I am using QGraphicsSvgItem subclass, that reads some content from a file, places content into a QDomDocument, does some initial processing, then sets the processed DOM onto a renderer.
During program processing, additional changes are required on a copy of pre-processed DOM, so the DOM is stored in class. After changes, the DOM is placed on renderer.
class MyGraphicsSvgItem : public QGraphicsSvgItem
{
public:
MyGraphicsSvgItem (QGraphicsItem *parent = 0):
QGraphicsSvgItem(parent),
_svgXML() {}
~MyGraphicsSvgItem () { delete renderer(); }
void CheckAndChangeSomeThings() {}
void LoadStuff (QString fileName)
{
QFile file(fileName);
file.open(QFile::ReadOnly | QFile::Text);
QTextStream in(&file);
QString svgContent = in.readAll();
file.close();
_svgXML.setContent(svgContent);
CheckAndChangeSomeThings(); // this modifies _svgXML
QByteArray _data = _svgXML.toByteArray();
setSharedRenderer(new QSvgRenderer(_data)); // very slow
}
void ChangeThingslater();
void ChangeSomeThingslater()
{
ChangeThingslater(); // this modifies _svgXML
renderer()->load(_svgXML.toByteArray()); // very slow - no file involved
}
protected:
QDomDocument _svgXML;
};
There seems to be a significant slow processing during the lines that assign the DOM to the renderer.
QByteArray _data = _svgXML.toByteArray();
setSharedRenderer(new QSvgRenderer(_data));
If I skip the DOM processing - if I set the renderer to the file - the code speeds up considerably:
Leaving all the code in, but replacing
setSharedRenderer(new QSvgRenderer(_data)); // VERY SLOW
with
setSharedRenderer(new QSvgRenderer(fileName)); // FAST
So it seems the bottleneck is loading the svg renderer from QByteArray.
I looked for alternatives... there is no mention of performance in documentation
QSvgRenderer::QSvgRenderer(const QString & filename, QObject * parent = 0)
Constructs a new renderer with the given parent and loads the contents of the SVG file with the specified filename.QSvgRenderer::QSvgRenderer(const QByteArray & contents, QObject * parent = 0)
Constructs a new renderer with the given parent and loads the SVG data from the byte array specified by contents.QSvgRenderer::QSvgRenderer(QXmlStreamReader * contents, QObject * parent = 0)
Constructs a new renderer with the given parent and loads the SVG data using the stream reader specified by contents.
Looking in QXmlStreamReader Class, I find that its constructors are similar ! In addition, it says
In some cases it might also be a faster and more convenient alternative for use in applications that would otherwise use a DOM tree
I seem to be going in circles, and even though having already a well formed xml in the DOM, it seems I cannot take advantage of it !
What are my alternatives, to either loading the renderer from the pre-processed DOM, or to a different way of pre-processing the xml - using something other than DOM that the renderer can read fast ?
qt 4.8. c++
