October 28, 2012

...Learn TDD with Codemanship

Refactoring Legacy Code #2 - Making Web Apps More Unit-Testable

Following on from that last post about refactoring legacy classes that depend on external systems (like a database) - which has been read by literally dozens of people, and that's no idle boast - I also get asked a lot about making web applications unit-testable.

Taking classic ASP.NET as a typical example - and again using a toy but typical example - the problem is also external dependencies. When we reference ASP.NET objects like Session and Request, we tie our code to the ASP.NET process and the lifecycle of our web forms. We can't just create na instance of a web form's class and start invoking methods on the controls on our page, because outside of ASP.NET, those objects won't be there.

Our goal in making our legacy code unit-testable is to be able to test as much of the logic of our app as possible quickly and effectively, and to do this we need to isolate as much code as we can from external dependencies like these.

I'm a big believer that server pages and web forms should do as little as possible. Really, they should just be a very thin film of glue that binds the logic of user interactions and the display - which, if we think about it, is only marginally about Session and Request and HTML controls - with the meat and potatoes seperated away from knowledge of those details.

I might start to refactor this by extracting the meat and potatoes, complete with ASP.NET dependencies, into its own method.

Next, if I'm looking for some way to write the order data to the page without actually referencing the page or any of its controls, I need to extract methods that I can use to delegate this work through.

Now, for the magic. You'll like this. Not a lot, but you'll like it. If I make these helper methods for writing customer data to the web form public, I can extract an interface on the form's class, and have our controlling method speak to the form through that interface.

Next, we need to tackle that reference to Session. There are many different ways of breaking this dependency, but the simplest here might be to hide it behind another extracted helper method as a stepping stone to where I want to go next.

Now, I could just extact another interface on our form's class and pass that in. But I'm guessing we may want to have a shared abstraction we can reuse in a wider set of situations. Basically, imagine we don't want to implement SetSessionVariable (and, presumably, GetSessionVariable) on every web form. So, I'm going to extract a new class, and then extract an interface on that class.

Now we have a DisplayCustomerWithOrders method that depends only on abstractions for Session and for the web form - importantly, abstractions we control.

Next, I would extract this method into its own class. if you like, we can all it a "controller". (Let's make that one sacrifice to appease the gods of enterprise architecture.)

Now we're really getting somewhere. As it stands we could move CustomerController into a new .NET library, along with the interfaces it depends on, and this would all be unit-testable without the need to be running in the ASP.NET process.

We've got as bit of tweaking to do, first, though. For starters, if we follow the rule (not blindly, but with sound reason) that objects should be born with their collaborators, then let's refactor CustomerController along those lines, so any other controller methods we add can access the userSession and the view.

And while we're about it, we should make it possible for us to inject our DataRepository, so we can write unit tests that won't hit a real database.

We now have a controller that's isolated from the front and the back end of this application, and can be unit-tested using, for example, mock objects to check that it calls for the right customer and tells the view to set the right customer field and order values on the GUI.

A little bit of clean-up in our web form's class, just to tie up the loose ends...

The observant among you will have noticed that our refactored ASP.NET web form class is not smaller than it was. This is because my example is very simple in terms of business and control logic, and also because we only had one event to deal with. If this web form had multiple event handlers, and our business logic was more sophisticated, like in a real application, then the ratio of unit-testable code to web form code would normally start to tip in our favour.

It's often feasible to end up with 90% or more of our code to end up in unit-testable classes when we abstract away the external stuff like GUIs and databases, and make them substitutible for testing and other purposes.

Again, while all this refactoring was going on, I was disciplined enough to run a basic Selenium test script after each individual step to make sure the app was still working. But at the earliest opportunity, I would start writing unit tests to check the logic. Selenium's dandy and all, but when you have 10,000 business rules to check, testing them through a web browser requires a lot of down-time.

Posted 8 years, 3 months ago on October 28, 2012