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<p>
  The classes in this package are used for development of accessibility service that
  provide alternative or augmented feedback to the user.
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<p>
  An {@link android.accessibilityservice.AccessibilityService} runs in the background and
  receives callbacks by the system when {@link android.view.accessibility.AccessibilityEvent}s
  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 extends this class and implements its abstract methods.
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<p>
  An {@link android.accessibilityservice.AccessibilityServiceInfo} describes an
  {@link android.accessibilityservice.AccessibilityService}. The system notifies an
  AccessibilityService for {@link android.view.accessibility.AccessibilityEvent}s
  according to the information encapsulated in this class.
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