public abstract class

AccessibilityService

extends Service
java.lang.Object
   ↳ android.content.Context
     ↳ android.content.ContextWrapper
       ↳ android.app.Service
         ↳ android.accessibilityservice.AccessibilityService

Class Overview

An accessibility service runs in the background and receives callbacks by the system when AccessibilityEvents are fired. Such events denote some state transition in the user interface, for example, the focus has changed, a button has been clicked, etc. Such a service can optionally request the capability for querying the content of the active window. Development of an accessibility service requires extending this class and implementing its abstract methods.

Lifecycle

The lifecycle of an accessibility service is managed exclusively by the system and follows the established service life cycle. Additionally, starting or stopping an accessibility service is triggered exclusively by an explicit user action through enabling or disabling it in the device settings. After the system binds to a service it calls onServiceConnected(). This method can be overriden by clients that want to perform post binding setup.

Declaration

An accessibility is declared as any other service in an AndroidManifest.xml but it must also specify that it handles the "android.accessibilityservice.AccessibilityService" Intent. Failure to declare this intent will cause the system to ignore the accessibility service. Following is an example declaration:

 <service android:name=".MyAccessibilityService">
     <intent-filter>
         <action android:name="android.accessibilityservice.AccessibilityService" />
     </intent-filter>
     . . .
 </service>

Configuration

An accessibility service can be configured to receive specific types of accessibility events, listen only to specific packages, get events from each type only once in a given time frame, retrieve window content, specify a settings activity, etc.

There are two approaches for configuring an accessibility service:

  • Providing a meta-data entry in the manifest when declaring the service. A service declaration with a meta-data tag is presented below:
     <service android:name=".MyAccessibilityService">
         <intent-filter>
             <action android:name="android.accessibilityservice.AccessibilityService" />
         </intent-filter>
         <meta-data android:name="android.accessibilityservice" android:resource="@xml/accessibilityservice" />
     </service>

    Note: This approach enables setting all properties.

    For more details refer to SERVICE_META_DATA and <accessibility-service>.

  • Calling setServiceInfo(AccessibilityServiceInfo). Note that this method can be called any time to dynamically change the service configuration.

    Note: This approach enables setting only dynamically configurable properties: eventTypes, feedbackType, flags, notificationTimeout, packageNames

    For more details refer to AccessibilityServiceInfo.

Retrieving window content

An service can specify in its declaration that it can retrieve the active window content which is represented as a tree of AccessibilityNodeInfo. Note that declaring this capability requires that the service declares its configuration via an XML resource referenced by SERVICE_META_DATA.

For security purposes an accessibility service can retrieve only the content of the currently active window. The currently active window is defined as the window from which was fired the last event of the following types: TYPE_WINDOW_STATE_CHANGED, TYPE_VIEW_HOVER_ENTER, TYPE_VIEW_HOVER_EXIT, In other words, the last window that was shown or the last window that the user has touched during touch exploration.

The entry point for retrieving window content is through calling AccessibilityEvent.getSource() of the last received event of the above types or a previous event from the same window (see AccessibilityEvent.getWindowId()). Invoking this method will return an AccessibilityNodeInfo that can be used to traverse the window content which represented as a tree of such objects.

Note An accessibility service may have requested to be notified for a subset of the event types, thus be unaware that the active window has changed. Therefore accessibility service that would like to retrieve window content should:

  • Register for all event types with no notification timeout and keep track for the active window by calling getWindowId() of the last received event and compare this with the getWindowId() before calling retrieval methods on the latter.
  • Prepare that a retrieval method on AccessibilityNodeInfo may fail since the active window has changed and the service did not get the accessibility event yet. Note that it is possible to have a retrieval method failing even adopting the strategy specified in the previous bullet because the accessibility event dispatch is asynchronous and crosses process boundaries.

Notification strategy

For each feedback type only one accessibility service is notified. Services are notified in the order of registration. Hence, if two services are registered for the same feedback type in the same package the first one wins. It is possible however, to register a service as the default one for a given feedback type. In such a case this service is invoked if no other service was interested in the event. In other words, default services do not compete with other services and are notified last regardless of the registration order. This enables "generic" accessibility services that work reasonably well with most applications to coexist with "polished" ones that are targeted for specific applications.

Note: The event notification timeout is useful to avoid propagating events to the client too frequently since this is accomplished via an expensive interprocess call. One can think of the timeout as a criteria to determine when event generation has settled down.

Event types

Feedback types

Summary

Constants
String SERVICE_INTERFACE The Intent that must be declared as handled by the service.
String SERVICE_META_DATA Name under which an AccessibilityService component publishes information about itself.
[Expand]
Inherited Constants
From class android.app.Service
From class android.content.Context
From interface android.content.ComponentCallbacks2
Public Constructors
AccessibilityService()
Public Methods
abstract void onAccessibilityEvent(AccessibilityEvent event)
Callback for AccessibilityEvents.
final IBinder onBind(Intent intent)
Implement to return the implementation of the internal accessibility service interface.
abstract void onInterrupt()
Callback for interrupting the accessibility feedback.
final void setServiceInfo(AccessibilityServiceInfo info)
Sets the AccessibilityServiceInfo that describes this service.
Protected Methods
void onServiceConnected()
This method is a part of the AccessibilityService lifecycle and is called after the system has successfully bound to the service.
[Expand]
Inherited Methods
From class android.app.Service
From class android.content.ContextWrapper
From class android.content.Context
From class java.lang.Object
From interface android.content.ComponentCallbacks
From interface android.content.ComponentCallbacks2

Constants

public static final String SERVICE_INTERFACE

Since: API Level 4

The Intent that must be declared as handled by the service.

Constant Value: "android.accessibilityservice.AccessibilityService"

public static final String SERVICE_META_DATA

Since: API Level 14

Name under which an AccessibilityService component publishes information about itself. This meta-data must reference an XML resource containing an <accessibility-service> tag. This is a a sample XML file configuring an accessibility service:

 <accessibility-service
     android:accessibilityEventTypes="typeViewClicked|typeViewFocused"
     android:packageNames="foo.bar, foo.baz"
     android:accessibilityFeedbackType="feedbackSpoken"
     android:notificationTimeout="100"
     android:accessibilityFlags="flagDefault"
     android:settingsActivity="foo.bar.TestBackActivity"
     android:canRetrieveWindowContent="true"
     . . .
 />

Constant Value: "android.accessibilityservice"

Public Constructors

public AccessibilityService ()

Since: API Level 4

Public Methods

public abstract void onAccessibilityEvent (AccessibilityEvent event)

Since: API Level 4

Callback for AccessibilityEvents.

Parameters
event An event.

public final IBinder onBind (Intent intent)

Since: API Level 4

Implement to return the implementation of the internal accessibility service interface.

Parameters
intent The Intent that was used to bind to this service, as given to Context.bindService. Note that any extras that were included with the Intent at that point will not be seen here.
Returns
  • Return an IBinder through which clients can call on to the service.

public abstract void onInterrupt ()

Since: API Level 4

Callback for interrupting the accessibility feedback.

public final void setServiceInfo (AccessibilityServiceInfo info)

Since: API Level 4

Sets the AccessibilityServiceInfo that describes this service.

Note: You can call this method any time but the info will be picked up after the system has bound to this service and when this method is called thereafter.

Parameters
info The info.

Protected Methods

protected void onServiceConnected ()

Since: API Level 4

This method is a part of the AccessibilityService lifecycle and is called after the system has successfully bound to the service. If is convenient to use this method for setting the AccessibilityServiceInfo.