从屏幕继承的单元测试ViewModel（Caliburn.Micro）

Question

I've started learning TDD, and I bumped into ViewModel testing. I'm using Caliburn.Micro as my MVVM framework, and my ViewModels are inheriting from Screen.

When I create try to test a ViewModel, I get this error:

The type 'Caliburn.Micro.Screen' is defined in an assembly that is not referenced. You must add a reference to assembly 'Caliburn.Micro'

Is this OK? I think isolating my classes from the framework is the right path, but I don't know how should I do it (preserving the benefits the framework gives by inheriting from Screen)

Thanks, Martín

Answer 1

Yes it is okay to add the reference. The only alternative is dynamically creating the type, which does not really offer you much advantage.

I assume your ViewModel will not try to display any screens. If it does you will need to subclass it to get a version that can be unit tested.

I haven't used Caliburn.Micro but it sounds a bit odd that your ViewModel inherits from Screen. Normally with MVVM the ViewModel knows about the View but does not inherit from one. A quick Google search suggests that in Caliburn.Micro the ViewModel implements the IScreen interface, as opposed to inheriting from Screen.

Answer 2

Does the dependency on Screen pull in a bunch of other things too... if not, might not be a big deal.

The ViewModels are part of the presentation layer and are usually coupled with the GUI framework that you use.. Corollary: the ViewModels might need to be reworked/thrown away when the GUI framework changes unless you design for it (separate the business logic from the presentation and push it down). If you're incensed by this para, check out Robert Martin's talk on Clean architecture at NDC Oslo (available online)