[Qgis-developer] Rule-based renderer updates

[Martin Dobias]

Hi all

I have just merged improved rule-based renderer to master branch.
There are various changes in both backend and GUI:

* Rules now form a hierarchical structure: any rule can contain
further "child" rules. When rendering, child rules are applied only
when their parent rule matches. This allows users to build a logical
trees of rules instead of keeping all rules in a flat list with
complex filters.

* The options "use only first rule" and "use symbol levels" have been
removed: now all the matching rules and applied and at the same time
the rendering order is given by the symbol levels.

* GUI: rules are shown as a tree. They can be organized easily with
drag and drop (multiple selection is supported, too). Let me emphasize
again that the tree of rules does not say anything about the rendering
order (because in SLD and Mapnik the order of rules matters). The
rendering order can be set by the button in bottom-right corner of the
renderer widget - it triggers the same dialog as used for setting
symbol levels.

* GUI: inline editing of labels, filters and scale range is now
possible - use F2 -or- click again a selected item to enter inline
editing.

* Rules may not contain a symbol: usually parent rules are not
assigned a symbol because their child rules take care of rendering.

* symbol levels dialog for other renderers is now accessible from a
menu from their "advanced" button - to avoid some clutter,
additionally renderers indicate whether they support symbol levels or
not.

Originally I wanted to design the rule-based renderer to follow SLD /
Mapnik logic - the order of rules (and group of rules) determines the
rendering order. However this is not very user friendly: the user has
to prepare the rules in a way that is convenient to a software, not
the user. For example, when drawing a map of roads with nicely
rendered road outlines and varying Z-levels (bridges / tunnels), the
user has to split the road symbols into multiple rules and move them
away from each other - further editing of rules and symbols becomes
complex. In our approach the user can keep the rules in a hierarchy
that is most convenient and then define the rendering order
independently. So there is not a 1:1 relation between QGIS rule-based
rendering and SLD (or Mapnik) rules, however the import/export
capabilities can be implemented without problems.

Please test, I would like to hear your comments, suggestions and bug
reports. I am especially interested to hear any notes related to
usability - I have some doubts regarding GUI, e.g.:
- expand all rules when GUI is opened?
- delete parent rule's symbol when refining it? or completely forbid
rules to contain a symbol if they have children?

There are still some smaller things that might be added in future:
- cloning of rules
- filtering of rules in GUI - by label / filter expression / scale
- "else" rule - will be used only if no other sibling rule matches
- checkable rules?
- better item delegates for inline editing - e.g. filter editor with
"..." button to open query builder

Finally, I would like to thank Ville de Morges, Switzerland for
sponsoring this work.

Regards
Martin