Collapse / expand optgroup using select2

Question

I'm trying to use the Select2 jquery plugin to make the optgroups in my select drop down menu collapsible/expandable.

I found what looks to be a good and straightforward solution (https://github.com/select2/select2/issues/730) but it's not working for me and I think it's because it's from an older version of select2.

I've figured out how to update the css from

.select2-result-sub > li.select2-result {
    display: none;
}

to

#select-container .select2-results__options--nested > li.select2-results__option {
    display: none;
}

(where i've also put my select object in a container per How to override .select2-results .select2-highlighted color)

But I'm at a loss for how to update the required javascript:

$('.select2-results').on('click', 'li', function(){
    $(this).find('li').show();
});

It seems to me like I would just need to replace select2-results with select2-results__options or select2-results__option, but I've tried a number of things, with and without using the container and I just can't get it to work. I don't have much experience in javascript/jquery so I don't really understand what I'm doing wrong with trying to select the optgroup element.

Also, here's my html:

<div id="select-container">
    <select id="select-test" multiple="multiple" style="width:300px">
            <optgroup label="Group 1">
                <option value="1">Option 1</option>
                <option value="2">Option 2</option>
                <option value="3">Option 3</option>
            </optgroup>
            <optgroup label="Group 2">    
                <option value="4">Option 4</option>
                <option value="5">Option 5</option>
                <option value="6">Option 6</option>
            </optgroup>
     </select>     
 </div>  

Any help is appreciated, thanks!!

Answer 1

Here's a simple demo.

$("#select-test").select2();
$("body").on('click', '.select2-results__group', function() {
  $(this).siblings().toggle();
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.5/js/select2.full.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.5/css/select2.min.css" />
<div id="select-container">
  <select id="select-test" multiple="multiple" style="width:300px">
    <optgroup label="Group 1">
      <option value="1">Option 1</option>
      <option value="2">Option 2</option>
      <option value="3">Option 3</option>
    </optgroup>
    <optgroup label="Group 2">
      <option value="4">Option 4</option>
      <option value="5">Option 5</option>
      <option value="6">Option 6</option>
    </optgroup>
  </select>
</div>

Edit:

We can start the optgroup as collapsed by using select2:open event. I add an object to record the collapsed state of each optgroup.

Set opacity to 0 and set it back to 1 after determining the collapsed state of each optgroup.

Note that select2:open triggered before the .select2-results__group created, so I use setTimeout to wait for a few miliseconds.

$("#select-test").select2();

let optgroupState = {};

$("body").on('click', '.select2-container--open .select2-results__group', function() {
  $(this).siblings().toggle();
  let id = $(this).closest('.select2-results__options').attr('id');
  let index = $('.select2-results__group').index(this);
  optgroupState[id][index] = !optgroupState[id][index];
})

$('#select-test').on('select2:open', function() {
  $('.select2-dropdown--below').css('opacity', 0);
  setTimeout(() => {
    let groups = $('.select2-container--open .select2-results__group');
    let id = $('.select2-results__options').attr('id');
    if (!optgroupState[id]) {
      optgroupState[id] = {};
    }
    $.each(groups, (index, v) => {
      optgroupState[id][index] = optgroupState[id][index] || false;
      optgroupState[id][index] ? $(v).siblings().show() : $(v).siblings().hide();
    })
    $('.select2-dropdown--below').css('opacity', 1);
  }, 0);
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.5/js/select2.full.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.5/css/select2.min.css" />
<div id="select-container">
  <select id="select-test" multiple="multiple" style="width:300px">
    <optgroup label="Group 1">
      <option value="1">Option 1</option>
      <option value="2">Option 2</option>
      <option value="3">Option 3</option>
    </optgroup>
    <optgroup label="Group 2">
      <option value="4">Option 4</option>
      <option value="5">Option 5</option>
      <option value="6">Option 6</option>
    </optgroup>
  </select>
</div>