Scrollviews need to announce which items of how many are shown #10595

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1. goderbauer

   added

   a: accessibilityAccessibility, e.g. VoiceOver or TalkBack. (aka a11y)Accessibility, e.g. VoiceOver or TalkBack. (aka a11y)

   on Jun 9, 2017
2. goderbauer

   added this to the 5: Make Hixie proudThis milestone has been deleted milestone on Dec 20, 2017
3. jonahwilliams commented on Jul 19, 2018

   

   jonahwilliams

   on Jul 19, 2018

   Last edited by jonahwilliams

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   on Android, the announcement is produced from the following method in talkback: https://github.com/google/talkback/blob/e69d4731fce02bb9e69613d0e48c29033cad4a98/talkback/src/main/java/eventprocessor/ProcessorScrollPosition.java#L246.

   This uses the getFromIndex and getToIndex of an accessibility record to produce the event. There is also some logic to determine the name of the page.

   In theory this could be supported for any List, but I'm not sure yet how it considers items that are outside of the viewport. Since we prune lists, if we wanted to handle the offscreen count we would need to provide the total number of children and current start index.
4. jonahwilliams commented on Jul 19, 2018

   

   jonahwilliams

   on Jul 19, 2018

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   The only change that needs to made on the engine side is trivial - add two more members describing the total number of children and the index of the first node in the child array, then Talkback handles the rest:

   event.setFromIndex(object.firstChildIndex);
   event.setToIndex(object.firstChildIndex + object.childrenInHitTestOrder.size() - 1);
   event.setItemCount(object.totalChildCount);

   The difficulty I've had is how exactly to get this information onto a single semantics node. Currently the scroll events are dispatched from the node created by Scrollable - but this only knows about the viewport and has no concept of children. Really what I would like so for a child that knows the child count, like the SliverList to tell this scrollable the child information. Haven't found a way to do this yet.

   cc @goderbauer
5. jonahwilliams commented on Sep 11, 2018

   

   jonahwilliams

   on Sep 11, 2018

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   So I've spent some time on this and figured out there may just be too many issues blocking a fully general automagic solution for CustomScrollView:

   • We don't know how many slivers can be rendered
   • We don't know which ones will actually contribute semantics
   • We don't know where one RenderSliverMuli* ends and another ends.
   • Indexes may not start at 0, or even increase.

   Even though all of the bits are there, they are spread out and not always available at the right time. I've prototyped some solutions that work by encoding the values into the semantics tree, but it quickly becomes very cumbersome - i.e. traversing 2 or 3 levels of semantics nodes to collect information. I don't feel confident that this is actually a generally solution.

   As an alternative we could not fix it, but I think this degrades the accessibility experience for fairly common cases like ListViews. My solution is fairly simple, but requires developer specific for more complex cases.

   1. CustomScrollView takes an 'int scroll(semantics)Children' parameter which declares the total number of scroll children which contribute semantics. This can be supplied by default for ListView.

   2. For indexes we require the user to wrap children in a SemanticsIndex(child: child, index: index). This can be done automatically for ListView. This does run the risk of marking dividers as important for semantics, but I still think this is better than the alternative.

   We can still try and work on a better API, or perhaps find more cases where we can automatically infer indexes in the mean time.

   Would appreciate any thoughts on this: @goderbauer, @yjbanov
6. jonahwilliams

   closed this as completedin #21764on Oct 9, 2018
7. github-actions commented on Aug 15, 2021

   

   github-actionsbot

   on Aug 15, 2021

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8. github-actions

   locked as resolved and limited conversation to collaborators on Aug 15, 2021