Creating a "collapsible" Views Exposed Filter
When building sites for our customers we usually create some administrative views (like for content or user administration) to make it easier for editors to work with the site. For a little more user experience we modify these views (especially the exposed form providing the filters). One of these modifications is to create a "collapsible" filter dropdown.
Think of a content administration view with filters for content type, status, author and so on. Normally the filter for the content type allows multiple selections and would look similar to the one in the image.
But we want the filter to act as a single dropdown that could be expanded to a multi-select list if the user wants to filter for several types.
To achieve this we need a small custom module and alter the exposed form:
Unfortunately we have to do some more magic to avoid errors after selecting the new option "All". Because we manually added the option in the form-alter, Views does not know about it and would throw an error after selecting it. The simplest way to avoid it, is to remove the filter value before displaying the results:
After you have all this together, users will be able to choose whether to select a single type from a dropdown or multiple values from a list. If a user selected multiple values the filter automatically displays as select list.