Chapter 1: Your First Unit Tests

When the iPhone platform was first introduced, applications were small and focused only on one feature. It was easy to make money with an app that only did one thing (for example, a flashlight app that only showed a white screen). The code of these early apps only had a few hundred lines and could easily be tested by tapping the screen for a few minutes.

Since then, the App Store and the available apps have changed a lot. There are still small apps with a clear focus in the App Store, but it's much harder to make money from them. A common app has many features but still needs to be easy to use. There are companies with several developers working on one app full-time. These apps sometimes have a feature set that is normally found in desktop applications. It is very difficult and time-consuming to test all the features in such apps manually for every update.

One reason for this is that manual testing needs to be done through a user interface (UI), and it takes time to load the app to be tested. In addition to this, human beings are very slow compared to the capabilities of computers for tasks such as testing and verifying computer programs. Most of the time, a computer (or a smartphone) waits for the user's next input. If we could let a computer insert values, testing could be drastically accelerated. In fact, a computer can run several hundred tests within a few seconds. This is exactly what unit tests are all about.

A unit test is a piece of code that executes some other code and checks whether the result is what the developer expected. The word "unit" means that the test executes a small unit of code. Usually, that is one function of a class or some similar type of structure. How big the unit actually is depends on the feature to be tested and on the person who is writing the test.

Writing unit tests seems hard at first because for most developers, it's a new concept. This chapter is aimed at helping you get started with writing your first simple unit tests.

These are the main topics we will cover in the chapter:

• Building your first automatic unit test
• Assert functions in the XCTest framework
• Understanding the difference from other kinds of tests