You can think of a fragment as a modular section of an activity, which has its own lifecycle,
receives its own input events, and which you can add or remove while the activity is running (sort
of like a "sub activity" that you can reuse in different activities). This lesson shows how to
extend the Fragment
class using the Support Library so your app
remains compatible with devices running system versions as low as Android 1.6.
Note: If you decide that the minimum
API level your app requires is 11 or higher, you don't need to use the Support
Library and can instead use the framework's built in Fragment
class and related
APIs. Just be aware that this lesson is focused on using the APIs from the Support Library, which
use a specific package signature and sometimes slightly different API names than the versions
included in the platform.
Before you begin this lesson, you must set up your Android project to use the Support Library.
If you have not used the Support Library before, set up your project to use the v4
library by following the Support Library
Setup document. However, you can also include the action bar in your activities by instead using the
v7 appcompat library, which is compatible with Android 2.1 (API level 7)
and also includes the Fragment
APIs.
Create a Fragment Class
To create a fragment, extend the Fragment
class, then override
key lifecycle methods to insert your app logic, similar to the way you would with an Activity
class.
One difference when creating a Fragment
is that you must use the
onCreateView()
callback to define the layout.
In fact, this is the only callback you need in order to get a fragment running. For
example, here's a simple fragment that specifies its own layout:
import android.os.Bundle; import android.support.v4.app.Fragment; import android.view.LayoutInflater; import android.view.ViewGroup; public class ArticleFragment extends Fragment { @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { // Inflate the layout for this fragment return inflater.inflate(R.layout.article_view, container, false); } }
Just like an activity, a fragment should implement other lifecycle callbacks that allow you to
manage its state as it is added or removed from the activity and as the activity transitions
between its lifecycle states. For instance, when the activity's onPause()
method is called, any fragments in the activity also receive a call
to onPause()
.
More information about the fragment lifecycle and callback methods is available in the Fragments developer guide.
Add a Fragment to an Activity using XML
While fragments are reusable, modular UI components, each instance of a Fragment
class must be associated with a parent FragmentActivity
. You can achieve this association by defining each
fragment within your activity layout XML file.
Note: FragmentActivity
is a
special activity provided in the Support Library to handle fragments on system versions older than
API level 11. If the lowest system version you support is API level 11 or higher, then you can use a
regular Activity
.
Here is an example layout file that adds two fragments to an activity when the device
screen is considered "large" (specified by the large
qualifier in the directory
name).
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="horizontal" android:layout_width="fill_parent" android:layout_height="fill_parent"> <fragment android:name="com.example.android.fragments.HeadlinesFragment" android:id="@+id/headlines_fragment" android:layout_weight="1" android:layout_width="0dp" android:layout_height="match_parent" /> <fragment android:name="com.example.android.fragments.ArticleFragment" android:id="@+id/article_fragment" android:layout_weight="2" android:layout_width="0dp" android:layout_height="match_parent" /> </LinearLayout>
Tip: For more about creating layouts for different screen sizes, read Supporting Different Screen Sizes.
Then apply the layout to your activity:
import android.os.Bundle; import android.support.v4.app.FragmentActivity; public class MainActivity extends FragmentActivity { @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.news_articles); } }
If you're using the v7
appcompat library, your activity should instead extend ActionBarActivity
, which is a subclass of FragmentActivity
(for more information,
read Adding the Action Bar).
Note: When you add a fragment to an activity layout by defining the fragment in the layout XML file, you cannot remove the fragment at runtime. If you plan to swap your fragments in and out during user interaction, you must add the fragment to the activity when the activity first starts, as shown in the next lesson.