What to test

What should be tested? When using TDD and following the rules mentioned in the previous sections, the answer is easy--everything. You only write code because there is a failing test.

In practice, it's not that easy. For example, should the position and color of a button be tested? Should the view hierarchy be tested? Probably not; the color and exact position of the button is not important for the functioning of an app. In the early stages of development, these kinds of things tend to change. With the auto layout and different localizations of the app, the exact position of buttons and labels depend on many parameters.

In general, you should test the features that make the app useful for a user and those that need to work. The ...

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