[Development] Qt Contributor Summit 2019 sessions
Uwe.Rathmann at tigertal.de
Fri Nov 8 10:50:39 CET 2019
On 11/8/19 9:51 AM, Richard Weickelt wrote:
>> doing scene graph node composition ? > What is the latter?
It is mostly about having a lower memory footprint.
With QML you only have access to ~heavy objects like QQuickItem and the
controls are usually compositions of basic items like text/image/rectangle.
F.e a page indicator with n bullets from QC2 is made of n + 3
QQuickItems. Each item has an impact on the scene graph as it adds at
least an additional transform node. Each bullet is a QQuickRectangle,
that has a gradient made of stops. Each gradient and each stop of each
rectangle is a QObject.
I leave it up you to sum things up:
sizeof( QSGNode ): 80
sizeof( QSGTransformNode ): 216
sizeof( QObjectPrivate ): 112
sizeof( QObject ): 16
sizeof( QQuickItemPrivate ): 320
sizeof( QQuickItem ) : 32
The page indicator of QSkinny is exactly 1 QQuickitem that creates
exactly one node per bullet.
The same technique is used for almost all QSkinny controls - f.e a push
button is made of Panel/Image/Text nodes. The maybe most extreme example
is the listbox that is also one QQuickItem only.
Having access to the nodes also allows a more memory aware approach
about when to create/destroy nodes.
The application driving QSkinny is a beast of ~250K lines of code,
having ~2000 different images. For applications like these you simply
have to be careful about resources.
> I'm not sure if that is what You have in mind, but I think that QML's
> language features:
No, I don't want to make a statement about QML vs. C++. This is
something that needs to be decided based on the situation of a project
and the preferences of the development team.
But of course you need to do C++ when accessing the scene graph or its
More information about the Development