, I created a script object called "scoManageChoices".
PreOpen Event of each drop down list
scoManageChoices.restrictChoices(this);
Script Object "scoManageChoices"
// setup E4X XML variable containing viable choices
var choices = <choices>
<choice>Canyoning</choice>
<choice>Flying Fox</choice>
<choice>High Ropes</choice>
<choice>White Water Rafting</choice>
</choices>
function restrictChoices(dropdown) {
// collect choices contained in other dropdowns
var dropdowns = dropdown.all; // get all dropdowns with same name/hierarchy
var otherChoices = []; // initialize empty array to save other choices
for (var i = 0; i < dropdowns.length; i++) {
var dd = dropdowns.item(i); // retrieve dropdown from list
if (dd.isNull || dd == dropdown) continue; // skip empty or current dropdowns
otherChoices.push(dd.rawValue); // save value of other non-empty dropdown into array
}
// fill current dropdown with choices
dropdown.clearItems(); // reset current dropdown
for each (var choice in choices.choice) { // loop through viable choices
if (otherChoices.indexOf(choice.toString()) > -1) { // skip adding to dropdown list if other dropdown already selected the choice
continue;
}
dropdown.addItem(choice.toString()); // add to dropdown
}
}
To make things easier in the script I placed all drop down lists within the same subform and gave all of them the same name, so that I can find them with the "all" property of the field (see the code example above).
I specified the viable choices in a format called E4X that is supported by Acrobat/Reader's Javascript engine and documented here (don't be scared by the deprecation notice, it continues to work in Acrobat/Reader).
Hope that helps,
Martin
