Complex text input in Plasma

Martin Gräßlin mgraesslin at
Sat Apr 8 19:43:54 UTC 2017

Am 2017-04-08 21:21, schrieb Weng Xuetian:
> On Saturday, 8 April 2017 09:46:17 PDT,Martin Gräßlin wrote:
>> Am 2017-04-08 17:26, schrieb Weng Xuetian:
>> > You're wrong about the QT_IM_MODULE stuff. To make application to use
>> > the
>> > wayland protocol to type (text-input), the implementation must be done
>> > with
>> > QT_IM_MODULE=wayland. I don't mind if it is set to certain application
>> > but set
>> > it in general won't work. Also to have real virtual keyboard , you need
>> > to let
>> > the input method daemon to provides a virtual keyboard implementation.
>> No you are wrong about that one :-) It might be that it used to be 
>> like
>> that, but Wayland is the default if no QT_IM_MODULE is specified. See
>> .cpp#n142
> What I want to say is, if you set QT_IM_MODULE to qtvirtualkeyboard, 
> that app
> can't talk to im daemon via the wayland protocol. The automatic 
> selection
> procedure is not really a problem there.

And that's not the case. Our virtual keyboard is using the Wayland text 
input protocol and does not require setting the QT_IM_MODULE at all.

Please believe a little bit in what I'm writing. I coded this stuff.

> Also, running it inside kwin could still be problematic. It'll also 
> need to
> display UI. From what I heard about, to run a qml window inside kwin 
> could
> even be a problem.

I don't know what you have heard but all our UI in KWin is written in 
Qml nowadays.

> The races you pointed about is not really a problem. Weston has already 
> done
> it well. Unless you want to reinvent a new protocol, using the current 
> wayland
> protocol essentially does what I said. And if such a thing would 
> interfere
> with kwin's rendering procedure, then probably kwin should refactor 
> about
> that.

Just because Weston doesn't think of this problem doesn't mean it 
doesn't exist. Weston is not perfect and I found several bugs in the way 
they implemented the protocols they developed and Weston is the 
reference for.

> And let me give you some number here, a modern input method for Chinese 
> could
> takes about 30ms to handle one single key (it is THAT complex and we 
> are all
> limited by the complexity, though I measured it about 5 years ago). You 
> really
> want to have kwin block on that and wait for such a long time? And just
> because it is such a slow thing, the IPC latency is not even 
> comparable. If
> you want good prediction or handwriting, it will be slow. There's no im 
> use
> neural network on linux right now, but iOS's virtual keyboard is 
> already doing
> that. Which might be the future, but also mean the im processing time 
> might
> not slow down.

I don't want to base any decisions on 5 year old numbers. 30 msec would 
be bad, but if it is with modern hardware just 10 it wouldn't be that 

> Also the "input method always on" idea is not saying that every single 
> key
> will be send to input method, it just mean whenever user need type 
> text, it
> will be forwarded to input method.
> I don't know whether I could persuade you, but please look around the 
> world,
> iOS, android, sailfish (also use wayland), mac OS , (windows used to 
> run im
> directly inside application.. but now they also have a service for that 
> ), no
> one would run a im daemon inside the compositor. Are they all ignore 
> the ipc
> overhead?

I'm using sailfish and Android and both keyboards suck bad times. 
Sailfish is really bad, well it's maliit so no surprise there. With both 
systems I see problems in UI positioning because it's not in the 
compositor. Both don't feel perfect especially not in the "every frame 
perfect sense" we are aiming for on Wayland.


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