In part 2, I made cascading dropdowns by calling selectFromAjax javascript function from my view:
This function has 3 arguments: address of service which provides data based on second argument – formData (in this case only id of selected item in current dropdown), and target – select list into which items from service are injected, replacing its current contents. These three parameters are almost enough to know that there is dependency between two dropdowns. Why almost? Because there is one implicit parameter – this function is part of “change” event of another dropdown – source of this event. So, source dropdown is fourth parameter. That four things are everything that client needs to know to be able to implement intended behavior of UI.
I will make this functionality unobtrusive by placing necessary information into target dropdown. Why? Because it seems most straightforward and “unobtrusive” to me, that dropdown list says “My contents depend on item selected in dropdown list X, and you need to call service Y to find out what should I show”. It is not ok to have information about MY behavior in some other dropdown X 
. Besides, with this approach, I’m eliminating one parameter, as I have only two now: dropdown X and service Y. Third was value of dropdown X, and fourth is target dropdown itself, into which I’m adding these attributes. Value of dropdown X I can read if I know which is dropdown X, so this parameter for function anyway must be determined at the moment of changing value in X.
So, if I make my select look like
then my dropdown contains all information needed to remove script from view, and place it in global js file, or make jQuery plugin. To make this information automatically embedded into my dropdown, I need to make html helper for this. If wee look at first overload of DropDownListFor in System.Web.Mvc.Html.SelectExtensions, we can easily see how new method should look:
My html helper will have another two arguments – depends on and load from:
It is using DropDownListFor helper from Microsoft, and I won’t render my own html code (and so make more space for bugs ).
So, view from part 2 should now look like this:
and generated html is:
<select cascading-dependson="ProductType" cascading-loadfrom="/Home/Manufacturers" id="Manufacturer" name="Manufacturer"></select>
Now, all that is left, is to automatically attach event handler onto “master” dropdown. It is easy using jQuery:
This is adding initializeCascader to list of functions that are run on document load. This function goes through each select that has cascading attributes and attaches onchange event to element with id in cascading-depenson attribute, with self as a target for selectFromAjax function (from part 2), and value of cascading-loadfrom attribute as a url.
This is very simple concept, but can be used for chaining random number of dropdowns, and can easily be expanded to make dropdown values dependent not only on “master” dropdown, but to depend on whole form. Further improvement of this could be custom data annotation attribute, which could be used for decorating property of model, so html helper does not have to be explicitly called.
Source code for this post can be downloaded here.
UPDATE: I have uploaded source code to bitbucket.