NPAPI:NativeAccessibility

Problem Summary

Plugins cannot currently provide accessible information about their contents on non-Windows platforms. This is Mozilla bug 78414.

Glossary

• Accessibility - the degree to which a product is accessible by as many people as possible.
• Assistive Technology (AT) - a piece of software or hardware that connects to the system's accessibility services to provide assistance to the user. This assistance can come in the form of a head tracking system for a user unable to use a mouse, a screen reader for a person with vision impairments, etc.
• Microsoft Active Accessibility (MSAA) - the standard accessibility API for Windows 95 through Windows XP. Accessible objects in MSAA inherit from the IAccessible interface.
• Accessibility Toolkit (ATK) - the standard accessibility API on Linux.

Related Approaches

On Windows, plugins can provide their own accessibility support by responding to the WM_GETOBJECT WinEvent and returning an instance of IAccessible. Firefox (and other browsers) will call this WinEvent on the plugin object and if an IAccessible is returned, register the accessible into the accessibility hierarchy.

API Requirements

• Plugins should be able to easily provide an accessible implementation (in the native accessibility toolkit) to the Browser
• The Browser should inject the plugin's accessible into its accessible hierarchy
• The lifetime of the plugin's accessible should be the same as the lifetime of the plugin.

Current Proposal

Specification

NPAPI Native Accessibility Support

• Last modified: June 26, 2009
• Author: Brad Taylor, Novell, Inc.
• Contributors:

Plugins have the need to provide accessibility support to the browser so they can be accessed by assistive technologies like screen readers and testing tools. Most common web browsers already provide an accessible implementation of DOM content, but thus far, can only export a blank (and therefore useless) object when it comes across a plugin.

When a plugin's accessible is requested from an assistive technology, or when the plugin is first created, the web browser should call the NPN_GetValue method, passing NPNVNativeAccessible as the variable parameter.

If the plugin sets value to NULL, the browser should use an empty accessible to represent the plugin.

If value is not NULL, the browser should cast it to the base type of the native accessibility toolkit, and insert the object into the accessibility hierarchy as a child of the plugin's DOM parent.

If the accessibility toolkit supports reference counting, the plugin accessible's reference should be incremented right after the call to NPN_GetValue, and decremented when the plugin is freed. If the accessible object provided is a pointer, the value should not be copied to ensure that the plugin can maintain the state of the plugin object.

This accessible does not need to be updated, so the browser may cache the object for the life of the plugin. Plugins that need to regularly remove and re-add their root accessible object should return a container-type object from NPN_GetValue and make their root accessible a child of the container.