Prevent select2 from flipping the dropdown upward

问题

As per title, is there a way to force select2 to always create a dropdown instead of a drop-up?

There also appears to be some javascript that is either causing the flip when you scroll above the dropdown, adding a new CSS class "select2-drop-above", or both.

EDIT: I should've specified that I'm pulling the library in via select2-rails. I'm hoping there is a way around this that doesn't involve pulling the whole select2 lib in myself and editing the select2.js file directly.

回答1:

Modifying the plugin is not preferred as you mention. I had a similar issue and couldn't find an way to use select2 options to force the dropdown to stay below. The solution I ended up with is the following:

$("#mySelect2").select2({ ...options... })
    .on('select2-open', function() {

        // however much room you determine you need to prevent jumping
        var requireHeight = 600;
        var viewportBottom = $(window).scrollTop() + $(window).height();

        // figure out if we need to make changes
        if (viewportBottom < requireHeight) 
        {           
            // determine how much padding we should add (via marginBottom)
            var marginBottom = requireHeight - viewportBottom;

            // adding padding so we can scroll down
            $(".aLwrElmntOrCntntWrppr").css("marginBottom", marginBottom + "px");

            // animate to just above the select2, now with plenty of room below
            $('html, body').animate({
                scrollTop: $("#mySelect2").offset().top - 10
            }, 1000);
        }
    });

This code determines if there is enough room to place the dropdown at the bottom and if not, creates it by adding margin-bottom to some element on the page. It then scrolls to just above the select2 so that the dropdown won't flip.

回答2:

You can just edit select2.js


Where it says

enoughRoomBelow = dropTop + dropHeight <= viewportBottom,
enoughRoomAbove = (offset.top - dropHeight) >= this.body().scrollTop(),

just change it to

enoughRoomBelow = true,
enoughRoomAbove = false,

回答3:

Since modifying the source code is not an option and adding a hook to the select2:open event is not very elegant, especially when you have multiple select2 instances in the same page, I have written a small extension for the Select2 plugin.

My implementation is inspired by a PR from the plugin's repository (https://github.com/select2/select2/pull/4618) that is not yet merged.

Basically, the following code overrides the original plugin function that handles the dropdown positioning and adds a new option (dropdownPosition) to force the dropdown positioning above/below.

The new dropdownPosition option can take the following values: - below - the dropdown is always displayed at the bottom of the input; - above - the dropdown is always displayed at the top of the input; - auto (default) - it uses the old behavior.

Just insert the following code after select2.js file:

(function($) {

  var Defaults = $.fn.select2.amd.require('select2/defaults');

  $.extend(Defaults.defaults, {
    dropdownPosition: 'auto'
  });

  var AttachBody = $.fn.select2.amd.require('select2/dropdown/attachBody');

  var _positionDropdown = AttachBody.prototype._positionDropdown;

  AttachBody.prototype._positionDropdown = function() {

    var $window = $(window);

    var isCurrentlyAbove = this.$dropdown.hasClass('select2-dropdown--above');
    var isCurrentlyBelow = this.$dropdown.hasClass('select2-dropdown--below');

    var newDirection = null;

    var offset = this.$container.offset();

    offset.bottom = offset.top + this.$container.outerHeight(false);

    var container = {
        height: this.$container.outerHeight(false)
    };

    container.top = offset.top;
    container.bottom = offset.top + container.height;

    var dropdown = {
      height: this.$dropdown.outerHeight(false)
    };

    var viewport = {
      top: $window.scrollTop(),
      bottom: $window.scrollTop() + $window.height()
    };

    var enoughRoomAbove = viewport.top < (offset.top - dropdown.height);
    var enoughRoomBelow = viewport.bottom > (offset.bottom + dropdown.height);

    var css = {
      left: offset.left,
      top: container.bottom
    };

    // Determine what the parent element is to use for calciulating the offset
    var $offsetParent = this.$dropdownParent;

    // For statically positoned elements, we need to get the element
    // that is determining the offset
    if ($offsetParent.css('position') === 'static') {
      $offsetParent = $offsetParent.offsetParent();
    }

    var parentOffset = $offsetParent.offset();

    css.top -= parentOffset.top
    css.left -= parentOffset.left;

    var dropdownPositionOption = this.options.get('dropdownPosition');

    if (dropdownPositionOption === 'above' || dropdownPositionOption === 'below') {
      newDirection = dropdownPositionOption;
    } else {

      if (!isCurrentlyAbove && !isCurrentlyBelow) {
        newDirection = 'below';
      }

      if (!enoughRoomBelow && enoughRoomAbove && !isCurrentlyAbove) {
        newDirection = 'above';
      } else if (!enoughRoomAbove && enoughRoomBelow && isCurrentlyAbove) {
        newDirection = 'below';
      }

    }

    if (newDirection == 'above' ||
    (isCurrentlyAbove && newDirection !== 'below')) {
        css.top = container.top - parentOffset.top - dropdown.height;
    }

    if (newDirection != null) {
      this.$dropdown
        .removeClass('select2-dropdown--below select2-dropdown--above')
        .addClass('select2-dropdown--' + newDirection);
      this.$container
        .removeClass('select2-container--below select2-container--above')
        .addClass('select2-container--' + newDirection);
    }

    this.$dropdownContainer.css(css);

  };

})(window.jQuery);

The initialize the plugin with as follows:

$(document).ready(function() {
  $(".select-el").select2({
    dropdownPosition: 'below'
  });
});

Fiddle here: https://jsfiddle.net/byxj73ov/

Github repository: https://github.com/andreivictor/select2-dropdownPosition

---

UPDATE December 30, 2019

Fiddle with the latest Select2 Version (v4.0.12): https://jsfiddle.net/g4maj9ox/

回答4:

You can do it by overwriting CSS like so:

.select-dropdown {
  position: static;
}
.select-dropdown .select-dropdown--above {
      margin-top: 336px;
}

回答5:

I used to find an easier/faster solution for that:

        $("select").select2({

            // Options

        }).on('select2:open',function(){

            $('.select2-dropdown--above').attr('id','fix');
            $('#fix').removeClass('select2-dropdown--above');
            $('#fix').addClass('select2-dropdown--below');

        });

It's simple, you just change the .select2-dropdown--above to .select2-dropdown--below in the opening event (select2:open).

It will only works under the event, and there could be many other ways to perform it just changing classes when the select is opened.

ps. It won't work if you try to populate your select using jquery.

回答6:

Updated from [shanabus] answer

jQuery("#line_item").select2({
            formatResult: Invoice.formatLineItem
        })
        .on('select2-open', function() {
            // however much room you determine you need to prevent jumping
            var requireHeight = $("#select2-drop").height()+10;
            var viewportBottom = $(window).height() - $("#line_item").offset().top;

            // figure out if we need to make changes
            if (viewportBottom < requireHeight)
            {
                // determine how much padding we should add (via marginBottom)
                var marginBottom = requireHeight - viewportBottom;

                // adding padding so we can scroll down
                $(".aLwrElmntOrCntntWrppr").css("marginBottom", marginBottom + "px");

                // animate to just above the select2, now with plenty of room below
                $('html, body').animate({
                    scrollTop: $("#select2-drop").offset().top - 10
                }, 1000);
            }
        });

来源：https://stackoverflow.com/questions/19983601/prevent-select2-from-flipping-the-dropdown-upward