android_frameworks_base/docs/html/training/basics/fragments/fragment-ui.jd

page.title=Building a Flexible UI
parent.title=Building a Dynamic UI with Fragments
parent.link=index.html

trainingnavtop=true
previous.title=Create a Fragment
previous.link=creating.html
next.title=Communicating with Other Fragments
next.link=communicating.html

@jd:body

<div id="tb-wrapper">
  <div id="tb"> 
    <h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#AddAtRuntime">Add a Fragment to an Activity at Runtime</a></li>
  <li><a href="#Replace">Replace One Fragment with Another</a></li>
</ol>
    
    <h2>You should also read</h2>
    <ul>
      <li><a href="{@docRoot}guide/components/fragments.html">Fragments</a></li>
      <li><a href="{@docRoot}guide/practices/tablets-and-handsets.html">Supporting Tablets and
Handsets</a></li>
    </ul>

<h2>Try it out</h2>

<div class="download-box">
 <a href="http://developer.android.com/shareables/training/FragmentBasics.zip"
class="button">Download the sample</a>
 <p class="filename">FragmentBasics.zip</p>
</div>

  </div>
</div>


<p>When designing your application to support a wide range of screen sizes, you can reuse your
fragments in different layout configurations to optimize the user experience based on the available
screen space.</p>

<p>For example, on a handset device it might be appropriate to display just one fragment at a time
for a single-pane user interface.  Conversely, you may want to set fragments side-by-side on a
tablet which has a wider screen size to display more information to the user.</p>

<img src="{@docRoot}images/training/basics/fragments-screen-mock.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> Two fragments, displayed in different
configurations for the same activity on different screen sizes. On a large screen, both fragment
fit side by side, but on a handset device, only one fragment fits at a time so the fragments must
replace each other as the user navigates.</p>

<p>The {@link android.support.v4.app.FragmentManager} class provides methods that allow you to add,
remove, and replace fragments to an activity at runtime in order to create a dynamic experience.</p>



<h2 id="AddAtRuntime">Add a Fragment to an Activity at Runtime</h2> 

<p>Rather than defining the fragments for an activity in the layout file&mdash;as shown in the
<a href="creating.html">previous lesson</a> with the {@code &lt;fragment>} element&mdash;you can add
a fragment to the activity during the activity runtime. This is necessary
if you plan to change fragments during the life of the activity.</p>

<p>To perform a transaction such as add or 
remove a fragment, you must use the {@link android.support.v4.app.FragmentManager} to create a
{@link android.support.v4.app.FragmentTransaction}, which provides APIs to add, remove, replace,
and perform other fragment transactions.</p>

<p>If your activity allows the fragments to be removed and replaced, you should add the
initial fragment(s) to the activity during the activity's
{@link android.app.Activity#onCreate onCreate()} method.</p>

<p>An important rule when dealing with fragments&mdash;especially those that you add at
runtime&mdash;is that the fragment must have a container {@link android.view.View} in the layout in
which the fragment's layout will reside.</p>

<p>The following layout is an alternative to the layout shown in the <a
href="creating.html">previous lesson</a> that shows only one fragment at a time. In order to replace
one fragment with another, the activity's layout
includes an empty {@link android.widget.FrameLayout} that acts as the fragment container.</p>

<p>Notice that the filename is the same as the layout file in the previous lesson, but the layout
directory does <em>not</em> have the <code>large</code> qualifier, so this layout is used when the
device screen is smaller than <em>large</em> because the screen does not fit both fragments at
the same time.</p>

<p><code>res/layout/news_articles.xml:</code></p>
<pre>
&lt;FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />
</pre>

<p>Inside your activity, call {@link
android.support.v4.app.FragmentActivity#getSupportFragmentManager()} to get a {@link
android.support.v4.app.FragmentManager} using the Support Library APIs. Then call {@link
android.support.v4.app.FragmentManager#beginTransaction} to create a {@link
android.support.v4.app.FragmentTransaction} and call {@link
android.support.v4.app.FragmentTransaction#add add()} to add a fragment.</p>

<p>You can perform multiple fragment transaction for the activity using the same {@link
android.support.v4.app.FragmentTransaction}. When you're ready to make the changes, you must call
{@link android.support.v4.app.FragmentTransaction#commit()}.</p>

<p>For example, here's how to add a fragment to the previous layout:</p>

<pre>
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;

public class MainActivity extends FragmentActivity {
    &#64;Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.news_articles);

        // Check that the activity is using the layout version with
        // the fragment_container FrameLayout
        if (findViewById(R.id.fragment_container) != null) {

            // However, if we're being restored from a previous state,
            // then we don't need to do anything and should return or else
            // we could end up with overlapping fragments.
            if (savedInstanceState != null) {
                return;
            }

            // Create an instance of ExampleFragment
            HeadlinesFragment firstFragment = new HeadlinesFragment();
            
            // In case this activity was started with special instructions from an Intent,
            // pass the Intent's extras to the fragment as arguments
            firstFragment.setArguments(getIntent().getExtras());
            
            // Add the fragment to the 'fragment_container' FrameLayout
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.fragment_container, firstFragment).commit();
        }
    }
}
</pre>

<p>Because the fragment has been added to the {@link android.widget.FrameLayout} container at
runtime&mdash;instead of defining it in the activity's layout with a {@code &lt;fragment>}
element&mdash;the activity can remove the fragment and replace it with a different one.</p>



<h2 id="Replace">Replace One Fragment with Another</h2>

<p>The procedure to replace a fragment is similar to adding one, but requires the {@link
android.support.v4.app.FragmentTransaction#replace replace()} method instead of {@link
android.support.v4.app.FragmentTransaction#add add()}.</p>

<p>Keep in mind that when you perform fragment transactions, such as replace or remove one, it's
often appropriate to allow the user to navigate backward and "undo" the change. To allow the user
to navigate backward through the fragment transactions, you must call {@link
android.support.v4.app.FragmentTransaction#addToBackStack addToBackStack()} before you commit the
{@link android.support.v4.app.FragmentTransaction}.</p>

<p class="note"><strong>Note:</strong> When you remove or replace a fragment and add the transaction
to the back stack, the fragment that is removed is stopped (not destroyed). If the user navigates
back to restore the fragment, it restarts. If you <em>do not</em> add the transaction to the back
stack, then the fragment is destroyed when removed or replaced.</p>

<p>Example of replacing one fragment with another:</p>

<pre>
// Create fragment and give it an argument specifying the article it should show
ArticleFragment newFragment = new ArticleFragment();
Bundle args = new Bundle();
args.putInt(ArticleFragment.ARG_POSITION, position);
newFragment.setArguments(args);

FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

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

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

<p>The {@link android.support.v4.app.FragmentTransaction#addToBackStack addToBackStack()} method
takes an optional string parameter that specifies a unique name for the transaction. The name isn't
needed unless you plan to perform advanced fragment operations using the {@link
android.support.v4.app.FragmentManager.BackStackEntry} APIs.</p>