20.9 Summary

All activities are derived from the Android Activity class which, in turn, contains a number of lifecycle methods that are designed to be called by the runtime system when the state of an activity changes. Similarly, the Fragment class contains a number of comparable methods. By overriding these methods, activities and fragments can respond to state changes and, where necessary, take steps to save and restore the current state of both the activity and the application. Lifecycle state can be thought of as taking two forms. The persistent state refers to data that needs to be stored between application invocations (for example to a file or database). Dynamic state, on the other hand, relates instead to the current appearance of the user interface.

Although lifecycle methods have a number of limitations that can be avoided by making use of lifecycle-aware components, an understanding of these methods is important in order to fully understand the new approaches to lifecycle...