Retired Document
Important: This document describes the old, key-based accessibility API. The use of this API is discouraged. Use the new, method-based API instead. For more information on the new accessibility API, see Accessibility Programming Guide for OS X.
Hit-Testing and Keyboard Focus
Assistive applications frequently need to be able to access the user interface object that is at a specific screen location or that currently has keyboard focus. The NSAccessibility protocol defines methods an assistive application uses to get this information from your application.
Hit-Testing
Hit-testing identifies the deepest member of the accessibility hierarchy that is located at a particular point on the screen. For example, an assistive application may want to identify the user-accessible object that lies beneath the current cursor position.
The NSAccessibility protocol defines the accessibilityHitTest: method which, similarly to the standard hitTest: view method, returns the farthest descendent of the accessibility hierarchy that contains the specified point (in bottom-left relative screen coordinates). Unlike the standard hitTest: view method, however, the accessibilityHitTest: method is never called unless the Cocoa accessibility implementation has already determined that the point lies within the receiver. This means that a class needs to override this method only when it contains child accessibility objects; childless accessibility objects never have to override this method.
When an accessibility object receives the accessibilityHitTest: method, therefore, it knows that the specified point lies somewhere within it. The accessibility object then has the opportunity to perform deeper hit-testing by identifying which child accessibility object, if any, contains the point. NSMatrix, for example, identifies which of its cells contains the point and propagates the accessibilityHitTest: message to it. Eventually, a childless accessibility object acknowledges that it is the object at the specified point and returns self from the method. An ignored accessibility object without any children does not return self; instead, it returns its first unignored ancestor.
Determining Keyboard Focus
Focus-testing identifies the deepest member of the accessibility hierarchy that has the keyboard focus. The NSAccessibility protocol defines the accessibilityFocusedUIElement method an assistive application can send to determine which object has keyboard focus.
As with the accessibilityHitTest: method, the accessibilityFocusedUIElement method is never called unless the Cocoa accessibility implementation has already determined that the receiver has keyboard focus somewhere within it. This means that a class needs to override this method only when it contains child accessibility objects; childless accessibility objects never have to override this method.
When an accessibility object receives the accessibilityFocusedUIElement message, therefore, it knows that it (or one of its children) has keyboard focus. The accessibility object then has the opportunity to perform deeper focus-testing by determining which of its children has keyboard focus and sending to it the accessibilityFocusedUIElement message. NSMatrix, for example, identifies which of its cells has the focus and propagates the accessibilityFocusedUIElement message to it. Eventually, a childless accessibility object acknowledges that it has keyboard focus and returns self from the method. An ignored object without any children returns its first unignored ancestor instead of self.
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