向现有的旧版 WinForm 添加演示者以添加单元测试？

Question

At my work, we have a large legacy codebase written using winforms.

The legacy code was not written with any form of separation of concern and highly sensitive and fragile business logic is intertwined with button click events, grid row events, etc.

I would like to start bringing some of this logic under unit tests to give myself a better understanding of the logic and (hopefully) allow for a future migration away from winforms to be done with sligtly greater confidence and speed (is that a pipe dream?)

For begining to add unit tests, I am aware of Michael Feather's great book 'Working With Legacy Code' and the concept of introducing seams in the code that allow for testing. As I understand it, this is done by introducing services that can be injected into the forms to perform the business logic outside of the ui layer. This seems like a great approach for the buisness logic, but it doesn't seem to me to work well for testing more mundane ui elements, such as setting the visible state of user controls based on some user action.

For the above example, I was wondering if it makes sense to introduce a Presenter class for a legacy winform, pass the user interaction up to the newly created presenter, and have the presenter perform the correct logic and set the view state via a view-interface. This would then allow me to write a test on the presenter's method with a mocked view interface.

So something like the following

public class LegacyPresenter
{
  ILegacyView view;
  
  public void SetViewState(string value)
  {
    switch (value)
    {
      case "case 1":
        view.Button1Visible = true;
      case "case 2"
        view.Button1Visible = false;
    }
  }
}

public class LegacyView
{
  public LegacyPresenter Presenter;
  ...
  private void SomeEvent_Clicked()
  {
    this.Presenter.SetViewState("case 2");
  }
}

Currently in our code, there is a defect in the above 'SetViewState' method that exists inside the WinForm. Does it make sense to introduce a Presenter class for this legacy form, move the 'SetViewState' from the view to the new presenter, and then write unit tests against the presenter?

The form is currently 4k lines of code so I would only refactor this one method at this time to the presenter. Does this make any sense or would I just be wasting my time on legacy code?

Answer 1

To answer your question, yes it makes sense to create a partial implementation. This is then a starting point to move on from. When you need to make changes to the View, or if you have some time, you can move other pieces of logic to that Presenter. Over time, the View will become more and more light-weight and you can unit test most parts of your Presenter.

As you progress, try to keep your Presenter clean (adhering to basic SOLID principles) so that you don't end up with a Presenter with 4k lines of code. How you do that depends on your code of course.

But, going on a limb here, I would say that a unit-tested class of 4k lines of code is still better than a non-tested class of 4k lines of code.