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From: Robert Csok <robert.csok@gmx.de>
Newsgroups: comp.lang.tcl
Subject: Re: Can Tk 9.0 handle touch events of a touch display?
Date: Sat, 1 Mar 2025 00:41:20 +0100
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Hello Harald,
hello Christian,

Thank you very much for your answers. I wanted to get back to you with 
some new information. First of all, I came across the Touchégg tool 
(https://github.com/JoseExposito/touchegg), but this tool is 
insufficient as it does not provide coordinates. It is more suitable for 
indirect touch devices such as touchpads, but not for touch-enabled 
displays. Then I tried to develop a solution that does not require any 
modification of Tk, and this solution works surprisingly well. Maybe I 
can make the code available. But perhaps the following hints are already 
useful for some of you:

So the idea is (a bit similar to Touchégg) to start a helper program 
(e.g. "touchHelper") in the background for each window/widget that needs 
touch events. Within a Tk application, the window could be a canvas with 
a map, for example. The "touchHelper" acts as an additional X client 
that selects for touch events on a specific window and forwards them to 
the associated Tk application using Tk_SendObjCmd(), i.e. Tk's send command.

The "touchHelper" uses libXi to select for the touch events (see 
XISelectEvents() from X11/extensions/XInput2.h). The events 
XI_TouchBegin, XI_TouchUpdate and XI_TouchEnd are selected on the 
specific window (ID) of the Tk application (determined by [winfo id 
window] in the Tk application and passed as a command line argument to 
the "touchHelper" together with [tk appname]). On the basis of these 
three events, a gesture recognition (swipe, pinch-to-zoom etc.) can be 
realized quite easily in Tcl.

Care must be taken to ensure that the queue of the Tk application is not 
flooded with events from the "touchHelper"; this can be done in the 
"touchHelper" and/or in the Tk application, namely by ignoring 
XI_TouchUpdate events that are triggered too frequently (taking the 
XIDeviceEvent::time field into account). Furthermore, one must ensure 
that a touch event does not trigger an additional click in the Tk 
application (button 1 event), as it may interfere. This can be done 
using a grab in the "touchHelper", e.g. via XIGrabButton().

I am aware that this is only a workaround for "legacy" Tk applications 
(on the X Window System). The long-term goal should of course be to make 
at least these three events (TouchBegin, TouchUpdate and TouchEnd) 
available in core Tk (via the bind command)

I found the following blog posts helpful, even if they don't go too much 
into detail:
http://who-t.blogspot.com/2009/05/xi2-recipes-part-1.html
http://who-t.blogspot.com/2010/11/high-level-overview-of-grabs.html

Best regards,
Robert

Am 21.12.24 um 17:45 Uhr schrieb Harald Oehlmann:
> Robert,
> 
> I have authored the TIP in 2020 and the described knowledge there has 
> not changed.
> As it is clear that this in an important feature, there was never any 
> movement here.
> Eventually, undrowish by Christian Werner has some support on Linux.
> 
> On Windows, we only have the button press emulation, that's all.
> 
> And the branch by Peter Sputh did not get any love since 2022.
> Aparently, Peters work also has some Mac aspects.
> 
> Any initiative appreciated - we need that !
> 
> Take care,
> Harald
> 
> Am 21.12.2024 um 13:57 schrieb Robert Csok:
>> Hello everyone,
>>
>> While reading through the “Highlights of Tk 9.0” I noticed the point 
>> “Platform Features and Conventions: many improvements, including two- 
>> finger gesture support where available”. I was initially pleased 
>> because I thought that Tcl 9 could react to touch events of a touch 
>> display. This function would make it easier to operate our cadastre 
>> application on devices with a touch-capable display (“Pinch To Zoom” 
>> etc.).
>>
>> However, I had to realize that this message probably refers more to 
>> the possibility of scrolling with two fingers on a touchpad (new event 
>> type “TouchpadScroll”). I also came across TIP 570, which describes 
>> the desired feature:
>>
>> https://core.tcl-lang.org/tips/doc/trunk/tip/570.md
>>
>> The proposal has been in a draft state since 2020.
>>
>> Can anyone give a hint on a possibility, trick or workaround to react 
>> to touch events of a touch display in a Tk application under Linux?
>>
>> Best regards
>>
>> Robert
>