Fragment represents a
reusable portion of your app's UI. A fragment defines and manages its own
layout, has its own lifecycle, and can handle its own input events. Fragments
cannot live on their own--they must be hosted by an activity or another
fragment. The fragment’s view hierarchy becomes part of, or attaches to,
the host’s view hierarchy.
Fragments introduce modularity and reusability into your activity’s UI by allowing you to divide the UI into discrete chunks. Activities are an ideal place to put global elements around your app's user interface, such as a navigation drawer. Conversely, fragments are better suited to define and manage the UI of a single screen or portion of a screen.
Consider an app that responds to various screen sizes. On larger screens, the app should display a static navigation drawer and a list in a grid layout. On smaller screens, the app should display a bottom navigation bar and a list in a linear layout. Managing all of these variations in the activity can be unwieldy. Separating the navigation elements from the content can make this process more manageable. The activity is then responsible for displaying the correct navigation UI while the fragment displays the list with the proper layout.
Dividing your UI into fragments makes it easier to modify your activity's
appearance at runtime. While your activity is in the
lifecycle state or
higher, fragments can be added, replaced, or removed. You can keep a record
of these changes in a back stack that is managed by the activity, allowing
the changes to be reversed.
You can use multiple instances of the same fragment class within the same activity, in multiple activities, or even as a child of another fragment. With this in mind, you should only provide a fragment with the logic necessary to manage its own UI. You should avoid depending on or manipulating one fragment from another.
For more documentation and resources related to fragments, see the following.
- Fragment manager
- Fragment transactions
- Animate transitions between fragments
- Fragment lifecycle
- Saving state with fragments
- Communicate between fragments and activities
- Working with the app bar
- Displaying dialogs with DialogFragment
- Debug your fragments
- Test your fragments
- Single Activity: Why, When, and How (Android Dev Summit ‘18)
- Fragments: Past, present, and future (Android Dev Summit '19)