xref: /frameworks/base/docs/html/guide/topics/ui/accessibility/services.jd

page.title=Building Accessibility Services
parent.title=Accessibility
parent.link=index.html
@jd:body

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

  <h2>Topics</h2>
  <ol>
    <li><a href="#manifest">Manifest Declarations and Permissions</a>
      <ol>
        <li><a href="#service-declaration">Accessibility service declaration</a></li>
        <li><a href="#service-config">Accessibility service configuration</a></li>
      </ol>
    </li>
    <li><a href="#methods">AccessibilityService Methods</a></li>
    <li><a href="#event-details">Getting Event Details</a></li>
    <li><a href="#examples">Example Code</a></li>
  </ol>

  <h2>Key classes</h2>
  <ol>
    <li>{@link android.accessibilityservice.AccessibilityService}</li>
    <li>{@link android.accessibilityservice.AccessibilityServiceInfo}</li>
    <li>{@link android.view.accessibility.AccessibilityEvent}</li>
    <li>{@link android.view.accessibility.AccessibilityRecord}</li>
    <li>{@link android.view.accessibility.AccessibilityNodeInfo}</li>
  </ol>

  <h2>See also</h2>
  <ol>
    <li><a href="{@docRoot}training/accessibility/index.html">Implementing Accessibility</a></li>
  </ol>

</div>
</div>

<p>An accessibility service is an application that provides user interface enhancements to
assist users with disabilities, or who may temporarily be unable to fully interact with a device.
For example, users who are driving, taking care of a young child or attending a very loud party
might need additional or alternative interface feedback.</p>

<p>Android provides standard accessibility services, including TalkBack, and developers can
create and distribute their own services. This document explains the basics of building an
accessibility service.</p>

<p>The ability for you to build and deploy accessibility services was introduced with Android
1.6 (API Level 4) and received significant improvements with Android 4.0 (API Level 14). The Android
Support Library was also updated with the release of Android 4.0 to provide support for these
enhanced accessibility features back to Android 1.6. Developers aiming for widely compatible
accessibility services are encouraged to use the
<a href="{@docRoot}tools/extras/support-library.html">Support Library</a> and develop for the more
advanced accessibility features introduced in Android 4.0.</p>


<h2 id="manifest">Manifest Declarations and Permissions</h2>

<p>Applications that provide accessibility services must include specific declarations in their
 application manifests in order to be treated as an accessibility service by an Android system.
 This section explains the required and optional settings for accessibility services.</p>


<h3 id="service-declaration">Accessibility service declaration</h3>

<p>In order to be treated as an accessibility service, your application must include the
{@code service} element (rather than the {@code activity} element) within the {@code application}
element in its manifest. In addition, within the {@code service} element, you must also include an
accessibility service intent filter, as shown in the following sample:</p>

<pre>
&lt;application&gt;
  &lt;service android:name=&quot;.MyAccessibilityService&quot;
      android:label=&quot;@string/accessibility_service_label&quot;&gt;
    &lt;intent-filter&gt;
      &lt;action android:name=&quot;android.accessibilityservice.AccessibilityService&quot; /&gt;
    &lt;/intent-filter&gt;
  &lt;/service&gt;
&lt;/application&gt;
</pre>

<p>These declarations are required for all accessibility services deployed on Android 1.6 (API Level
 4) or higher.</p>


<h3 id="service-config">Accessibility service configuration</h3>

<p>Accessibility services must also provide a configuration which specifies the types of
accessibility events that the service handles and additional information about the service. The
configuration of an accessibility service is contained in the {@link
android.accessibilityservice.AccessibilityServiceInfo} class. Your service can build and set a
configuration using an instance of this class and {@link
android.accessibilityservice.AccessibilityService#setServiceInfo setServiceInfo()} at runtime.
However, not all configuration options are available using this method.</p>

<p>Beginning with Android 4.0, you can include a {@code &lt;meta-data&gt;} element in your manifest
with a reference to a configuration file, which allows you to set the full range of options for
your accessibility service, as shown in the following example:</p>

<pre>
&lt;service android:name=&quot;.MyAccessibilityService&quot;&gt;
  ...
  &lt;meta-data
    android:name=&quot;android.accessibilityservice&quot;
    android:resource=&quot;@xml/accessibility_service_config&quot; /&gt;
&lt;/service&gt;
</pre>

<p>This meta-data element refers to an XML file that you create in your application’s resource
directory ({@code &lt;project_dir&gt;/res/xml/accessibility_service_config.xml}). The following code
shows example contents for the service configuration file:</p>

<pre>
&lt;accessibility-service xmlns:android=&quot;http://schemas.android.com/apk/res/android";
    android:description=&quot;@string/accessibility_service_description&quot;
    android:packageNames=&quot;com.example.android.apis&quot;
    android:accessibilityEventTypes=&quot;typeAllMask&quot;
    android:accessibilityFlags=&quot;flagDefault&quot;
    android:accessibilityFeedbackType=&quot;feedbackSpoken&quot;
    android:notificationTimeout=&quot;100&quot;
    android:canRetrieveWindowContent=&quot;true&quot;
    android:settingsActivity=&quot;com.example.android.accessibility.ServiceSettingsActivity&quot;
/&gt;
</pre>

<p>One of the most important functions of the accessibility service configuration parameters is to
allow you to specify what types of accessibility events your service can handle. Being able to
specify this information enables accessibility services to cooperate with each other, and allows you
as a developer the flexibility to handle only specific events types from specific applications. The
event filtering can include the following criteria:</p>

<ul>
  <li><strong>Package Names</strong> - Specify the package names of applications whose accessibility
events you want your service to handle. If this parameter is omitted, your accessibility service is
considered available to service accessibility events for any application. This parameter can be set
in the accessibility service configuration files with the {@code android:packageNames} attribute as
a comma-separated list, or set using the {@link
android.accessibilityservice.AccessibilityServiceInfo#packageNames
AccessibilityServiceInfo.packageNames} member.</li>
  <li><strong>Event Types</strong> - Specify the types of accessibility events you want your service
to handle. This parameter can be set in the accessibility service configuration files with the
{@code android:accessibilityEventTypes} attribute as a comma-separated list, or set using the
{@link android.accessibilityservice.AccessibilityServiceInfo#eventTypes
AccessibilityServiceInfo.eventTypes} member. </li>
</ul>

<p>For more information about the XML attributes which can be used in the accessibility service
 configuration file, follow these links to the reference documentation:</p>

<ul>
  <li><a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_description">{@code android:description}</a></li>
  <li><a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_packageNames">{@code android:packageNames}</a></li>
  <li><a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_accessibilityEventTypes">{@code android:accessibilityEventTypes}</a></li>
  <li><a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_accessibilityFlags">{@code android:accessibilityFlags}</a></li>
  <li><a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_accessibilityFeedbackType">{@code android:accessibilityFeedbackType}</a></li>
  <li><a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_notificationTimeout">{@code android:notificationTimeout}</a></li>
  <li><a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_canRetrieveWindowContent">{@code android:canRetrieveWindowContent}</a></li>
  <li><a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_settingsActivity">{@code android:settingsActivity}</a></li>
</ul>

<p>For more information about which configuration settings can be dynamically set at runtime, see
the {@link android.accessibilityservice.AccessibilityServiceInfo} reference documentation.</p>


<h2 id="methods">AccessibilityService Methods</h2>

<p>An application that provides accessibility service must extend the {@link
android.accessibilityservice.AccessibilityService} class and override the following methods from
that class. These methods are presented in the order in which they are called by the Android system,
from when the service is started
({@link android.accessibilityservice.AccessibilityService#onServiceConnected onServiceConnected()}),
while it is running ({@link android.accessibilityservice.AccessibilityService#onAccessibilityEvent
onAccessibilityEvent()},
{@link android.accessibilityservice.AccessibilityService#onInterrupt onInterrupt()}) to when it is
shut down ({@link android.accessibilityservice.AccessibilityService#onUnbind onUnbind()}).</p>

<ul>
  <li>{@link android.accessibilityservice.AccessibilityService#onServiceConnected
onServiceConnected()} - (optional) This system calls this method when it successfully connects to
your accessibility service. Use this method to do any one-time setup steps for your service,
including connecting to user feedback system services, such as the audio manager or device vibrator.
If you want to set the configuration of your service at runtime or make one-time adjustments, this
is a convenient location from which to call {@link
android.accessibilityservice.AccessibilityService#setServiceInfo setServiceInfo()}.</li>

  <li>{@link android.accessibilityservice.AccessibilityService#onAccessibilityEvent
onAccessibilityEvent()} - (required) This method is called back by the system when it detects an
{@link android.view.accessibility.AccessibilityEvent} that matches the event filtering parameters
specified by your accessibility service. For example, when the user clicks a button or focuses on a
user interface control in an application for which your accessibility service is providing feedback.
When this happens, the system calls this method of your service with the associated {@link
android.view.accessibility.AccessibilityEvent}, which you can then interpret and provide feedback to
the user. This method may be called many times over the lifecycle of your service.</li>

  <li>{@link android.accessibilityservice.AccessibilityService#onInterrupt onInterrupt()} -
(required) This method is called when the system wants to interrupt the feedback your service is
providing, usually in response to a user taking action, such as moving focus to a different user
interface control than the one for which you are currently providing feedback. This method may be
called many times over the lifecycle of your service.</li>

  <li>{@link android.accessibilityservice.AccessibilityService#onUnbind onUnbind()} - (optional)
This method is called when the system is about to shutdown the accessibility service. Use this
method to do any one-time shutdown procedures, including de-allocating user feedback system
services, such as the audio manager or device vibrator.</li>
</ul>

<p>These callback methods provide the basic structure for your accessibility service. It is up to
you to decide on how to process data provided by the Android system in the form of {@link
android.view.accessibility.AccessibilityEvent} objects and provide feedback to the user.</p>


<h2 id="event-details">Getting Event Details</h2>

<p>The Android system provides information to accessibility services about the user interface
interaction through {@link android.view.accessibility.AccessibilityEvent} objects. Prior to Android
4.0, the information available in an accessibility event, while providing a significant amount of
detail about a user interface control selected by the user, typically provided limited contextual
information. In many cases, this missing context information might be critical to understanding the
meaning of the selected control.</p>

<p>A typical example of an interface where context is of critical importance is a calendar or day
planner. If a user selects a 4:00 PM time slot in a Monday to Friday day list and the accessibility
service announces “4 PM”, but fails to indicate this is a Friday a Monday, the month or day, this is
hardly ideal feedback for the user. In this case, the context of a user interface control is of
critical importance to a user who wants to schedule a meeting.</p>

<p>Android 4.0 significantly extends the amount of information that an accessibility service can
obtain about an user interface interaction by composing accessibility events based on the view
hierarchy. A view hierarchy is the set of user interface components that contain the component (its
parents) and the user interface elements that may be contained by that component (its children). In
this way, the Android system can provide much richer detail about accessibility events, allowing
accessibility services to provide more useful feedback to users.</p>

<p>An accessibility service gets information about an user interface event through an {@link
android.view.accessibility.AccessibilityEvent} passed by the system to the service’s
{@link android.accessibilityservice.AccessibilityService#onAccessibilityEvent
onAccessibilityEvent()} callback method. This object provides details about the event, including the
type of object being acted upon, its descriptive text and other details. Starting in Android 4.0
(and supported in previous releases through the {@link
android.support.v4.view.accessibility.AccessibilityEventCompat} object in the Support Library), you
can obtain additional information about the event using these calls:</p>

<ul>
  <li>{@link android.view.accessibility.AccessibilityEvent#getRecordCount
AccessibilityEvent.getRecordCount()} and {@link
android.view.accessibility.AccessibilityEvent#getRecord getRecord(int)} - These methods allow you to
retrieve the set of {@link android.view.accessibility.AccessibilityRecord} objects which contributed
to the {@link android.view.accessibility.AccessibilityEvent} passed to you by the system, which can
provide more context for your accessibility service.</li>

  <li>{@link android.view.accessibility.AccessibilityEvent#getSource
AccessibilityEvent.getSource()} - This method returns an {@link
android.view.accessibility.AccessibilityNodeInfo} object. This object allows you to request the
parents and children of the component that originated the accessibility event and investigate their
contents and state in order to provide

    <p class="caution"><strong>Important:</strong> The ability to investigate the full view
hierarchy from an {@link android.view.accessibility.AccessibilityEvent} potentially exposes private
user information to your accessibility service. For this reason, your service must request this
level of access through the accessibility <a href="#service-config">service configuration XML</a>
file, by including the {@code canRetrieveWindowContent} attribute and setting it to {@code true}. If
you do not include this setting in your service configuration xml file, calls to {@link
android.view.accessibility.AccessibilityEvent#getSource getSource()} fail.</p>
  </li>
</ul>


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

<p>The API Demo project contains two samples which can be used as a starting point for generating
accessibility services
({@code &lt;sdk&gt;/samples/&lt;platform&gt;/ApiDemos/src/com/example/android/apis/accessibility}):
</p>

<ul>
  <li><a href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/accessibility/ClockBackService.html">ClockBackService</a>
 - This service is based on the original implementation of {@link
android.accessibilityservice.AccessibilityService} and can be used as a base for developing basic
accessibility services that are compatible with Android 1.6 (API Level 4) and higher.</li>
  <li><a href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/accessibility/TaskBackService.html">TaskBackService</a>
 - This service is based on the enhanced accessibility APIs introduced in Android 4.0 (API Level
14). However, you can use the Android <a href="{@docRoot}tools/extras/support-library.html">Support
Libary</a> to substitute classes introduced in later API levels (e.g.,
{@link android.view.accessibility.AccessibilityRecord},
{@link android.view.accessibility.AccessibilityNodeInfo}
) with equivalent support package classes (e.g.,
{@link android.support.v4.view.accessibility.AccessibilityRecordCompat},
{@link android.support.v4.view.accessibility.AccessibilityNodeInfoCompat}
) to make this example work with API versions back to Android 1.6 (API Level 4).</li>
</ul>