topics/resources/runtime-changes.jd

page.title=Handling Runtime Changes
parent.title=Application Resources
parent.link=index.html
@jd:body

<div id="qv-wrapper">
<div id="qv">

  <h2>In this document</h2>
  <ol>
    <li><a href="#RetainingAnObject">Retaining an Object During a Configuration Change</a></li>
    <li><a href="#HandlingTheChange">Handling the Configuration Change Yourself</a>
  </ol>

  <h2>See also</h2>
  <ol>
    <li><a href="providing-resources.html">Providing Resources</a></li>
    <li><a href="accessing-resources.html">Accessing Resources</a></li>
    <li><a href="{@docRoot}resources/articles/faster-screen-orientation-change.html">Faster Screen
Orientation Change</a></li>
  </ol>
</div>
</div>

<p>Some device configurations can change during runtime
(such as screen orientation, keyboard availability, and language). When such a change occurs,
Android restarts the running
Activity ({@link android.app.Activity#onDestroy()} is called, followed by {@link
android.app.Activity#onCreate(Bundle) onCreate()}). The restart behavior is designed to help your
application adapt to new configurations by automatically reloading your application with
alternative resources.</p>

<p>To properly handle a restart, it is important that your Activity restores its previous
state through the normal <a href="{@docRoot}guide/topics/fundamentals.html#lcycles">Activity
lifecycle</a>, in which Android calls
{@link android.app.Activity#onSaveInstanceState(Bundle) onSaveInstanceState()} before it destroys
your Activity so that you can save data about the application state. You can then restore the state
during {@link android.app.Activity#onCreate(Bundle) onCreate()} or {@link
android.app.Activity#onRestoreInstanceState(Bundle) onRestoreInstanceState()}. To test
that your application restarts itself with the application state intact, you should
invoke configuration changes (such as changing the screen orientation) while performing various
tasks in your application.</p>

<p>Your application should be able to restart at any time without loss of user data or
state in order to handle events such as when the user receives an incoming phone call and then
returns to your application (read about the
<a href="{@docRoot}guide/topics/fundamentals.html#lcycles">Activity lifecycle</a>).</p>

<p>However, you might encounter a situation in which restarting your application and
restoring significant amounts of data can be costly and create a poor user experience. In such a
situation, you have two options:</p>

<ol type="a">
  <li><a href="#RetainAnObject">Retain an object during a configuration change</a>
  <p>Allow your Activity to restart when a configuration changes, but carry a stateful
{@link java.lang.Object} to the new instance of your Activity.</p>

  </li>
  <li><a href="#HandlingTheChange">Handle the configuration change yourself</a>
  <p>Prevent the system from restarting your Activity during certain configuration
changes and receive a callback when the configurations do change, so that you can manually update
your Activity as necessary.</p>
  </li>
</ol>


<h2 id="RetainingAnObject">Retaining an Object During a Configuration Change</h2>

<p>If restarting your Activity requires that you recover large sets of data, re-establish a
network connection, or perform other intensive operations, then a full restart due to a
configuration change might
be an unpleasant user experience. Also, it may not be possible for you to completely
maintain your Activity state with the {@link android.os.Bundle} that the system saves for you during
the Activity lifecycle&mdash;it is not designed to carry large objects (such as bitmaps) and the
data within it must be serialized then deserialized, which can consume a lot of memory and make the
configuration change slow. In such a situation, you can alleviate the burden of reinitializing
your Activity by retaining a stateful Object when your Activity is restarted due to a configuration
change.</p>

<p>To retain an Object during a runtime configuration change:</p>
<ol>
  <li>Override the {@link android.app.Activity#onRetainNonConfigurationInstance()} method to return
the Object you would like to retain.</li>
  <li>When your Activity is created again, call {@link
android.app.Activity#getLastNonConfigurationInstance()} to recover your Object.</li>
</ol>

<p>Android calls {@link android.app.Activity#onRetainNonConfigurationInstance()} between {@link
android.app.Activity#onStop()} and {@link
android.app.Activity#onDestroy()} when it shuts down your Activity due to a configuration
change. In your implementation of {@link
android.app.Activity#onRetainNonConfigurationInstance()}, you can return any {@link
java.lang.Object} that you need in order to efficiently restore your state after the configuration
change.</p>

<p>A scenario in which this can be valuable is if your application loads a lot of data from the
web. If the user changes the orientation of the device and the Activity restarts, your application
must re-fetch the data, which could be slow. What you can do instead is implement
{@link android.app.Activity#onRetainNonConfigurationInstance()} to return an object carrying your
data and then retrieve the data when your Activity starts again with {@link
android.app.Activity#getLastNonConfigurationInstance()}. For example:</p>

<pre>
&#64;Override
public Object onRetainNonConfigurationInstance() {
    final MyDataObject data = collectMyLoadedData();
    return data;
}
</pre>

<p class="caution"><strong>Caution:</strong> While you can return any object, you
should never pass an object that is tied to the {@link android.app.Activity}, such as a {@link
android.graphics.drawable.Drawable}, an {@link android.widget.Adapter}, a {@link android.view.View}
or any other object that's associated with a {@link android.content.Context}. If you do, it will
leak all the Views and resources of the original Activity instance. (To leak the resources
means that your application maintains a hold on them and they cannot be garbage-collected, so
lots of memory can be lost.)</p>

<p>Then retrieve the {@code data} when your Activity starts again:</p>

<pre>
&#64;Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    final MyDataObject data = (MyDataObject) getLastNonConfigurationInstance();
    if (data == null) {
        data = loadMyData();
    }
    ...
}
</pre>

<p>In this case, {@link android.app.Activity#getLastNonConfigurationInstance()} retrieves
the data saved by {@link android.app.Activity#onRetainNonConfigurationInstance()}. If {@code data}
is null (which happens when the
Activity starts due to any reason other than a configuration change) then the data object is loaded
from the original source.</p>


<h2 id="HandlingTheChange">Handling the Configuration Change Yourself</h2>

<p>If your application doesn't need to update resources during a specific configuration
change <em>and</em> you have a performance limitation that requires you to
avoid the Activity restart, then you can declare that your Activity handles the configuration change
itself, which prevents the system from restarting your Activity.</p>

<p class="note"><strong>Note:</strong> Handling the configuration change yourself can make it much
more difficult to use alternative resources, because the system does not automatically apply them
for you. This technique should be considered a last resort and is not recommended for most
applications.</p>

<p>To declare that your Activity handles a configuration change, edit the appropriate <a
href="{@docRoot}guide/topics/manifest/activity-element.html">{@code &lt;activity&gt;}</a> element
in your manifest file to include the <a
href="{@docRoot}guide/topics/manifest/activity-element.html#config">{@code
android:configChanges}</a> attribute with a string value that represents the configuration that you
want to handle. Possible values are listed in the documentation for
the <a href="{@docRoot}guide/topics/manifest/activity-element.html#config">{@code
android:configChanges}</a> attribute (the most commonly used values are {@code orientation} to
handle when the screen orientation changes and {@code keyboardHidden} to handle when the
keyboard availability changes).  You can declare multiple configuration values in the attribute
by separating them with a pipe character ("|").</p>

<p>For example, the following manifest snippet declares an Activity that handles both the
screen orientation change and keyboard availability change:</p>

<pre>
&lt;activity android:name=".MyActivity"
          android:configChanges="orientation|keyboardHidden"
          android:label="@string/app_name">
</pre>

<p>Now when one of these configurations change, {@code MyActivity} is not restarted.
Instead, the Activity receives a call to {@link
android.app.Activity#onConfigurationChanged(Configuration) onConfigurationChanged()}. This method
is passed a {@link android.content.res.Configuration} object that specifies
the new device configuration. By reading fields in the {@link android.content.res.Configuration},
you can determine the new configuration and make appropriate changes by updating
the resources used in your interface. At the
time this method is called, your Activity's {@link android.content.res.Resources} object is updated
to return resources based on the new configuration, so you can easily
reset elements of your UI without the system restarting your Activity.</p>

<p>For example, the following {@link
android.app.Activity#onConfigurationChanged(Configuration) onConfigurationChanged()} implementation
checks the availability of a hardware keyboard and the current device orientation:</p>

<pre>
&#64;Override
public void onConfigurationChanged(Configuration newConfig) {
    super.onConfigurationChanged(newConfig);

    // Checks the orientation of the screen
    if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) {
        Toast.makeText(this, "landscape", Toast.LENGTH_SHORT).show();
    } else if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT){
        Toast.makeText(this, "portrait", Toast.LENGTH_SHORT).show();
    }
    // Checks whether a hardware keyboard is available
    if (newConfig.hardKeyboardHidden == Configuration.HARDKEYBOARDHIDDEN_NO) {
        Toast.makeText(this, "keyboard visible", Toast.LENGTH_SHORT).show();
    } else if (newConfig.hardKeyboardHidden == Configuration.HARDKEYBOARDHIDDEN_YES) {
        Toast.makeText(this, "keyboard hidden", Toast.LENGTH_SHORT).show();
    }
}
</pre>

<p>The {@link android.content.res.Configuration} object represents all of the current
configurations, not just the ones that have changed. Most of the time, you won't care exactly how
the configuration has changed and can simply re-assign all your resources that provide alternatives
to the configuration that you're handling. For example, because the {@link
android.content.res.Resources} object is now updated, you can reset
any {@link android.widget.ImageView}s with {@link android.widget.ImageView#setImageResource(int)}
and the appropriate resource for the new configuration is used (as described in <a
href="providing-resources.html#AlternateResources">Providing Resources</a>).</p>

<p>Notice that the values from the {@link
android.content.res.Configuration} fields are integers that are matched to specific constants
from the {@link android.content.res.Configuration} class. For documentation about which constants
to use with each field, refer to the appropriate field in the {@link
android.content.res.Configuration} reference.</p>

<p class="note"><strong>Remember:</strong> When you declare your Activity to handle a configuration
change, you are responsible for resetting any elements for which you provide alternatives. If you
declare your Activity to handle the orientation change and have images that should change
between landscape and portrait, you must re-assign each resource to each element during {@link
android.app.Activity#onConfigurationChanged(Configuration) onConfigurationChanged()}.</p>

<p>If you don't need to update your application based on these configuration
changes, you can instead <em>not</em> implement {@link
android.app.Activity#onConfigurationChanged(Configuration) onConfigurationChanged()}. In
which case, all of the resources used before the configuration change are still used
and you've only avoided the restart of your Activity. However, your application should always be
able to shutdown and restart with its previous state intact. Not only because
there are other configuration changes that you cannot prevent from restarting your application but
also in order to handle events such as when the user receives an incoming phone call and then
returns to your application.</p>

<p>For more about which configuration changes you can handle in your Activity, see the <a
href="{@docRoot}guide/topics/manifest/activity-element.html#config">{@code
android:configChanges}</a> documentation and the {@link android.content.res.Configuration}
class.</p>