What to test
What should be tested? When using TDD and following its ideology, the answer is easy – everything. You only write production 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 Auto Layout, different screen sizes, and different localizations of apps, the exact position of buttons and labels depends on many parameters.
In general, you should test the features that make the app useful for a user and those that need to work. Users don't care whether the button is exactly 20 points from the rightmost edge of the screen. Users seek apps that are easy to understand and a joy to use.
In addition to this, you should not test the whole app at once using...
