Double Loop TDD

Double loop TDD introduces the concept of acceptance tests in the Red -> Green -> Refactor classic TDD loop. The idea is to have a high-level test that covers a business requirement. As the name implies, we add a second loop to our TDD loop. This is an outer loop and also has Red -> Green -> Refactor stages. In this acceptance test, with the outside loop, we are effectively creating an executable definition of "done."

Figure 16.3: Double loop TDD

When starting a new feature, we begin by writing an acceptance test. This test will inevitably fail since we are lacking an implementation. Before we move on, we need to make sure our acceptance test is failing because it doesn't meet the acceptance criteria. This usually forces us to define the external API of our system.

Once we have a failing acceptance test, we move to the inner loop and stay on it until the acceptance test becomes green. The inner loop corresponds...