Custom C++-based QML components in Qt 5?

[fonzi337]

With the Qt 5 alpha release already out and the first production release coming in a few months, I'm wondering what is required to port existing Qt 4.x QDeclarativeItem-based visual classes to Qt 5. I know QQuickPaintedItem is included to facilitate this porting effort by providing the QPainter API similar to what QDeclarativeItem does today, but I'm wondering how custom visual C++-based QML components are developed in Qt 5. The documentation for QQuickItem does not list a virtual paint method as QDeclarativeItem does. If someone could describe what the overall process is for developing C++-based visual QML components in Qt 5 (and, even better, provide a trivial code example) I'd greatly appreciate it. So far I haven't found any information or examples on this topic and am very curious as to how custom painting will be accomplished in Qt 5/Qt Quick 2.

Edit: I forgot to mention that I've seen the QQuickCanvas QML element that allows for an HTML5-like API for painting in QML. However, I'm interested in how painting is done in C++.

[fonzi337]

Perhaps the way to do this is not yet known/documented for Qt 5? I still can't seem to find information on this.

[wysota]

The documentation for QQuickItem does not list a virtual paint method as QDeclarativeItem does.

There exists a virtual paint() method in QQuickPaintedItem.

If someone could describe what the overall process is for developing C++-based visual QML components in Qt 5 (and, even better, provide a trivial code example) I'd greatly appreciate it.

From what I see in general one reimplements QQuickItem::updatePaintNode().

[fonzi337]

Thanks wysota. It looks like the documentation for updatePaintNode was updated with a code snippet that clarifies things. Unfortunately, the API for drawing in the Scenegraph seems a little less user-friendly than the QPainter API (for instance, the code snippet under http://doc-snapshot.qt-project.org/5...metrynode.html that shows how to draw a straight line is fairly involved). Hopefully the Scenegraph API will grow to include facilities to simplify these common operations (or perhaps they are available but just haven't been documented yet).

[Brandybuck]

Wait.. that ugly code snippet is just to draw a red line? WTF? If that's how you draw a single red line, what the hell does a real world functional item look like? I fully understand the limitations of a sequential rasterized paint system, but this stuff looks makes it seem Nokia is intentionally making C++ use more difficult.

For perspective, here's how you draw a red line in traditional QPainter:

painter->setPen(Qt::red);
painter->drawLine(0, 0, width(), height());

[fonzi337]

Yeah, the API seems quite low-level (closer to OpenGL than high-level C++). Is it possible the QSGGeometryNode class is the wrong class to be looking at? I find it hard to believe that the highest-level drawing API provided for Qt 5 is what's found in QSGGeometryNode, but I haven't been able to find anything else so far.

[wysota]

I'm sure we'll get QPainter API too, probably with a warning that this causes slowdowns compared to scene graph approach.

[fonzi337]

In case anyone else is interested in this, I have found two small tutorials that show concrete examples of how custom C++-based elements are exposed to Qt Quick in Qt 5.

http://doc-snapshot.qt-project.org/5...mgeometry.html
http://wiki.qtlabs.org.br/projects/q...les/scenegraph

These examples are unfortunately still pretty basic/simple, but they at least show the basics.

[jsprenkle]

In case anyone else is interested in this, I have found two small tutorials that show concrete examples of how custom C++-based elements are exposed to Qt Quick in Qt 5.

http://doc-snapshot.qt-project.org/5...mgeometry.html
http://wiki.qtlabs.org.br/projects/q...les/scenegraph

The Qt documentation erasure fairy strikes again. Both links are now dead.