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Re: Identifying touch events that can trigger scrolling

From: Rick Byers <rbyers@chromium.org>
Date: Thu, 22 Jan 2015 11:05:10 -0500
Message-ID: <CAFUtAY9LNCgiZTm2KfgU-PaJw8=AUmuwx5k034VjSSzdeW3USg@mail.gmail.com>
To: Olli Pettay <olli@pettay.fi>
Cc: Jacob Rossi <Jacob.Rossi@microsoft.com>, "public-touchevents@w3.org" <public-touchevents@w3.org>, Mustaq Ahmed <mustaq@chromium.org>, Matt Gaunt <mattgaunt@chromium.org>, Jake Archibald <jakearchibald@chromium.org>, Daniel Freedman <dfreedm@chromium.org>
On Thu, Jan 22, 2015 at 7:24 AM, Olli Pettay <olli@pettay.fi> wrote:

> On 01/22/2015 05:34 AM, Rick Byers wrote:
>
>> Thanks Jacob!
>>
>> Other potential name ideas (I'm not interested in bike-shedding over it
>> either - any of these would be fine with me):
>> - triggersManipulation
>> - canStartScroll
>>
> is it 'can' or 'may'.
>

I'm not sure which is more correct.  It's "will attempt to scroll/zoom if
uncancelled".  Hence Ben's "defaultStarts..' suggestion.

Perhaps we don't want a boolean attribute but enum attribute to indicate
> which action the UA
> tries to do.
>
> enum {
>   "",
>   "horizontal-scroll",
>   "vertical-scroll",
>   "scroll"
> };
>
>
This is basically the same idea Ben proposed here
<https://bugs.webkit.org/show_bug.cgi?id=138151> and we discussed on the
call.  Jacob expressed concern with trying to be general like this because
we'd have to also include actions like "zoom" and we may quickly run up
against UA-specific behavior (eg. how gestures map to actions) that we
don't want to try to force UAs to agree on (and would be unable to
standardize in W3C given the current IP climate anyway).

I'm also not sure that's that much additional value to reporting the
different actions explicitly.  When used with touch-action you can suppress
the actions you want to implement yourself in JS and then you know that any
other "mayStartManipulation" event is for one of the remaining action types.

But I'd personally be happy to go down this route if we had a concrete
example where it is better for developers than the simpler bool option.


> [NoInterfaceObject] interface EventManipulation {
>   readonly attribute ManipulationType defaultManipulation;
> };
>
> TouchEvent implements EventManipulation;
> PointerEvent implements EventManipulation; // if we want something like
> this too.
>
>
>
> (Though, I don't like the word 'manipulation'. 'action' might be better)
>
>
> -Olli
>
>  - lastCancelableEvent
>>
> This makes really no sense ;)
>
>
>> Rick
>>
>> On Tue, Jan 20, 2015 at 1:41 AM, Jacob Rossi <Jacob.Rossi@microsoft.com
>> <mailto:Jacob.Rossi@microsoft.com>> wrote:
>>
>>     At first glance defaultStartsManipulationseems reasonable (not crazy
>> about the name, but even less crazy about bikeshedding over it). I want to
>>     take a look at some of the pages we’ve seen using crude JS gesture
>> recognizers & magic numbers to determine if a manipulation is going to occur
>>     and see how easy something like this is to swap in. ____
>>
>>     __ __
>>
>>     -Jacob____
>>
>>     __ __
>>
>>     *From:*rbyers@google.com <mailto:rbyers@google.com> [mailto:
>> rbyers@google.com <mailto:rbyers@google.com>] *On Behalf Of *Rick Byers
>>     *Sent:* Monday, January 19, 2015 8:32 AM
>>     *To:* public-touchevents@w3.org <mailto:public-touchevents@w3.org>
>>     *Cc:* Mustaq Ahmed; Matt Gaunt; Jake Archibald; Daniel Freedman
>>     *Subject:* Identifying touch events that can trigger scrolling____
>>
>>     __ __
>>
>>     In last week's TECG call <http://www.w3.org/2015/01/13-
>> touchevents-minutes.html>, we agreed that the below problem of
>> identifying touch events
>>     that can trigger a click is interesting mostly for browsers that
>> don't have an API for disabling the click delay and that adding such an API
>> is
>>     probably a superior solution for those scenarios.____
>>
>>     __ __
>>
>>     However, the very related (often inverse) problem of identifying
>> touch events which can cause a scroll to start is still very interesting
>> for all
>>     browsers.  We liked Ben's proposal <https://bugs.webkit.org/show_
>> bug.cgi?id=138151> of adding an API to touch events for indicating what
>> actions
>>     they can trigger, but prefer to start small focused on concrete
>> actions they can trigger.  In particular, an API defined in terms of touch
>>     gestures (like "pinch" and "double tap") is unlikely to be
>> standardizable in the current IP climate.____
>>
>>     __ __
>>
>>     Based on this discussion, I've written up a concrete proposal
>>     <https://docs.google.com/a/chromium.org/document/d/1Rf-
>> WafO91JehdjNVGRAkOQHCSYbFju8M90A4Xt1s-A8/edit#heading=h.ay2pmk927mpu>
>> for addressing the
>>     "when will scrolling start" problem.  Essentially this adds a boolean
>> to touchmove events called "defaultStartsManipulation" (with some connection
>>     to the touch-action specification).____
>>
>>     __ __
>>
>>     Jake/Matt: in particular I expect this to address the concern
>>     <https://groups.google.com/a/chromium.org/forum/#!searchin/
>> input-dev/mattgaunt/input-dev/Bo5FyTLljwY/aZVWhHV04AwJ> you raised
>> awhile back about
>>     custom side-swipe and the lack of touchcancel events - especially if
>> we can get a common API supported in all major browsers.____
>>
>>     __ __
>>
>>     Feedback (here or as comments in the doc)?____
>>
>>         Rick____
>>
>>     __ __
>>
>>     On Mon, Jan 5, 2015 at 12:48 PM, Rick Byers <rbyers@chromium.org
>> <mailto:rbyers@chromium.org>> wrote:____
>>
>>         Sometimes it's useful to detect a 'tap' gesture from JavaScript
>> from the touch events, before the mouse events (click, etc.) are fired.  For
>>         example, popular libraries like FastClick <
>> https://github.com/ftlabs/fastclick> need to do this for browsers that
>> don't have a better way to
>>         disable the 300ms click delay (but there are other scenarios as
>> well).____
>>
>>         __ __
>>
>>         Many apps / libraries implement a simple heuristic for this like
>> "look for a first-finger touchstart, followed by 0 or more touchmoves which
>>         are less than N pixels from the start, then a touchend" This is
>> relatively simple and works alright in practice most of the time.  But the
>>         developer's intention is that it should match the browser's tap
>> detection logic EXACTLY.  If the logic mis-identifies a tap, then you could
>>         get double handling (JS behaving as a tap but the browser
>> triggering a scroll). If, on the other hand, the logic fails to identify a
>> tap it
>>         may feel harder to activate a control, or activation behavior may
>> be inconsistent.____
>>
>>         __ __
>>
>>         There are a number of reasons why such application logic can
>> never be perfect:____
>>
>>          1. The value of N can vary.  Eg. on Android, device OEMs can
>> control the ViewConfiguration::getScaledTouchSlop
>>             <http://developer.android.com/reference/android/view/
>> ViewConfiguration.html#getScaledTouchSlop()> value used by browsers to
>> implement tap
>>             detection.____
>>          2. The browser may use an algorithm more complex than a simple
>> bounding box.  Extreme example: in iOS UIWebView (and WKWebView) the native
>>             application has control over the gesture recognizer graph and
>> can arbitrarily influence detection logic.____
>>
>>         All in all, it seems web developers should really have a way of
>> asking "will this touchend cause a click event to be generated if it's not
>>         canceled?".____
>>
>>         __ __
>>
>>         I discussed this problem a bit with Benjamin Poulin on the Safari
>> team here: https://bugs.webkit.org/show_bug.cgi?id=138151.  He proposes a
>>         property on TouchEvent ("eventCanStartDefault") which somehow
>> indicates which gestures would be started by the default action of the event
>>         ("Tap", "Pan", "Pinch", etc.).  I'd be happy to add something
>> like this to chromium as well (due to a recent re-architecture it's actually
>>         quite easy for us to implement).____
>>
>>         __ __
>>
>>         Thoughts?____
>>
>>             Rick____
>>
>>         __ __
>>
>>     __ __
>>
>>
>>
>
Received on Thursday, 22 January 2015 16:05:58 UTC

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