How can I validate a multiselect InputField using its onchange event?

Question

I have an inputField bound to a multi-select custom Field.

I want to validate that the user picks a max of 3 items from the list, and I want to show a message or an alert when the user selects a forth item right away(not on submission of the form). I found that the actionSupport can help me with that, but I'm not sure if it can allow me to keep track of how many items are selected

<apex:inputField value="{!Agency_Profile.Destination_ Specialties}">
<apex:actionSupport event="onchange" action="{}" />
</apex:inputField>

Answer 1

It's easy to use a JavaScript function. Add this to your page between <script> and </script> tags:

var selected = {};

function validate(obj)
{
  for (var i = 0; i < obj.options.length; i++) {
    if (obj.options[i].selected) {
      selected[obj.id+'_'+obj.options[i].value] = obj.options[i].label; 
    }
    else {
      selected[obj.id+'_'+obj.options[i].value] = null;
    }
  }

  var size = 0;
  for (var key in selected) {
    if (selected[key] != null) { size++; }
  }

  //console.log(size);

  if(size > 3){
    alert("You can only select a maximum of three options.");
  }
}

Then change your <apex:inputField> to look like this:

<apex:inputField value="{!Agency_Profile.Destination_ Specialties}" onchange="validate(this);"></apex:inputField>

Here's a complete sample Visualforce page:

<apex:page >
  <script>
    var selected = {};

    function validate(obj)
    {
      for (var i = 0; i < obj.options.length; i++) {
        if (obj.options[i].selected) {
          selected[obj.id+'_'+obj.options[i].value] = obj.options[i].label; 
        }
        else {
          selected[obj.id+'_'+obj.options[i].value] = null;
        }
      }

      var size = 0;
      for (var key in selected) {
        if (selected[key] != null) { size++; }
      }

      //console.log(size);

      if(size > 3){
        alert("You can only select a maximum of three options.");
      }
    }
  </script>
  <apex:form >
    <apex:selectList id="chooseColor1" size="5" multiselect="true" onchange="validate(this);">
      <apex:selectOption itemValue="white" itemLabel="White"/>
      <apex:selectOption itemValue="red" itemLabel="Red"/>
      <apex:selectOption itemValue="orange" itemLabel="Orange"/>
      <apex:selectOption itemValue="green" itemLabel="Green"/>
      <apex:selectOption itemValue="blue" itemLabel="Blue"/>
    </apex:selectList> 
    <apex:selectList id="chooseColor2" size="5" multiselect="true" onchange="validate(this);">
      <apex:selectOption itemValue="white" itemLabel="White"/>
      <apex:selectOption itemValue="red" itemLabel="Red"/>
      <apex:selectOption itemValue="orange" itemLabel="Orange"/>
      <apex:selectOption itemValue="green" itemLabel="Green"/>
      <apex:selectOption itemValue="blue" itemLabel="Blue"/>
    </apex:selectList> 
  </apex:form>
</apex:page>