Checking in on this old issue.
We have drawFocusRing(element) in the Canvas 2d spec for keyboard
accessibility,
we're looking to add setClickableRegion(element) for pointer
event/spatial accessibility.
Primarily, for apps like VoiceOver on Mobile Safari, and ZoomText on
windows, we need
the a11y tree to stash bounding box information for interactive elements
in the shadow dom,
and it needs to be done in a way that's efficient. That information is
shared by drawFocusRing,
only applies when an element has active focus. setClickableRegion would
apply
to an element as long as it is in the canvas shadow dom; using an empty
path could also
reset the bounding information.
As part of encouraging Canvas developers (and as requested by several
canvas developers)
to use the method, we've discussed using the method on the UA side as
well, to capture
and forward pointer events. I've found some precedent for delegating
click events/using an onclick handler,
though most developers would like to see all pointer events bubble up
from the canvas shadow dom.
The highest priority is to fill in the bounding box information for the
accessibility tree;
and when the shadow dom in webkit is activated, this should be a trivial
task. This is not
"retained mode" canvas, as the scripting environment has no access to
that information, it
simply passes that info to the accessibility API.
Links galore:
Precedent for ".click()", the pointer-event analogue similar to ".focus()":
https://dev.mozilla.jp/localmdc/localmdc_6373.html#Using_hidden_file_input_elements_using_the_click%28%29_method
I believe click() is passed to shadow dom elements in IE9 with focus()
events, as part of an obligation to the ARIA specs.
Proposals for something like setClickableRegion:
Charles Pritchard (me):
http://lists.w3.org/Archives/Public/public-canvas-api/2011JulSep/0187.html
Jonas Sickling:
http://lists.w3.org/Archives/Public/public-canvas-api/2011JulSep/0195.html
Richard Schwerdtfeger:
http://lists.w3.org/Archives/Public/public-canvas-api/2011JanMar/0081.html
More abstract:
David Singer:
http://lists.w3.org/Archives/Public/public-canvas-api/2011JulSep/0179.html
Edward O'Connor:
http://lists.w3.org/Archives/Public/public-canvas-api/2011JulSep/0024.html
More concrete:
WebIDL demonstrating Edward's proposal, as well as options for
Richard/Jonas proposals:
(search for Supplemental)
http://lists.w3.org/Archives/Public/public-canvas-api/2011AprJun/0047.html
Richard's earlier proposal:
http://lists.w3.org/Archives/Public/public-canvas-api/2011JanMar/0090.html
http://lists.w3.org/Archives/Public/public-canvas-api/2011JanMar/att-0090/Canvas2DClickableRegion.html#focus-management
CanvasRenderingContext2D implementations:
http://trac.webkit.org/browser/trunk/Source/WebCore/html/canvas/CanvasRenderingContext2D.cpp
https://hg.mozilla.org/mozilla-central/file/ec809c159ad2/content/canvas/src/nsCanvasRenderingContext2D.cpp
Please let me know if there's anything I can clear up about this issue.
The list of links and discussion is rather lengthy,
but the issue itself is quite simple.
My original thread was about adding a touchhover event, which would
allow the scripting environment
to manage hit testing for VoiceOver on Mobile Safari. Though it's a
relatively easy to implement, it does not solve
the underlying issue that VoiceOver and other ATs would do better to
have bounding box information ahead of time.
At some point, touch screens may benefit from a touchhover event;
tablets are capable of penpressure and penhover events,
though they simply pass them as mouse events (see Wacom's Bamboo as an
example) with a serial interface to pressure.
-Charles
On 3/21/2011 12:06 PM, James Craig wrote:
The canvas "sub-DOM" proposal should allow some of this, but AFAIK, it is not
implemented in WebKit yet. I believe we were the first to propose it, though to date I
think it is only implemented in IE9. I have not checked IE to verify support, but they
have claimed support in IE9. Basically the ARIA-enhanced subtree is accessible to the
keyboard and assistive technology, and the DOM structure mimics the UI visible in the
canvas. It requires a focus model change to WebKit because the canvas subtree was not
originally intended to be accessible in any modality.
http://lists.w3.org/Archives/Public/public-canvas-api/2009OctDec/0026.html
I'm not sure I agree with the need for a touchhover event, but I'd be
interested to hear how you think it should work.
James
On Mar 11, 2011, at 3:30 PM, Chris Fleizach wrote:
There's no established API to handle this, but we are working on a W3C proposal
http://lists.w3.org/Archives/Public/wai-xtech/2010Aug/att-0079/UserInterfaceIndependence.html
to address this.
In the meantime, VoiceOver on iOS will call .focus() every time it "hovers" on
an item, so you can use that monitor where VO is at any moment.
If that doesn't work with<canvas> tags please file bugs at bugs.webkit.org and
CC me
On Mar 11, 2011, at 9:43 AM, Charles Pritchard wrote:
Recently, while working on code review / accessibility for a large canvas
application, I found that the iOS VoiceOver AT
does not play well with the html canvas element. It can work, but it's not a
natural transition from html to
canvas (with "buttons" in it). Using transparent [button] tags work, but it is
rather awkward.
It occurred to me that, when VoiceOver is on, on these mobile devices, that a new
"touch" type is active.
For the time being, I'm proposing calling it "touchhover".
Currently, VoiceOver AT captures touch events, waiting until the user has set
virtual focus to an element,
then tapped twice, to set event focus on that element. It works quite well for
html elements.
My thinking is that this state, where it's not passing
touchstart/touchmove/touchend events, is
very much like the traditional "hover" events we've used with mice.
If touchhover events were passed through Mobile Safari, I'd be able to use hit
detection on canvas
elements, and set the canvas element to the appropriate title matching whatever
the user has focus upon.
Does this make sense? Should I provide more examples? The purpose here is to
enable UI elements
in Canvas to work appropriately with touch-based AT, such as the iOS VoiceOver
extension.
-Charles
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