What's the right way, for testability, to add functionality to a ComboBox?

Question

The desired functionality of the 'enhanced' combo box is a quick find method. Each item in the combobox has a ToString() method, such that they can be displayed in the drop down list. On clicking an item in the drop down list, the combobox's observers are notified of the selection.

Also, every time the typed text in the combobox changes, a list of "Candidates" is generated, made of all those items in the drop down list which contain the text typed in so far. Hitting enter takes you to the first candidate in that list, repeatedly hitting enter cycles you through the list.

I've implemented this functionality by deriving from ComboBox - I figured this made sense as I'm still functionally left with a combobox, it just has this "QuickFind" feature added. However, the logic to create the candidate list and cycle through it, whilst simple, isn't totally trivial, and would enjoy some testing.

However, as seen here it doesn't seem to be all that easy to test a ComboBox just by constructing it and prodding the extra routines I've added - it needs to exist on a form for it to behave the same way as it does in the application. This seems altogether too much effort to test a simple addition to a simple combo box!

Nothing in the code to cycle through the candidates is specific to my application however - I have created a general control that may be used in any number of contexts, the only requirement being that the objects in the combobox have a ToString() methiod, which is the same restriction placed on objects going into normal comboboxes anyway, and is guaranteed by C# .NET.

So, with testability in mind, where would you put the enhanced functionality?

Answer 1

As with the post you references: seperating the logic from the gui elements is also the solution here.

You should consider using a controller-like class which exposes a list of items which you can databind to your ComboBox' DataSource. The controller itself is responsible for maintaining this list.

So whenever you type a letter in the ComboBox you call a function on the controller, let's say UpdateList(string typedString). In this way you have seperated the logic of filling the list with 'candidates'.

Now you can easily write a number of tests which each call UpdateList() with different arguments and inspect the list of items afterwards. No GUI stuff needed to do the testing, you're only testing the algorithm itself.