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Architecture and Design

UML Class Diagram

The goal of this design is to allow us to provide customized interfaces for different accessibility roles in a concise and readable way that does not compromise on flexibility and possible future OSX API changes.


mozAccessible Protocol

This consists of the API necessary to talk to platform accessibility (accessibilityAttributeValue, etc.) and widget code (representedView, etc.)

MOXAccessible Protocol

This inner protocol's API is a mapping to Apple's attributes and actions. It allows us to have simple getter and setter methods for each attribute and a method for performing each action. All methods are optional, so an implementing object gets to choose which subset of these methods it supports. This gets reflected in what is returned to platform in accessibilityAttributeNames and friends. An instance can also programatically block a method if it is defined and it will be excluded from the mapping.



The abstract base class for all our platform accessibles. This class:

  • Implements the platform and widget API defined in the MOXAccessible protocol. The platform API it implements should be considered "final" (there is no way in Objective C to enforce this).
  • Forwards platform methods to the mapped inner protocol's methods.
  • Filters ignored accessibles and ensures they are not exposed to the platform.
  • Manages the lifecycle of the accessible as it relates to the platform (ie. notify on destroy)

A subclass should be able to implement a handful of inner protocol getters and have a well-behaved platform accessible. An example of that is MOXColumnAccessible.


The base class for all accessibles that wrap a Gecko accessible. This class:

  • Maps gecko roles to platform roles, subroles, and role descriptions.
  • Caches and utilizes gecko states.
  • Forwards gecko accessible events to platform notifications.
  • Provides parent and children getters via gecko's hierarchy.
  • Provides a base set of attributes and actions that most gecko accessibles support.

MOXAccessible subclasses

Subclasses of MOXAccessible are mainly gecko role-based and typically provide additional attributes that the type uses. For example a mathml subclass might provide mathml related attributes via implemented inner protocol methods (eg. mathRootRadicand). An accessible that backs a slider might implement special actions via the inner protocol, like performIncrement.