Accessibility/TextAttributes

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Introduction

This article is targeted to reflect a support of text attributes by Gecko accessibility API. There is no implementation in Gecko yet. This is a draft of specification how we will do it.

Proposed API

Summary

This is the internal nsIAccessibleText method (not the ATK or IA2 method -- the implementation for those will consume this method).

The following method returns a collection of text attributes at the given offset, and calculates the range where returned attributes are stretched.

 nsIPersistentProperties getTextAttributes(in long offset,
                                           out long rangeStartOffset,
                                           out long rangeEndOffset);

Parameters

offset

[in] the given offset

rangeStartOffset

[out] the start offset of the result range

rangeEndOffset

[out] the end offset of the result range

Ranges are not nested. They are consecutive, so that AT's always know when to ask for the next range. For example, for text 'abc

<b>

def

</b>

ghi', there are three ranges (0, 3), (3, 6) and (6, 9).

Return Value

A collection of text attributes (the list of pairs consisted from name and value). All text attributes that apply to the current range are exposed, but in order to conserve the amount of data passed, most text attributes will have reasonable defaults and will not be exposed in the list when they are equal to the default. For example, if font-style is "normal" it will not be exposed in the return value. See the list of text attributes below to find the defaults.

The following method returns a collection of text attributes that apply to the entire text object:

 nsIPersistentProperties getDefaultTextAttributes()

A collection of text attributes (the list of pairs consisted from name and value). All text attributes that apply to the current accessible text object are exposed. Since IA2 does not support default attributes, the IA2 implementation will combine the default attributes into the range attributes before exposing IA2::attributes to the AT.

Events

When a certain text attribute is changed then 'EVENT_TEXT_ATTRIBUTE_CHANGED' event should be fired.

In IA2, there is no data to specify what attribute changed, and for what offsets. Therefore, the AT must cache attributes of any objects that it will want to know about text attribute changes for. For example, it can cache the the text attribute ranges for the current text accessible, and compare the previous text attribute runs with the current text attribute runs.

Events will be fired for the topic object to which they apply, in order to avoid floods of too many events. For example, if someone selects all and then boldfaces the selection -- every object in the accessible tree is essentially getting an attribute change. We will reduce the number of events by only firing the text attribute change event on the root accessible for the change.

For Firefox 3, events will not be fired for style changes that are not caused by DOM changes. At the moment we are focused on the rich text editing use case. For rich text editing there will always be a DOM change.

List of supported attributes

Gecko specific

Attribute name	Attribute description	Default value if attribute not exposed
direction	rtl or ltr for bidi text	ltr
border	not sure how best to expose these properties, or whether to (same with outline CSS property)	no border
language	What language is this text in, e.g. en-US (do we want this or do we use IA2::locale and make sure each locale change gets separate accessible object?)	same as doc itself?
misspelled	"true" if it is marked by the inline spell checker for being incorrectly spelled. Attribute not present in all other cases.	Not misspelled
static	"true" for list bullet/numbering text or layout-inserted text (such as via CSS pseudo styles :before or :after)	false
z-index	a number which indicates how low/high an element is	0 (normal height)

Linux Foundation Proposed (ATK + IA2)

Note: the following characters in names and values need to be escaped with a backslash: backslash, colon, comma, equals, and semicolon.

Attribute name	Attribute description	Default value if attribute not exposed
background-color	Background color as u, u, u	no default, always specified?
color	Foreground color as u, u, u	no default, always specified?
font-family	The computed font name	no default, always specified?
font-size	Font size in pixels	no default, always specified?
font-style	italic (oblique not currently supported)	normal
font-weight	The computed font weight (100, 200, 300, 400, 500, 600, 700, 800, 900; normal = 400, bold = 700)	400
text-line-through-style	solid (only exposed if if there is a line through)	no line through
text-underline-style	solid (only exposed if if there is an underline)	no underline
line-height	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever? What units should be used here?)	no default, always specified?
margin-bottom	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever? What units should be used here?)	no default, always specified?
margin-left	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever? What units should be used here?)	no default, always specified?
margin-right	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever? What units should be used here?)	no default, always specified?
text-align	left/center/right/justify	left
text-indent	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever? What units should be used here?)	no indent
text-position (for sup/sub)	?	?

Currently unsupported

The following text attributes, which are in the IAccessible2 text attribute specification, are not currently supported due to today's limitations of text formatting on the web:

• text-line-through-mode
• text-line-through-type
• text-underline-type
• text-underline-width
• text-underline-mode
• list -- the list bullet text is exposed via the accessible text interface, with the text attribute static=true
• tab-stop
• text-shadow

References

• IA2 Text Attributes - IAccessible2 Text Attributes Documentation