Introduction

In the previous chapters, you learned how to write unit tests. The question is: what can you unit test? Can you unit test activities and fragments? They are pretty hard to unit test on your machine because of the way they are built. Testing would be easier if you could move the code away from activities and fragments.

Also, consider the situation where you are building an application that supports different orientations, such as landscape and portrait, and supports multiple languages. What tends to happen in these scenarios by default is that when the user rotates the screen, the activities and fragments are recreated for the new display orientation. Now, imagine that happens while your application is in the middle of processing data. You have to keep track of the data you are processing, you have to keep track of what the user was doing to interact with your screens, and you have to avoid causing a context leak.

Note

A context leak occurs when your destroyed...