To reach as much practical relevance as possible, this book shows how to implement a real-world scenario driven by unit tests. This way of proceeding allows us to explain the various concepts and techniques under the light of a coherent requirement. Thus, we kick off with a modest specification of what our example application will be all about. However, before finally descending into the depths of development practices, we will go ahead and clarify the basic characteristics of unit testing and test-first practices in dedicated sections.
Let's assume that we have to write a simple timeline component as it is known from the various social networks, such as Twitter, Google+, Facebook, and the like. To make things a bit more interesting, the application has to run on different platforms (desktop, browser, and mobile) and allow the display of content from arbitrary sources. The wireframe in the following image gives an impression of the individual functional requirements of our timeline:
Timeline wireframe
The header contains a label indicating the source of the items displayed in the list under it. It also notifies the user if newer entries are available and allows the them to fetch and insert them at the top.
The list section is a sequence of chronologically ordered items, which can be browsed by a scrollbar. The component should allow us to load its entries page-wise. This means that it shows a maximum of, let's say, ten entries. If scrolling reaches the last one, the next ten items can be fetched from the provider. The newly loaded entries are added and the scrollbar is adjusted accordingly. To keep things in scope, a push button for manual fetching will be sufficient here.
An item type, in turn, comprises several text or image attributes that compose an entry's content. Note that the timestamp is considered mandatory as it is needed for chronological ordering. Apart from that, the depiction should be undetermined by the component itself and depend on the type of the underlying information source.
This means that a Twitter feed probably provides a different information structure than the commits of a branch in a Git repository. The following image shows what the running applications will look like. The JUnit items shown are commits taken from the master branch of the tool's project repository at GitHub.
Given the application description, it is important to note that the following chapters will focus on the unit testing aspects of the development process to keep the book on target. But this immediately raises the question: what exactly is a unit test?