The TDD workflow – red, green, refactor

The normal workflow of TDD comprises three steps – red, green, refactor. The following sections describe these steps in detail.

Red

You start by writing a failing test. It needs to test a required feature of the software product that is not already implemented or an edge case that you want to make sure is covered. The name red comes from the way most IDEs indicate a failing test. Xcode uses a red diamond with a white x on it as shown in the following figure:

Figure 2.1 – Xcode marks a failing test with a white cross in a red diamond

It is very important that the test you write in this step initially fails. Otherwise, you can't ensure that the test works and really tests the feature that you want to implement. It could be that you have written a test that always passes and is, therefore, useless. Or, it is possible that the feature is already implemented. Either way,...