Why so many QStyles
faure at kde.org
Sun Jan 4 12:51:15 UTC 2015
On Thursday 02 October 2014 22:10:05 Hugo Pereira Da Costa wrote:
> In Qt5 this seems not to be true anymore and I've seen up to 6 QStyle
> running concurently for a single Kate Window, increasing to 13 when
> opening 'open file' dialog, decreasing back to 6 when closing it, etc.
> Is this a known feature ? Will this stay ?
I'm surprised that nobody has thought of running the app in gdb and putting a
breakpoint on the QStyle constructor, to answer these questions with actual
So here I come.
1) first QStyle is created by the QApp initialization.
2) second QStyle is KLineEditStyle, a proxy style used by KLineEdit
But I suppose your question is about OxygenStyle instances, not other styles,
Ah, but then....
3) the first time the lineedit calls style(), qproxystyle.cpp does
100│ if (!baseStyle) // Use application desktop style
101├> baseStyle =
So it creates another instance of the main widget style (e.g. oxygen or
breeze), as the base style for the proxy.
BTW the code in Qt4 is the same, it's just that KLineEdit wasn't using
QProxyStyle but rather KdeUiProxyStyle.
This patch for kcompletion.git works around the issue by setting the base
style for the proxystyle to be the existing app style, rather than creating a
diff --git a/src/klineedit_p.h b/src/klineedit_p.h
index 0be32ba..cf63e09 100644
@@ -29,6 +29,7 @@
@@ -186,7 +187,7 @@ class KLineEditStyle : public QProxyStyle
- : QProxyStyle(),
+ : QProxyStyle(qApp->style()),
This leads me to the following thoughts:
1) I'm not sure why QProxyStyle defaults to creating another instance of the
app style rather than using the existing one. Maybe J-P Nurmi knows? Cc'ed.
2) I can't think of a reason against passing qApp->style() to the
KLineEditStyle ctor; the rest of the code in QProxyStyle::ensureBaseStyle is
about the -style cmdline override (which qApp->style() honours too).
3) In any case, Qt creates multiple instances of widget styles, AFAICS, in
many places. So yes, your style should be ready for that.
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE Frameworks 5
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