How can you limit the number of selections in a List box

OK, never say never.  Here's the solution:

The only script in the list box field is the following Mouse Up action:

this.getField("Decoy").setFocus();

Create a text field called Decoy and enter the following On Focus script:

var box=this.getField("List Box");
if(typeof box.value=="object")
{
if( box.value.length==3)
{
var vals=box.value;
}
else if (box.value.length>3)
{
box.value=vals;
app.alert("The maximum number of selections is 3.",1);
}
}
box.setFocus();

Decoy can't be hidden or read only or it will not receive the focus and script will not run.  So make the field kind of hidden by making it invisible by setting all its coordinates to zero.  Run the following script in the console:

this.getField("Decoy").rect=[0,0,0,0];

Hi @rarebit_7112 ,

+++Adding to the valuable guidance of @PDF Automation Station , 

List boxes can be quite challenging to navigate, as you mentioned, because they are designed to have a somewhat similar feel to button objects and also share characteristics with dropdown menus. However, they are fundamentally different from text field objects. Therefore, managing the field properties like you would with other PDF objects is not possible unless you employ some creative methods.

The primary issue with list boxes is that when they read and write values, the selected (or entered) values do not immediately reflect a change upon selection. That is why @PDF Automation Station employed a Blur Event action .

Additionally, the script he provided works well and addresses your inquiry. It may need to be slightly modified, to avoid randomly reverting to three selections after the alert is triggered.

That said, the items in a listbox are accessed using methods that utilize a zero-based index. So, when you query a list box object, it returns an array (in the case of multiple selections) in the order that they were listed when that field object was created, starting from 0.

And this being the case, If you are working on a validation script, you might need to use a hidden text field and place the validation script there, as this is not a field property of a list box, unlike dropdown menus. Moreover, implementing this approach contradicts the simplicity and purpose of your workflow.

In my example script below, I utilize the "currentValueIndices" method along with an App Alert that activates when more than three items are selected from the list box.

To make it function like a validation script, I incorporated the setFocus() method into the script. This, combined with the alert, ensures that the focus remains on the listbox field, preventing the user from proceeding to other fields until three items are selected. By doing this, the values are neither reverted nor reset, allowing the user to see their previous selections and adjust them as needed.

var f = this.getField("List Box");

var a = f.currentValueIndices;

for (var i = 0; i < a.length; i ++) {

 (a.length > 3) ? ([app.alert("Please choose only three options."), f.setFocus()]) : f.getItemAt(a,true);

 break;

} 

Let us know if this information is helpful.

Enter the following On Blur script in the list box field:

var vals=event.target.value;
if(typeof vals=="object")
{
var newVals=[];
for(var i=0;i<3;i++)
{
newVals.push(vals[i])
}
if(vals.length>3)
{app.alert("The maximum number of selections is 3.",1)}
event.target.value=newVals;
}

Change the three 3s to the actual maximum number of selections.  Multiple selections in a list box are not committed until the blur event.