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="#CarryingAnObject">Carrying 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's default behavior is to restart the running
Activity ({@link android.app.Activity#onDestroy()} is called, followed by {@link
android.app.Activity#onCreate(Bundle) onCreate()}). In doing so, the system re-queries your
application resources for alternatives that might apply to the new configuration.</p>

<p>It is important that your Activity safely handles restarts and restores its previous
state through the normal <a href="{@docRoot}guide/topics/fundamentals.html#lcycles">Activity
lifecycle</a>. In fact, it's a useful field test to invoke configuration changes (such as changing
the screen orientation) during various states of your application to be sure that it properly
restarts itself with the application state intact. So it's in the best interest of your application
to allow the system to restart your application during any configuration change&mdash;this behavior
is in place to help you by automatically handling configuration changes and adapting your
application as necessary.</p>

<p>However, you might encounter a situation in which restarting your application and
restoring significant amounts of data can be costly, create a slow user experience, and
using {@link android.app.Activity#onSaveInstanceState(Bundle) onSaveInstanceState()} does not
suffice. In such a situation, you have two options:</p>

<ol type="a">
  <li><a href="#CarryingAnObject">Carrying an Object During a Configuration Change</a>
  <p>Allow your
application to restart so that the appropriate configuration changes can take effect, but also
implement {@link android.app.Activity#onRetainNonConfigurationInstance()} paired with {@link
android.app.Activity#getLastNonConfigurationInstance()} to carry an {@link java.lang.Object} over
to the new instance of your Activity.</p>
  <p>This is the recommended technique if you're facing performance issues during the
configuration restart. It allows your Activity to properly restart and reload resources for
the new configuration and also allows you to carry your arbitrary data that may be expensive to
collect again.</p>
  </li>
  <li><a href="#HandlingTheChange">Handling the Configuration Change Yourself</a>
  <p>Declare that your
application will handle certain configuration changes and prevent the system from restarting your
application when such a change occurs. For example, you can declare in your manifest that your
Activity will handle configuration changes to the screen orientation. When the orientation
changes, your Activity will not be restarted and your Activity will receive a call to {@link
android.app.Activity#onConfigurationChanged(Configuration) onConfigurationChanged()} so that you can
perform necessary changes based on the new configuration.</p>
  <p>This technique should be considered a last resort and temporary solution, because not all
runtime configuration changes can be handled this way&mdash;your application will eventually
encounter a runtime configuration in which you cannot prevent the Activity from being restarted,
whereas the first option will handle all configuration changes.</p>
  </li>
</ol>

<p class="note"><strong>Note:</strong> Your application should always be able to successfully
restart at any time without any loss of user data or state in order to handle other 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>). The following
techniques for handling runtime configuration changes should only be necessary to optimize
performance during specific configuration changes.</p>


<h2 id="CarryingAnObject">Carrying an Object During a Configuration Change</h2>

<p>If your application has acquired significant amounts of data during its life, which would be
costly to recover due to a restart of the Activity, you can use {@link
android.app.Activity#onRetainNonConfigurationInstance()} paired with {@link
android.app.Activity#getLastNonConfigurationInstance()} to pass an {@link java.lang.Object}
to the new Activity instance. The {@link android.app.Activity#onRetainNonConfigurationInstance()}
method is called between {@link android.app.Activity#onStop()} and {@link
android.app.Activity#onDestroy()} when your Activity is being shut down due to a configuration
change. In your implementation of this method, you can return any {@link java.lang.Object} that you
need to efficiently restore your state after the configuration change. When your Activity is
created again, you can call {@link
android.app.Activity#getLastNonConfigurationInstance()} to retrieve the {@link
java.lang.Object}.</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
will need to re-fetch the data, which could be slow. What you can do is implement
{@link android.app.Activity#onRetainNonConfigurationInstance()} to return an object carrying your
data and then retrieve the data when your Activity restarts 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 get the {@code data} after the restart:</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()} is called to get
the data saved during the configuration change, and if it is null (which will happen if the
Activity is started in any case other than a configuration change) then the data 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 will prevent 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 will not automatically apply them
for you.</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>