5.3.2. Fragments

A fragment typically represents a reusable portion of an Activity’s user interface, but may also represent reusable program logic. This app uses fragments to create and manage portions of the app’s GUI. You can combine several fragments to create user interfaces that take advantage of tablet screen sizes. You also can easily interchange fragments to make your GUIs more dynamic—you’ll learn about this in Chapter 8.

Fragment (package android.app) is the base class of all fragments. The Flag Quiz app defines the following direct and indirect Fragment subclasses:

• Class QuizFragment (Section 5.6)—a direct subclass of Fragment—displays the quiz’s GUI and defines the quiz’s logic. Like an Activity, each Fragment has its own layout that’s typically defined as a layout resource, but can be created dynamically. In Section 5.4.8, you’ll build QuizFragment’s GUI. You’ll use the QuizFragment in MainActivity’s layouts—one for devices in portrait orientation and one for tablet devices in landscape orientation.

• Class SettingsFragment (Section 5.7) is a subclass of PreferenceFragment (package android.preference), which can automatically maintain an app’s user preferences in a SharedPreferences file on the device. As you’ll see, you can create an XML file that describes the user preferences and class PreferenceFragment can use that XML file to build an appropriate preferences GUI (Figs. 5.3–5.4).

• When you finish a quiz, the QuizFragment creates an anonymous inner class that extends DialogFragment (package android.app) and uses it to display an AlertDialog containing the quiz results (Section 5.6.9).

Fragments must be hosted by an Activity—they cannot execute independently. When this app runs in landscape orientation on a tablet, the MainActivity hosts all of the Fragments. In portrait orientation (on any device), the SettingsActivity (Section 5.8) hosts the SettingsFragment and the MainActivity hosts the others.

Though Fragments were introduced in Android 3.0, Fragments and other more recent Android features can be used in earlier versions via the Android Support Library. For more information, visit: