[Interest] QML: Use singleton types instead of pragma library scripts

Dmitry Volosnykh dmitry.volosnykh at gmail.com
Fri Dec 26 11:22:13 CET 2014

Dmitry, you can read more on that in this wiki article:
Primarily, you are interested in section "Nesting QtObjects", but reading
the whole article would be useful too.

In our application, we found it more convenient to do that in slightly
different manner. We instantiate a single instance of that component
(rather than making it be a true singleton) which is placed in the root
.qml file and provided with some id (say, 'appStyle') which is supposed by
our internal convention to be a global name used to refer from any other
QML component. This trick allows us to live code UI components. True
singletons lack this ability since they are not re-created at the time UI
gets reloaded due to changes to .qml files. My collegue has noted that such
implicit referring to an id out of component definition may impact QML
engine performance. Maybe some one can elaborate more on this.


On Fri, Dec 26, 2014 at 1:12 PM, Dmitry Q <linuxsquirrel.dev at gmail.com>

> Hi. In the official QML documentation we see the following suggestion (
> http://doc.qt.io/qt-5/qtquick-performance.html):
> >>>
> Use singleton types instead of pragma library scripts
> If you are using a pragma library script to store application-wide
> instance data, consider using a QObject singleton type instead. This should
> result in better performance, and will result in less JavaScript heap
> memory being used.
> <<<
> What does is mean? How should we use QObject singletons? For example, I
> have the following ".pragma library" JS to store application-wide constants:
> .pragma library
> var X = 10;
> ... many other declarations...
> How should I rewrite this with QObject singletons?
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