page.title=Fragments
parent.title=Activities
parent.link=activities.html
@jd:body

<div id="qv-wrapper">
<div id="qv">
  <h2>In this document</h2>
  <ol>
    <li><a href="#Design">Design Philosophy</a></li>
    <li><a href="#Creating">Creating a Fragment</a>
      <ol>
        <li><a href="#UI">Adding a user interface</a></li>
        <li><a href="#Adding">Adding a fragment to an activity</a></li>
      </ol>
    </li>
    <li><a href="#Managing">Managing Fragments</a></li>
    <li><a href="#Transactions">Performing Fragment Transactions</a></li>
    <li><a href="#CommunicatingWithActivity">Communicating with the Activity</a>
      <ol>
        <li><a href="#EventCallbacks">Creating event callbacks to the activity</a></li>
        <li><a href="#ActionBar">Adding items to the Action Bar</a></li>
      </ol>
    </li>
    <li><a href="#Lifecycle">Handling the Fragment Lifecycle</a>
      <ol>
        <li><a href="#CoordinatingWithActivity">Coordinating with the activity lifecycle</a></li>
      </ol>
    </li>
    <li><a href="#Example">Example</a></li>
  </ol>

  <h2>Key classes</h2>
  <ol>
    <li>{@link android.app.Fragment}</li>
    <li>{@link android.app.FragmentManager}</li>
    <li>{@link android.app.FragmentTransaction}</li>
  </ol>
  
  <h2>See also</h2>
  <ol>
    <li><a href="{@docRoot}training/basics/fragments/index.html">Building a Dynamic UI with Fragments</a></li>
    <li><a href="{@docRoot}guide/practices/tablets-and-handsets.html">Supporting Tablets
and Handsets</a></li>
  </ol>
</div>
</div>

<p>A {@link android.app.Fragment} represents a behavior or a portion of user interface in an
{@link android.app.Activity}. You can combine multiple fragments in a single activity to build a
multi-pane UI and reuse a fragment in multiple activities. 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).</p>

<p>A fragment must always be embedded in an activity and the fragment's lifecycle is directly
affected by the host activity's lifecycle. For example, when the activity is paused, so are all
fragments in it, and when the activity is destroyed, so are all fragments. However, while an
activity is running (it is in the <em>resumed</em> <a
href="{@docRoot}guide/components/activities.html#Lifecycle">lifecycle state</a>), you can
manipulate each fragment independently, such as add or remove them. When you perform such a
fragment transaction, you can also add it to a back stack that's managed by the
activity&mdash;each back stack entry in the activity is a record of the fragment transaction that
occurred. The back stack allows the user to reverse a fragment transaction (navigate backwards),
by pressing the <em>Back</em> button.</p>

<p>When you add a fragment as a part of your activity layout, it lives in a {@link
android.view.ViewGroup} inside the activity's view hierarchy and the fragment defines its own view
layout.
You can insert a fragment into your activity layout by declaring the fragment in the activity's
layout file, as a {@code &lt;fragment&gt;} element, or from your application code by adding it to an
existing {@link android.view.ViewGroup}. However, a fragment is not required to be a part of the
activity layout; you may also use a fragment without its own UI as an invisible worker for the
activity.</p>

<p>This document describes how to build your application to use fragments, including
how fragments can maintain their state when added to the activity's back stack, share
events with the activity and other fragments in the activity, contribute to the activity's action
bar, and more.</p>


<h2 id="Design">Design Philosophy</h2>

<p>Android introduced fragments in Android 3.0 (API level 11), primarily to support more
dynamic and flexible UI designs on large screens, such as tablets. Because a
tablet's screen is much larger than that of a handset, there's more room to combine and
interchange UI components. Fragments allow such designs without the need for you to manage complex
changes to the view hierarchy. By dividing the layout of an activity into fragments, you become able
to modify the activity's appearance at runtime and preserve those changes in a back stack
that's managed by the activity.</p>

<p>For example, a news application can use one fragment to show a list of articles on the
left and another fragment to display an article on the right&mdash;both fragments appear in one
activity, side by side, and each fragment has its own set of lifecycle callback methods and handle
their own user input events. Thus, instead of using one activity to select an article and another
activity to read the article, the user can select an article and read it all within the same
activity, as illustrated in the tablet layout in figure 1.</p>

<p>You should design each fragment as a modular and reusable activity component. That is, because
each fragment defines its own layout and its own behavior with its own lifecycle callbacks, you can
include one fragment in multiple activities, so you should design for reuse and avoid directly
manipulating one fragment from another fragment. This is especially important because a modular
fragment allows you to change your fragment combinations for different screen sizes. When designing
your application to support both tablets and handsets, you can reuse your fragments in different
layout configurations to optimize the user experience based on the available screen space. For
example, on a handset, it might be necessary to separate fragments to provide a single-pane UI when
more than one cannot fit within the same activity.</p>

<img src="{@docRoot}images/fundamentals/fragments.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> An example of how two UI modules defined by
fragments can be combined into one activity for a tablet design, but separated for a
handset design.</p>

<p>For example&mdash;to continue with the news application example&mdash;the application can embed
two fragments in <em>Activity A</em>, when running on a tablet-sized device. However, on a
handset-sized screen, there's not enough room for both fragments, so <em>Activity A</em> includes
only the fragment for the list of articles, and when the user selects an article, it starts
<em>Activity B</em>, which includes the second fragment to read the article. Thus, the application
supports both tablets and handsets by reusing fragments in different combinations, as illustrated in
figure 1.</p>

<p>For more information about designing your application with different fragment combinations for
different screen configurations, see the guide to <a
href="{@docRoot}guide/practices/tablets-and-handsets.html">Supporting Tablets and Handsets</a>.</p>


<h2 id="Creating">Creating a Fragment</h2>

<div class="figure" style="width:327px">
<img src="{@docRoot}images/fragment_lifecycle.png" alt="" />
<p class="img-caption"><strong>Figure 2.</strong> The lifecycle of a fragment (while its
activity is running).</p>
</div>

<p>To create a fragment, you must create a subclass of {@link android.app.Fragment} (or an existing
subclass of it). The {@link android.app.Fragment} class has code that looks a lot like
an {@link android.app.Activity}. It contains callback methods similar to an activity, such
as {@link android.app.Fragment#onCreate onCreate()}, {@link android.app.Fragment#onStart onStart()},
{@link android.app.Fragment#onPause onPause()}, and {@link android.app.Fragment#onStop onStop()}. In
fact, if you're converting an existing Android application to use fragments, you might simply move
code from your activity's callback methods into the respective callback methods of your
fragment.</p>

<p>Usually, you should implement at least the following lifecycle methods:</p>

<dl>
  <dt>{@link android.app.Fragment#onCreate onCreate()}</dt>
  <dd>The system calls this when creating the fragment. Within your implementation, you should
initialize essential components of the fragment that you want to retain when the fragment is
paused or stopped, then resumed.</dd>
  <dt>{@link android.app.Fragment#onCreateView onCreateView()}</dt>
  <dd>The system calls this when it's time for the fragment to draw its user interface for the
first time. To draw a UI for your fragment, you must return a {@link android.view.View} from this
method that is the root of your fragment's layout. You can return null if the fragment does not
provide a UI.</dd>
  <dt>{@link android.app.Activity#onPause onPause()}</dt>
  <dd>The system calls this method as the first indication that the user is leaving the
fragment (though it does not always mean the fragment is being destroyed). This is usually where you
should commit any changes that should be persisted beyond the current user session (because
the user might not come back).</dd>
</dl>

<p>Most applications should implement at least these three methods for every fragment, but there are
several other callback methods you should also use to handle various stages of the
fragment lifecycle. All the lifecycle callback methods are discussed in more detail in the section
about <a href="#Lifecycle">Handling the Fragment Lifecycle</a>.</p>


<p>There are also a few subclasses that you might want to extend, instead of the base {@link
android.app.Fragment} class:</p>

<dl>
  <dt>{@link android.app.DialogFragment}</dt>
  <dd>Displays a floating dialog. Using this class to create a dialog is a good alternative to using
the dialog helper methods in the {@link android.app.Activity} class, because you can
incorporate a fragment dialog into the back stack of fragments managed by the activity,
allowing the user to return to a dismissed fragment.</dd>

  <dt>{@link android.app.ListFragment}</dt>
  <dd>Displays a list of items that are managed by an adapter (such as a {@link
android.widget.SimpleCursorAdapter}), similar to {@link android.app.ListActivity}. It provides
several methods for managing a list view, such as the {@link
android.app.ListFragment#onListItemClick(ListView,View,int,long) onListItemClick()} callback to
handle click events.</dd>

  <dt>{@link android.preference.PreferenceFragment}</dt>
  <dd>Displays a hierarchy of {@link android.preference.Preference} objects as a list, similar to
{@link android.preference.PreferenceActivity}. This is useful when creating a "settings"
activity for your application.</dd>
</dl>


<h3 id="UI">Adding a user interface</h3>

<p>A fragment is usually used as part of an activity's user interface and contributes its own
layout to the activity.</p>

<p>To provide a layout for a fragment, you must implement the {@link
android.app.Fragment#onCreateView onCreateView()} callback method, which the Android system calls
when it's time for the fragment to draw its layout. Your implementation of this method must return a
{@link android.view.View} that is the root of your fragment's layout.</p>

<p class="note"><strong>Note:</strong> If your fragment is a subclass of {@link
android.app.ListFragment}, the default implementation returns a {@link android.widget.ListView} from
{@link android.app.Fragment#onCreateView onCreateView()}, so you don't need to implement it.</p>

<p>To return a layout from {@link
android.app.Fragment#onCreateView onCreateView()}, you can inflate it from a <a
href="{@docRoot}guide/topics/resources/layout-resource.html">layout resource</a> defined in XML. To
help you do so, {@link android.app.Fragment#onCreateView onCreateView()} provides a
{@link android.view.LayoutInflater} object.</p>

<p>For example, here's a subclass of {@link android.app.Fragment} that loads a layout from the
{@code example_fragment.xml} file:</p>

<pre>
public static class ExampleFragment extends Fragment {
    &#64;Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.example_fragment, container, false);
    }
}
</pre>

<div class="sidebox-wrapper">
<div class="sidebox">
  <h3>Creating a layout</h3>
  <p>In the sample above, {@code R.layout.example_fragment} is a reference to a layout resource
named {@code example_fragment.xml} saved in the application resources. For information about how to
create a layout in XML, see the <a href="{@docRoot}guide/topics/ui/index.html">User Interface</a>
documentation.</p>
</div>
</div>

<p>The {@code container} parameter passed to {@link android.app.Fragment#onCreateView
onCreateView()} is the parent {@link android.view.ViewGroup} (from the activity's layout) in which
your fragment layout
will be inserted. The {@code savedInstanceState} parameter is a {@link android.os.Bundle} that
provides data about the previous instance of the fragment, if the fragment is being resumed
(restoring state is discussed more in the section about <a href="#Lifecycle">Handling the
Fragment Lifecycle</a>).</p>

<p>The {@link android.view.LayoutInflater#inflate(int,ViewGroup,boolean) inflate()} method takes
three arguments:</p>
<ul>
  <li>The resource ID of the layout you want to inflate.</li>
  <li>The {@link android.view.ViewGroup} to be the parent of the inflated layout. Passing the {@code
container} is important in order for the system to apply layout parameters to the root view of the
inflated layout, specified by the parent view in which it's going.</li>
  <li>A boolean indicating whether the inflated layout should be attached to the {@link
android.view.ViewGroup} (the second parameter) during inflation. (In this case, this
is false because the system is already inserting the inflated layout into the {@code
container}&mdash;passing true would create a redundant view group in the final layout.)</li>
</ul>

<p>Now you've seen how to create a fragment that provides a layout. Next, you need to add
the fragment to your activity.</p>


<h3 id="Adding">Adding a fragment to an activity</h3>

<p>Usually, a fragment contributes a portion of UI to the host activity, which is embedded as a part
of the activity's overall view hierarchy. There are two ways you can add a fragment to the activity
layout:</p>

<ul>
  <li><b>Declare the fragment inside the activity's layout file.</b>
<p>In this case, you can
specify layout properties for the fragment as if it were a view. For example, here's the layout
file for an activity with two fragments:</p>
<pre>
&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent"&gt;
    &lt;fragment android:name="com.example.news.ArticleListFragment"
            android:id="@+id/list"
            android:layout_weight="1"
            android:layout_width="0dp"
            android:layout_height="match_parent" /&gt;
    &lt;fragment android:name="com.example.news.ArticleReaderFragment"
            android:id="@+id/viewer"
            android:layout_weight="2"
            android:layout_width="0dp"
            android:layout_height="match_parent" /&gt;
&lt;/LinearLayout&gt;
</pre>
  <p>The {@code android:name} attribute in the {@code &lt;fragment&gt;} specifies the {@link
android.app.Fragment} class to instantiate in the layout.</p>

<p>When the system creates this activity layout, it instantiates each fragment specified in the
layout and calls the {@link android.app.Fragment#onCreateView onCreateView()} method for each one,
to retrieve each fragment's layout. The system inserts the {@link android.view.View} returned by the
fragment directly in place of the {@code &lt;fragment&gt;} element.</p>

<div class="note">
  <p><strong>Note:</strong> Each fragment requires a unique identifier that
the system can use to restore the fragment if the activity is restarted (and which you can use to
capture the fragment to perform transactions, such as remove it). There are three ways to provide an
ID for a fragment:</p>
  <ul>
    <li>Supply the {@code android:id} attribute with a unique ID.</li>
    <li>Supply the {@code android:tag} attribute with a unique string.</li>
    <li>If you provide neither of the previous two, the system uses the ID of the container 
view.</li>
  </ul>
</div>
  </li>

  <li><b>Or, programmatically add the fragment to an existing {@link android.view.ViewGroup}.</b>
<p>At any time while your activity is running, you can add fragments to your activity layout. You
simply need to specify a {@link
android.view.ViewGroup} in which to place the fragment.</p>
  <p>To make fragment transactions in your activity (such as add, remove, or replace a
fragment), you must use APIs from {@link android.app.FragmentTransaction}. You can get an instance
of {@link android.app.FragmentTransaction} from your {@link android.app.Activity} like this:</p>

<pre>
FragmentManager fragmentManager = {@link android.app.Activity#getFragmentManager()}
FragmentTransaction fragmentTransaction = fragmentManager.{@link android.app.FragmentManager#beginTransaction()};
</pre>

<p>You can then add a fragment using the {@link
android.app.FragmentTransaction#add(int,Fragment) add()} method, specifying the fragment to add and
the view in which to insert it. For example:</p>

<pre>
ExampleFragment fragment = new ExampleFragment();
fragmentTransaction.add(R.id.fragment_container, fragment);
fragmentTransaction.commit();
</pre>

  <p>The first argument passed to {@link android.app.FragmentTransaction#add(int,Fragment) add()}
is the {@link android.view.ViewGroup} in which the fragment should be placed, specified by
resource ID, and the second parameter is the fragment to add.</p>
  <p>Once you've made your changes with
{@link android.app.FragmentTransaction}, you must
call {@link android.app.FragmentTransaction#commit} for the changes to take effect.</p>
  </li>
</ul>


<h4 id="AddingWithoutUI">Adding a fragment without a UI</h4>

<p>The examples above show how to add a fragment to your activity in order to provide a UI. However,
you can also use a fragment to provide a background behavior for the activity without presenting
additional UI.</p>

<p>To add a fragment without a UI, add the fragment from the activity using {@link
android.app.FragmentTransaction#add(Fragment,String)} (supplying a unique string "tag" for the
fragment, rather than a view ID). This adds the fragment, but, because it's not associated with a
view in the activity layout, it does not receive a call to {@link
android.app.Fragment#onCreateView onCreateView()}. So you don't need to implement that method.</p>

<p>Supplying a string tag for the fragment isn't strictly for non-UI fragments&mdash;you can also
supply string tags to fragments that do have a UI&mdash;but if the fragment does not have a
UI, then the string tag is the only way to identify it. If you want to get the fragment from the
activity later, you need to use {@link android.app.FragmentManager#findFragmentByTag
findFragmentByTag()}.</p>

<p>For an example activity that uses a fragment as a background worker, without a UI, see the {@code
FragmentRetainInstance.java} sample, which is included in the SDK samples (available through the
Android SDK Manager) and located on your system as 
<code>&lt;sdk_root&gt;/APIDemos/app/src/main/java/com/example/android/apis/app/FragmentRetainInstance.java</code>.</p>


<h2 id="Managing">Managing Fragments</h2>

<p>To manage the fragments in your activity, you need to use {@link android.app.FragmentManager}. To
get it, call {@link android.app.Activity#getFragmentManager()} from your activity.</p>

<p>Some things that you can do with {@link android.app.FragmentManager} include:</p>

<ul>
  <li>Get fragments that exist in the activity, with {@link
android.app.FragmentManager#findFragmentById findFragmentById()} (for fragments that provide a UI in
the activity layout) or {@link android.app.FragmentManager#findFragmentByTag
findFragmentByTag()} (for fragments that do or don't provide a UI).</li> 
  <li>Pop fragments off the back stack, with {@link
android.app.FragmentManager#popBackStack()} (simulating a <em>Back</em> command by the user).</li>
  <li>Register a listener for changes to the back stack, with {@link
android.app.FragmentManager#addOnBackStackChangedListener addOnBackStackChangedListener()}.</li>
</ul>

<p>For more information about these methods and others, refer to the {@link
android.app.FragmentManager} class documentation.</p>

<p>As demonstrated in the previous section, you can also use {@link android.app.FragmentManager}
to open a {@link android.app.FragmentTransaction}, which allows you to perform transactions, such as
add and remove fragments.</p>


<h2 id="Transactions">Performing Fragment Transactions</h2>

<p>A great feature about using fragments in your activity is the ability to add, remove, replace,
and perform other actions with them, in response to user interaction. Each set of changes that you
commit to the activity is called a transaction and you can perform one using APIs in {@link
android.app.FragmentTransaction}. You can also save each transaction to a back stack managed by the
activity, allowing the user to navigate backward through the fragment changes (similar to navigating
backward through activities).</p>

<p>You can acquire an instance of {@link android.app.FragmentTransaction} from the {@link
android.app.FragmentManager} like this:</p>

<pre>
FragmentManager fragmentManager = {@link android.app.Activity#getFragmentManager()};
FragmentTransaction fragmentTransaction = fragmentManager.{@link android.app.FragmentManager#beginTransaction()};
</pre>

<p>Each transaction is a set of changes that you want to perform at the same time. You can set
up all the changes you want to perform for a given transaction using methods such as {@link
android.app.FragmentTransaction#add add()}, {@link android.app.FragmentTransaction#remove remove()},
and {@link android.app.FragmentTransaction#replace replace()}. Then, to apply the transaction
to the activity, you must call {@link android.app.FragmentTransaction#commit()}.</p>
</dl>

<p>Before you call {@link
android.app.FragmentTransaction#commit()}, however, you might want to call {@link
android.app.FragmentTransaction#addToBackStack addToBackStack()}, in order to add the transaction
to a back stack of fragment transactions. This back stack is managed by the activity and allows
the user to return to the previous fragment state, by pressing the <em>Back</em> button.</p>

<p>For example, here's how you can replace one fragment with another, and preserve the previous
state in the back stack:</p>

<pre>
// Create new fragment and transaction
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);

// Commit the transaction
transaction.commit();
</pre>

<p>In this example, {@code newFragment} replaces whatever fragment (if any) is currently in the
layout container identified by the {@code R.id.fragment_container} ID. By calling {@link
android.app.FragmentTransaction#addToBackStack addToBackStack()}, the replace transaction is
saved to the back stack so the user can reverse the transaction and bring back the
previous fragment by pressing the <em>Back</em> button.</p>

<p>If you add multiple changes to the transaction (such as another {@link
android.app.FragmentTransaction#add add()} or {@link android.app.FragmentTransaction#remove
remove()}) and call {@link
android.app.FragmentTransaction#addToBackStack addToBackStack()}, then all changes applied
before you call {@link android.app.FragmentTransaction#commit commit()} are added to the
back stack as a single transaction and the <em>Back</em> button will reverse them all together.</p>

<p>The order in which you add changes to a {@link android.app.FragmentTransaction} doesn't matter,
except:</p>
<ul>
  <li>You must call {@link android.app.FragmentTransaction#commit()} last</li>
  <li>If you're adding multiple fragments to the same container, then the order in which
you add them determines the order they appear in the view hierarchy</li>
</ul>

<p>If you do not call {@link android.app.FragmentTransaction#addToBackStack(String)
addToBackStack()} when you perform a transaction that removes a fragment, then that fragment is
destroyed when the transaction is committed and the user cannot navigate back to it. Whereas, if you
do call {@link android.app.FragmentTransaction#addToBackStack(String) addToBackStack()} when
removing a fragment, then the fragment is <em>stopped</em> and will be resumed if the user navigates
back.</p>

<p class="note"><strong>Tip:</strong> For each fragment transaction, you can apply a transition
animation, by calling {@link android.app.FragmentTransaction#setTransition setTransition()} before
you commit.</p>

<p>Calling {@link android.app.FragmentTransaction#commit()} does not perform the transaction
immediately. Rather, it schedules it to run on the activity's UI thread (the "main" thread) as soon
as the thread is able to do so. If necessary, however, you may call {@link
android.app.FragmentManager#executePendingTransactions()} from your UI thread to immediately execute
transactions submitted by {@link android.app.FragmentTransaction#commit()}. Doing so is
usually not necessary unless the transaction is a dependency for jobs in other threads.</p>

<p class="caution"><strong>Caution:</strong> You can commit a transaction using {@link
android.app.FragmentTransaction#commit commit()} only prior to the activity <a
href="{@docRoot}guide/components/activities.html#SavingActivityState">saving its
state</a> (when the user leaves the activity). If you attempt to commit after that point, an
exception will be thrown. This is because the state after the commit can be lost if the activity
needs to be restored. For situations in which its okay that you lose the commit, use {@link
android.app.FragmentTransaction#commitAllowingStateLoss()}.</p>


<h2 id="CommunicatingWithActivity">Communicating with the Activity</h2>

<p>Although a {@link android.app.Fragment} is implemented as an object that's independent from an
{@link android.app.Activity} and can be used inside multiple activities, a given instance of
a fragment is directly tied to the activity that contains it.</p>

<p>Specifically, the fragment can access the {@link android.app.Activity} instance with {@link
android.app.Fragment#getActivity()} and easily perform tasks such as find a view in the
activity layout:</p>

<pre>
View listView = {@link android.app.Fragment#getActivity()}.{@link android.app.Activity#findViewById findViewById}(R.id.list);
</pre>

<p>Likewise, your activity can call methods in the fragment by acquiring a reference to the
{@link android.app.Fragment} from {@link android.app.FragmentManager}, using {@link
android.app.FragmentManager#findFragmentById findFragmentById()} or {@link
android.app.FragmentManager#findFragmentByTag findFragmentByTag()}. For example:</p>

<pre>
ExampleFragment fragment = (ExampleFragment) getFragmentManager().findFragmentById(R.id.example_fragment);
</pre>


<h3 id="EventCallbacks">Creating event callbacks to the activity</h3>

<p>In some cases, you might need a fragment to share events with the activity. A good way to do that
is to define a callback interface inside the fragment and require that the host activity implement
it. When the activity receives a callback through the interface, it can share the information with
other fragments in the layout as necessary.</p>

<p>For example, if a news application has two fragments in an activity&mdash;one to show a list of
articles (fragment A) and another to display an article (fragment B)&mdash;then fragment A must tell
the activity when a list item is selected so that it can tell fragment B to display the article. In
this case, the {@code OnArticleSelectedListener} interface is declared inside fragment A:</p>

<pre>
public static class FragmentA extends ListFragment {
    ...
    // Container Activity must implement this interface
    public interface OnArticleSelectedListener {
        public void onArticleSelected(Uri articleUri);
    }
    ...
}
</pre>

<p>Then the activity that hosts the fragment implements the {@code OnArticleSelectedListener}
interface and
overrides {@code onArticleSelected()} to notify fragment B of the event from fragment A. To ensure
that the host activity implements this interface, fragment A's {@link
android.app.Fragment#onAttach onAttach()} callback method (which the system calls when adding
the fragment to the activity) instantiates an instance of {@code OnArticleSelectedListener} by
casting the {@link android.app.Activity} that is passed into {@link android.app.Fragment#onAttach
onAttach()}:</p>

<pre>
public static class FragmentA extends ListFragment {
    OnArticleSelectedListener mListener;
    ...
    &#64;Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnArticleSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnArticleSelectedListener");
        }
    }
    ...
}
</pre>

<p>If the activity has not implemented the interface, then the fragment throws a
{@link java.lang.ClassCastException}.
On success, the {@code mListener} member holds a reference to activity's implementation of 
{@code OnArticleSelectedListener}, so that fragment A can share events with the activity by calling
methods defined by the {@code OnArticleSelectedListener} interface. For example, if fragment A is an
extension of {@link android.app.ListFragment}, each time
the user clicks a list item, the system calls {@link android.app.ListFragment#onListItemClick
onListItemClick()} in the fragment, which then calls {@code onArticleSelected()} to share
the event with the activity:</p>

<pre>
public static class FragmentA extends ListFragment {
    OnArticleSelectedListener mListener;
    ...
    &#64;Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        // Append the clicked item's row ID with the content provider Uri
        Uri noteUri = ContentUris.{@link android.content.ContentUris#withAppendedId withAppendedId}(ArticleColumns.CONTENT_URI, id);
        // Send the event and Uri to the host activity
        mListener.onArticleSelected(noteUri);
    }
    ...
}
</pre>

<p>The {@code id} parameter passed to {@link
android.app.ListFragment#onListItemClick onListItemClick()} is the row ID of the clicked item,
which the activity (or other fragment) uses to fetch the article from the application's {@link
android.content.ContentProvider}.</p>

<p><!--To see a complete implementation of this kind of callback interface, see the <a
href="{@docRoot}resources/samples/NotePad/index.html">NotePad sample</a>. -->More information about
using a content provider is available in the <a
href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a> document.</p>


<h3 id="ActionBar">Adding items to the Action Bar</h3>

<p>Your fragments can contribute menu items to the activity's <a
href="{@docRoot}guide/topics/ui/menus.html#options-menu">Options Menu</a> (and, consequently, the <a
href="{@docRoot}guide/topics/ui/actionbar.html">Action Bar</a>) by implementing
{@link android.app.Fragment#onCreateOptionsMenu(Menu,MenuInflater) onCreateOptionsMenu()}. In order
for this method to receive calls, however, you must call {@link
android.app.Fragment#setHasOptionsMenu(boolean) setHasOptionsMenu()} during {@link
android.app.Fragment#onCreate(Bundle) onCreate()}, to indicate that the fragment
would like to add items to the Options Menu (otherwise, the fragment will not receive a call to
{@link android.app.Fragment#onCreateOptionsMenu onCreateOptionsMenu()}).</p>

<p>Any items that you then add to the Options Menu from the fragment are appended to the existing
menu items. The fragment also receives callbacks to {@link
android.app.Fragment#onOptionsItemSelected(MenuItem) onOptionsItemSelected()} when a menu item
is selected.</p>

<p>You can also register a view in your fragment layout to provide a context menu by calling {@link
android.app.Fragment#registerForContextMenu(View) registerForContextMenu()}. When the user opens
the context menu, the fragment receives a call to {@link
android.app.Fragment#onCreateContextMenu(ContextMenu,View,ContextMenu.ContextMenuInfo)
onCreateContextMenu()}. When the user selects an item, the fragment receives a call to {@link
android.app.Fragment#onContextItemSelected(MenuItem) onContextItemSelected()}.</p>

<p class="note"><strong>Note:</strong> Although your fragment receives an on-item-selected callback
for each menu item it adds, the activity is first to receive the respective callback when the user
selects a menu item. If the activity's implementation of the on-item-selected callback does not
handle the selected item, then the event is passed to the fragment's callback. This is true for
the Options Menu and context menus.</p>

<p>For more information about menus, see the <a
href="{@docRoot}guide/topics/ui/menus.html">Menus</a> and <a
href="{@docRoot}guide/topics/ui/actionbar.html">Action Bar</a> developer guides.</p>


<h2 id="Lifecycle">Handling the Fragment Lifecycle</h2>

<div class="figure" style="width:350px">
<img src="{@docRoot}images/activity_fragment_lifecycle.png" alt=""/>
<p class="img-caption"><strong>Figure 3.</strong> The effect of the activity lifecycle on the fragment
lifecycle.</p>
</div>

<p>Managing the lifecycle of a fragment is a lot like managing the lifecycle of an activity. Like
an activity, a fragment can exist in three states:</p>

<dl>
  <dt><i>Resumed</i></dt>
    <dd>The fragment is visible in the running activity.</dd>

  <dt><i>Paused</i></dt>
    <dd>Another activity is in the foreground and has focus, but the activity in which this
fragment lives is still visible (the foreground activity is partially transparent or doesn't
cover the entire screen).</dd>

  <dt><i>Stopped</i></dt>
    <dd>The fragment is not visible. Either the host activity has been stopped or the
fragment has been removed from the activity but added to the back stack. A stopped fragment is
still alive (all state and member information is retained by the system). However, it is no longer
visible to the user and will be killed if the activity is killed.</dd>
</dl>

<p>Also like an activity, you can retain the state of a fragment using a {@link
android.os.Bundle}, in case the activity's process is killed and you need to restore the
fragment state when the activity is recreated. You can save the state during the fragment's {@link
android.app.Fragment#onSaveInstanceState onSaveInstanceState()} callback and restore it during
either {@link android.app.Fragment#onCreate onCreate()}, {@link
android.app.Fragment#onCreateView onCreateView()}, or {@link
android.app.Fragment#onActivityCreated onActivityCreated()}. For more information about saving
state, see the <a
href="{@docRoot}guide/components/activities.html#SavingActivityState">Activities</a>
document.</p>

<p>The most significant difference in lifecycle between an activity and a fragment is how one is
stored in its respective back stack. An activity is placed into a back stack of activities
that's managed by the system when it's stopped, by default (so that the user can navigate back
to it with the <em>Back</em> button, as discussed in <a
href="{@docRoot}guide/components/tasks-and-back-stack.html">Tasks and Back Stack</a>).
However, a fragment is placed into a back stack managed by the host activity only when you
explicitly request that the instance be saved by calling {@link
android.app.FragmentTransaction#addToBackStack(String) addToBackStack()} during a transaction that
removes the fragment.</p>

<p>Otherwise, managing the fragment lifecycle is very similar to managing the activity
lifecycle. So, the same practices for <a
href="{@docRoot}guide/components/activities.html#Lifecycle">managing the activity
lifecycle</a> also apply to fragments. What you also need to understand, though, is how the life
of the activity affects the life of the fragment.</p>

<p class="caution"><strong>Caution:</strong> If you need a {@link android.content.Context} object
within your {@link android.app.Fragment}, you can call {@link android.app.Fragment#getActivity()}.
However, be careful to call {@link android.app.Fragment#getActivity()} only when the fragment is
attached to an activity. When the fragment is not yet attached, or was detached during the end of
its lifecycle, {@link android.app.Fragment#getActivity()} will return null.</p>


<h3 id="CoordinatingWithActivity">Coordinating with the activity lifecycle</h3>

<p>The lifecycle of the activity in which the fragment lives directly affects the lifecycle of the
fragment, such that each lifecycle callback for the activity results in a similar callback for each
fragment. For example, when the activity receives {@link android.app.Activity#onPause}, each
fragment in the activity receives {@link android.app.Fragment#onPause}.</p>

<p>Fragments have a few extra lifecycle callbacks, however, that handle unique interaction with the
activity in order to perform actions such as build and destroy the fragment's UI. These additional
callback methods are:</p>

<dl>
  <dt>{@link android.app.Fragment#onAttach onAttach()}</dt>
    <dd>Called when the fragment has been associated with the activity (the {@link
android.app.Activity} is passed in here).</dd>
  <dt>{@link android.app.Fragment#onCreateView onCreateView()}</dt>
    <dd>Called to create the view hierarchy associated with the fragment.</dd>
  <dt>{@link android.app.Fragment#onActivityCreated onActivityCreated()}</dt>
    <dd>Called when the activity's {@link android.app.Activity#onCreate
onCreate()} method has returned.</dd>
  <dt>{@link android.app.Fragment#onDestroyView onDestroyView()}</dt>
    <dd>Called when the view hierarchy associated with the fragment is being removed.</dd>
  <dt>{@link android.app.Fragment#onDetach onDetach()}</dt>
    <dd>Called when the fragment is being disassociated from the activity.</dd>
</dl>

<p>The flow of a fragment's lifecycle, as it is affected by its host activity, is illustrated
by figure 3. In this figure, you can see how each successive state of the activity determines which
callback methods a fragment may receive. For example, when the activity has received its {@link
android.app.Activity#onCreate onCreate()} callback, a fragment in the activity receives no more than
the {@link android.app.Fragment#onActivityCreated onActivityCreated()} callback.</p>

<p>Once the activity reaches the resumed state, you can freely add and remove fragments to the
activity. Thus, only while the activity is in the resumed state can the lifecycle of a fragment
change independently.</p>

<p>However, when the activity leaves the resumed state, the fragment again is pushed through its
lifecycle by the activity.</p>


<h2 id="Example">Example</h2>

<p>To bring everything discussed in this document together, here's an example of an activity
using two fragments to create a two-pane layout. The activity below includes one fragment to
show a list of Shakespeare play titles and another to show a summary of the play when selected
from the list. It also demonstrates how to provide different configurations of the fragments,
based on the screen configuration.</p>

<p class="note"><strong>Note:</strong> The complete source code for this activity is available in
<a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.html">{@code
FragmentLayout.java}</a>.</p>

<p>The main activity applies a layout in the usual way, during {@link
android.app.Activity#onCreate onCreate()}:</p>

{@sample development/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.java main}

<p>The layout applied is {@code fragment_layout.xml}:</p>

{@sample development/samples/ApiDemos/res/layout-land/fragment_layout.xml layout}

<p>Using this layout, the system instantiates the {@code TitlesFragment} (which lists the play
titles) as soon as the activity loads the layout, while the {@link android.widget.FrameLayout}
(where the fragment for showing the play summary will go) consumes space on the right side of the
screen, but remains empty at first. As you'll see below, it's not until the user selects an item
from the list that a fragment is placed into the {@link android.widget.FrameLayout}.</p>

<p>However, not all screen configurations are wide enough to show both the list of
plays and the summary, side by side. So, the layout above is used only for the landscape
screen configuration, by saving it at {@code res/layout-land/fragment_layout.xml}.</p>

<p>Thus, when the screen is in portrait orientation, the system applies the following layout, which
is saved at {@code res/layout/fragment_layout.xml}:</p>

{@sample development/samples/ApiDemos/res/layout/fragment_layout.xml layout}

<p>This layout includes only {@code TitlesFragment}. This means that, when the device is in
portrait orientation, only the list of play titles is visible. So, when the user clicks a list
item in this configuration, the application will start a new activity to show the summary,
instead of loading a second fragment.</p>

<p>Next, you can see how this is accomplished in the fragment classes. First is {@code
TitlesFragment}, which shows the list of Shakespeare play titles. This fragment extends {@link
android.app.ListFragment} and relies on it to handle most of the list view work.</p>

<p>As you inspect this code, notice that there are two possible behaviors when the user clicks a
list item: depending on which of the two layouts is active, it can either create and display a new
fragment to show the details in the same activity (adding the fragment to the {@link
android.widget.FrameLayout}), or start a new activity (where the fragment can be shown).</p>

{@sample development/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.java titles}

<p>The second fragment, {@code DetailsFragment} shows the play summary for the item selected from
the list from {@code TitlesFragment}:</p>
 
{@sample development/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.java details}

<p>Recall from the {@code TitlesFragment} class, that, if the user clicks a list item and the
current layout does <em>not</em> include the {@code R.id.details} view (which is where the
{@code DetailsFragment} belongs), then the application starts the {@code DetailsActivity}
activity to display the content of the item.</p>

<p>Here is the {@code DetailsActivity}, which simply embeds the {@code DetailsFragment} to display
the selected play summary when the screen is in portrait orientation:</p>

{@sample development/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.java
details_activity}
 
<p>Notice that this activity finishes itself if the configuration is landscape, so that the main
activity can take over and display the {@code DetailsFragment} alongside the {@code TitlesFragment}.
This can happen if the user begins the {@code DetailsActivity} while in portrait orientation, but
then rotates to landscape (which restarts the current activity).</p>


<p>For more samples using fragments (and complete source files for this example),
see the API Demos sample app available in <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/index.html#Fragment">
ApiDemos</a> (available for download from the <a
href="{@docRoot}resources/samples/get.html">Samples SDK component</a>).</p>