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="#register">Registering for Accessibility Events</a></li>
    <li><a href="#methods">AccessibilityService Methods</a></li>
    <li><a href="#event-details">Getting Event Details</a></li>
    <li><a href="#act-for-users">Taking Action for Users</a>
      <ol>
        <li><a href="#detect-gestures">Listening for gestures</a></li>
        <li><a href="#using-actions">Using accessibility actions</a></li>
        <li><a href="#focus-types">Using focus types</a></li>
      </ol>
    </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">Training: 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
  <a href="{@docRoot}tools/extras/support-library.html">Support Library</a> 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 Support Library 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 to be treated as an accessibility service by the 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. For compatiblity with Android 4.1 and higher, the manifest
must also request the {@link android.Manifest.permission#BIND_ACCESSIBILITY_SERVICE} permission
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;uses-permission android:name="android.permission.BIND_ACCESSIBILITY_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 applications 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>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="register">Registering for Accessibility Events</h2>

<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 list separated by the {@code |} character
(for example {@code accessibilityEventTypes="typeViewClicked|typeViewFocused"}), or set using the
{@link android.accessibilityservice.AccessibilityServiceInfo#eventTypes
AccessibilityServiceInfo.eventTypes} member. </li>
</ul>

<p>When setting up your accessibility service, carefully consider what events your service is able
to handle and only register for those events. Since users can activate more than one accessibility
services at a time, your service must not consume events that it is not able to handle. Remember
that other services may handle those events in order to improve a user's experience.</p>

<p class="note"><strong>Note:</strong> The Android framework dispatches accessibility events to
more than one accessibility service if the services provide different
<a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_accessibilityFeedbackType">
feedback types</a>. However, if two or more services provide the same feedback type, then only the
first registered service receives the event.</p>


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

<p>An 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, passing the associated {@link
android.view.accessibility.AccessibilityEvent}, which the service can then interpret and use to
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 action such as moving focus to a different control. 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. For more
information about getting information from an accessibility event, see the
<a href="{@docRoot}training/accessibility/service.html">Implementing Accessibility</a> training.</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, offered limited contextual
information. In many cases, this missing context information might be critical to understanding the
meaning of the selected control.</p>

<p>An example of an interface where context is critical is a calendar or day planner. If the
user selects a 4:00 PM time slot in a Monday to Friday day list and the accessibility service
announces 4 PM, but does not announce the weekday name, the day of the month, or the month name,
the resulting feedback is confusing. In this case, the context of a user interface control is
critical 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 services
{@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. This level
of detail provides more context for the event that triggered 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 view
layout hierarchy (parents and children) of the component that originated the accessibility event.
This feature allows an accessibility service to investigate the full context of an event, including
the content and state of any enclosing views or child views.

<p class="caution"><strong>Important:</strong> The ability to investigate the 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>

<p class="note"><strong>Note:</strong> In Android 4.1 (API Level 16) and higher, the
{@link android.view.accessibility.AccessibilityEvent#getSource getSource()} method,
as well as {@link android.view.accessibility.AccessibilityNodeInfo#getChild
AccessibilityNodeInfo.getChild()} and
{@link android.view.accessibility.AccessibilityNodeInfo#getParent getParent()}, return only
view objects that are considered important for accessibility (views that draw content or respond to
user actions). If your service requires all views, it can request them by setting the
{@link android.accessibilityservice.AccessibilityServiceInfo#flags flags} member of the service's
{@link android.accessibilityservice.AccessibilityServiceInfo} instance to
{@link android.accessibilityservice.AccessibilityServiceInfo#FLAG_INCLUDE_NOT_IMPORTANT_VIEWS}.</p>
  </li>
</ul>


<h2 id="act-for-users">Taking Action for Users</h2>

<p>Starting with Android 4.0 (API Level 14), accessibility services can act on behalf
  of users, including changing the input focus and selecting (activating) user interface elements.
  In Android 4.1 (API Level 16) the range of actions has been expanded to include scrolling lists
  and interacting with text fields. Accessibility services can
  also take global actions, such as navigating to the Home screen, pressing the Back button, opening
  the notifications screen and recent applications list. Android 4.1 also includes a new type of
  focus, <em>Accessibilty Focus</em>, which makes all visible elements selectable by an
  accessibility service.</p>

<p>These new capabilities make it possible for developers of accessibility services to create
  alternative navigation modes such as
  <a href="{@docRoot}tools/testing/testing_accessibility.html#test-gestures">gesture navigation</a>,
  and give users with disabilities improved control of their Android devices.</p>


<h3 id="detect-gestures">Listening for gestures</h3>

<p>Accessibility services can listen for specific gestures and respond by taking action on behalf
  of a user. This feature, added in Android 4.1 (API Level 16), and requires that your
  accessibility service request activation of the Explore by Touch feature. Your service can
  request this activation by setting the
  {@link android.accessibilityservice.AccessibilityServiceInfo#flags flags} member of the services
  {@link android.accessibilityservice.AccessibilityServiceInfo} instance to
  {@link android.accessibilityservice.AccessibilityServiceInfo#FLAG_REQUEST_TOUCH_EXPLORATION_MODE},
  as shown in the following example.
  </p>

<pre>
public class MyAccessibilityService extends AccessibilityService {
    &#64;Override
    public void onCreate() {
        getServiceInfo().flags = AccessibilityServiceInfo.FLAG_REQUEST_TOUCH_EXPLORATION_MODE;
    }
    ...
}
</pre>

<p>Once your service has requested activation of Explore by Touch, the user must allow the
  feature to be turned on, if it is not already active. When this feature is active, your service
  receives notification of accessibility gestures through your service's
  {@link android.accessibilityservice.AccessibilityService#onGesture onGesture()} callback method
  and can respond by taking actions for the user.</p>


<h3 id="using-actions">Using accessibility actions</h3>

<p>Accessibility services can take action on behalf of users to make interacting with applications
  simpler and more productive. The ability of accessibility services to perform actions was added
  in Android 4.0 (API Level 14) and significantly expanded with Android 4.1 (API Level 16).</p>

<p>In order to take actions on behalf of users, your accessibility service must
  <a href="#register">register</a> to receive events from a few or many applications and request
  permission to view the content of applications by setting the
  <a href="{@docRoot}reference/android/R.styleable.html#AccessibilityService_canRetrieveWindowContent">
  {@code android:canRetrieveWindowContent}</a> to {@code true} in the
  <a href="#service-config">service configuration file</a>. When events are received by your
  service, it can then retrieve the
  {@link android.view.accessibility.AccessibilityNodeInfo} object from the event using
  {@link android.view.accessibility.AccessibilityEvent#getSource getSource()}.
  With the {@link android.view.accessibility.AccessibilityNodeInfo} object, your service can then
  explore the view hierarchy to determine what action to take and then act for the user using
  {@link android.view.accessibility.AccessibilityNodeInfo#performAction performAction()}.</p>

<pre>
public class MyAccessibilityService extends AccessibilityService {

    &#64;Override
    public void onAccessibilityEvent(AccessibilityEvent event) {
        // get the source node of the event
        AccessibilityNodeInfo nodeInfo = event.getSource();

        // Use the event and node information to determine
        // what action to take

        // take action on behalf of the user
        nodeInfo.performAction(AccessibilityNodeInfo.ACTION_SCROLL_FORWARD);

        // recycle the nodeInfo object
        nodeInfo.recycle();
    }
    ...
}
</pre>

<p>The {@link android.view.accessibility.AccessibilityNodeInfo#performAction performAction()} method
  allows your service to take action within an application. If your service needs to perform a
  global action such as navigating to the Home screen, pressing the Back button, opening the
  notifications screen or recent applications list, then use the
  {@link android.accessibilityservice.AccessibilityService#performGlobalAction performGlobalAction()}
  method.</p>


<h3 id="focus-types">Using focus types</h3>

<p>Android 4.1 (API Level 16) introduces a new type of user interface focus called <em>Accessibility
  Focus</em>. This type of focus can be used by accessibility services to select any visible user
  interface element and act on it. This focus type is different from the more well known <em>Input
  Focus</em>, which determines what on-screen user interface element receives input when a user
  types characters, presses <strong>Enter</strong> on a keyboard or pushes the center button of a
  D-pad control.</p>

<p>Accessibility Focus is completely separate and independent from Input Focus. In fact, it is
  possible for one element in a user interface to have Input Focus while another element has
  Accessibility Focus. The purpose of Accessibility Focus is to provide accessibility services with
  a method of interacting with any visible element on a screen, regardless of whether or not the
  element is input-focusable from a system perspective. You can see accessibility focus in action by
  testing accessibility gestures. For more information about testing this feature, see
  <a href="{@docRoot}tools/testing/testing_accessibility.html#test-gestures">Testing gesture
  navigation</a>.</p>

<p class="note">
  <strong>Note:</strong> Accessibility services that use Accessibility Focus are responsible for
  synchronizing the current Input Focus when an element is capable of this type of focus. Services
  that do not synchronize Input Focus with Accessibility Focus run the risk of causing problems in
  applications that expect input focus to be in a specific location when certain actions are taken.
  </p>

<p>An accessibility service can determine what user interface element has Input Focus or
  Accessibility Focus using the {@link android.view.accessibility.AccessibilityNodeInfo#findFocus
  AccessibilityNodeInfo.findFocus()} method. You can also search for elements that can be selected
  with Input Focus using the
  {@link android.view.accessibility.AccessibilityNodeInfo#focusSearch focusSearch()} method.
  Finally, your accessibility service can set Accessibility Focus using the
  {@link android.view.accessibility.AccessibilityNodeInfo#performAction
  performAction(AccessibilityNodeInfo.ACTION_SET_ACCESSIBILITY_FOCUS)} method.</p>


<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>