Tuesday, August 14, 2007

Tip:#2 Smell: Duplicate Tests Indicate a Missing Class

Smell: A class with two public methods on it, both perform the same functionality, or one is a subset of the other. The existence of a private method is a good indication of this.

To fully test these public methods you have to repeat a bunch of tests. There has to be a lazier simpler solution.

Recently I have been getting this smell a lot using MVC in a .Net winforms app.

.Net forms are hard to unit test. It is therefore helpful to keep your Views as thin as possible. Just use them to expose the form fields as a bunch of properties and to catch events and call the corresponding method on the controller. This moves the logic to the Controller where it is far more testable.

This can however lead to methods on the controller like "OnStartButtonClicked" and "OnStartMenuItemSelected". Both are going to perform the same actions. Both are going to need the same set of tests.

The solution is simple. Use the 'Extract Class' refactoring to pull the private method out to another class and use Dependency Injection to pass an instance of this new class back to the original class.

Following this above advice though you end up with another object. In my experience this split makes a lot of sense. I call this new class a Service. I rename the new thinner Controller to Presenter which better resembles it's remaining responsibilities.

(Note: I've read a few things on the differences between MVC and MVP, but I don't really get it. What I have here may be what is meant by the naming. Either way I like this design better.)

I'm really liking the new code. The view is really thin. The presenter translates UI events to service calls, and knows which views to update when the domain changes.

A piece of advice Steve Hayes often gives is "Design your UI layer so you could replace it with a command line and everything would keep working". I have struggled to do this with .Net, even with MVC. Having this new split however I can see keeping the Service and Model layers and replacing the UI would be easy.

So... Look out for duplicates tests... get lazy and write better code!

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