QScreen vs. KScreen in plasmashell

[David Edmundson]

On Fri, Oct 24, 2014 at 9:58 AM, Daniel Vratil <dvratil at redhat.com> wrote:

> Hi all,
>
> the current state of multi-monitor support in Plasma 5 is rather bad as
> both
> Plasma 5 and KWin keep crashing every time there's any change (+ the
> desktop
> is extremely sluggish for about 15 seconds after that). Many colleagues in
> my
> office would love to switch to Plasma 5 already, but this is one of the
> major
> blockers that scares most people away, so it needs to be fixed.
>
> I was looking into plasmashell code to fix it (and also for a different
> reason, more on that later) and realized that the code is mixing use of
> KScreen and QScreen, and I couldn't resist asking myself why. QScreen
> provides
> subset of information that KScreen and all I can see in the code is just
> endless conversion between QScreen and respective KScreen::Output and
> hoping
> they both behave the same. Would it make sense to get rid of QScreen
> completely and use KScreen exclusively? I think it would solve most of the
> crashes I'm currently getting.


I don't think it solves everything, there's a big XCB problem too.


> I'm of course volunteering to do all the work,
> but before I start, I want to make sure there's no real obscure reason for
> keeping both QScreen and KScreen.
>

There are two reasons I can think of.

 1) So that the attached Screen property has the right info (
http://qt-project.org/doc/qt-5/qml-qtquick-window-screen.html)

Related to this, MouseEventListener in kdeclarative returns a QScreen
object in the mouse event, this /could/ be turned into a KScreen output but
that requires work.

 2) For future High DPI support we can scale independently per screen. All
QSGNodes of an item are wiped when it moves between screens and when they
are recreated the scale factor /could/ be different.

(right now they're not, in 5.4 it's fixed between all screens, and plasma
currently does it's own thing anyway, so arguably this isn't a very good
reason)


>
> Cheers,
> Daniel
>
> (For the "other reason I was looking into plasmashell": I'm currently
> working
> on a big KScreen API and design changes, which includes using
> QSharedPointers,
> having async API, and running platform backends in a separate process to
> improve performance and stability, but I'll send a separate email on that
> once
> the API is finished, as that will affect all KScreen-enabled applications)
>
> --
> Daniel Vrátil | dvratil at redhat.com | dvratil on #kde-devel, #kontact,
> #akonadi
> Software Engineer - KDE Desktop Team, Red Hat Inc.
>
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