Delphi style: How to structure data modules for unit-testable code?

Question

I am looking for some advice about structuring Delphi programs for maintainability. I\'ve come to Delphi programming after a couple of decades of mostly C/C++ though I first lea

Answer 1

Firstly before you change anything you need some unit tests so you can ensure you don't break anything. I would attempt to write unit tests against the current GUI without changing anything. DUnit has support for GUI testing (along with traditional unit testing) and although it's a little clunky and can't handle modal dialogs it is functional.

Next, since your forms don't use data aware controls I would approach this by introducing another layer of data modules, a service layer if you will, between the forms and the existing global data modules.

For every form in your application I would create a corresponding new service layer data module. This may sound like a lot of data modules but they're very lightweight and you can consolidate them later if you want.

You could use ordinary TObjects rather than TDataModules for the service layer if you liked however using data modules gives you the flexibility of being able to place non-visual components on them later, for example a TClientDataSet and TDataSource if you went down the data-aware controls route at a later date.

Initially each service layer data module would merely act as a proxy for accessing the global data modules. Your goal at this point would be simply to remove the direct dependency of the forms on the global data modules.

Once the forms only indirectly accessed the global data modules via the service layer data modules then I would start to move functionality from the forms into the service layer. With this functionality in the service layer data modules you will find it much easier to write unit tests for new and existing code.

At this point you could also start consolidating the per-form service layer data modules. It will be much easier to consolidate them now after the logic extraction from the forms is complete than if you try do do it during that process.