Question

What happens behind, when i add a rectangle

Rectangle{
 width:40
 heigh: 40
 color:"blue"

}

in a qml file?

What invokes what and a blue rectangle has been drawn?
What is the mechanism? Can you advise something to read, i couldnot find.
Thanks

Answer 1

In general:

 -> parse Qml code 
 -> create QDeclarativeItems
 -> add to QDeclerativeView's scene
 -> call items standard slot like onCreated()
 -> call the QDeclarativeItems paint method etc     

You will get a warm belly feeling once you type QGraphicsRectItem into assistant-qt4 (the primary qt documentation assistant) and later realize that your qml code “Rectangle” is finally creating one. The Qt4 QtQuick implementation is currently a thin abstraction of QGraphicsView/QGraphicsScene. As above, the steps are the same:

 -> create QGraphicsItems
 -> add to QGraphicsView's scene 
 -> call QGraphicsItem paint method etc

These classes are exceptionally well documented, and once you understand their principle you are ready to go forward and study their evolution to QDeclarative* etc. Unfortunately the documentation of the latter is somewhat lacking in certain details, but overall very useful. Surely, there is more to it – but once your realize the QtQuick evolution originating from QGraphicsView/-Scene classes, everything else is much more logical. Then, a glimpse into the source code (where documentation was lacking) will make you feel much more oriented – as you have an idea how the parsed markup is finally converging into QGraphicsItem’s beeing created in your QGraphicsScene (QDeclarativeView,..).

Good Luck!