Computed Accessibility tree access (Phase 4)

[alice]

Some top of my head thoughts/questions:

• What use cases should we keep in mind to drive decisions?
  • Testing
  • Debugging
  • Portable assistive technology (for example, a screen reader written as a browser extension and/or JS library)
  • W3C spec tests
• From what scope/context should authors be able to use the phase 4 API?
  • WebDriver only, or at least WebDriver first?
  • Web worker only?
  • Main page context?
• Synchronous or asynchronous?
  • Almost certainly asynchronous, since computing the tree is likely to be expensive
• Same object type as phases 1-3 or different?
  • Nice to avoid having multiple almost-the-same types, however
  • Nice to be able to only think about needs for phase 4 when designing object
• How do we go about standardising the tree across browsers to create a consistent experience for devs?

[alice]

Total strawman proposal just to get some gut reactions to these choices:

• main page context
• asynchronous
• different type from AccessibleNode - maybe ComputedAccessibleNode (naming things is hard 😞)

let computedAXNode = await document.activeElement.accessibleNode.requestComputedNode();

// no need for label/labelledby etc, just get the computed name and description
console.log(computedAXNode.name, computedAXNode.description);  

// finally we can get the computed role!
console.log(computedAXNode.roleName);

// do we want a type taxonomy like DOM objects?
if (computedAXNode instanceof AccessibilitySlider) {
  console.log(computedAXNode.minValue);

// do some tree walking
let parent = computedAXNode.parent;
while (parent.parent)
  parent = parent.parent;
let rootAXNode = parent;

// get all the more obscure relations - 
// maybe this is a list of strings, maybe this is a dictionary?
var relations = computedAXNode.accessibleRelations;

[alice]

https://github.com/WICG/aom/issues/6 talks about use cases

[minorninth]

// no need for label/labelledby etc, just get the computed name and description console.log(computedAXNode.name, computedAXNode.description);

+1

// do we want a type taxonomy like DOM objects? if (computedAXNode instanceof AccessibilitySlider) { console.log(computedAXNode.minValue);

I vote no. If minValue doesn't apply to that role it should return undefined.

// do some tree walking let parent = computedAXNode.parent;

How about: let parent = await computedAXNode.requestParent()?

That way the implementation can be super lazy. For performance reasons we can try to return synchronously whenever possible, but we won't guarantee we will.

If we don't do that, what happens if you modify the accessibility tree and try to access the computed node's relationships synchronously - does it answer based on the previous snapshot, or does it block?

[alice]

Example WebDriver APIs which aren't available on the open web: https://www.w3.org/TR/webdriver/#pointer-actions https://www.w3.org/TR/webdriver/#dismiss-alert https://www.w3.org/TR/webdriver/#take-screenshot

[alice]

Would ComputedAccessibleNode be live?

• Returning a live object requires a live tree, which has serious performance implications.
• OTOH, if you're going to be heavily using the accessibility tree, it would be more performant to have a live object.
• Could follow pattern used by multi-process browsers
  • get a snapshot, but have a mechanism for learning when an update is available
  • this does still require a live tree!