[Qgis-developer] Expression based labeling

[Nathan Woodrow]

Thanks Andreas. Glad you like it.

>> Question 1: the output preview of the label does not seem to work when I
use a field - is there something I need to >>do to enable it? I would expect
it to pull in the first record or some random record to retrieve values?

Ah yes there seems to be a bug with grabbing the first record with some data
sources.  I get the top feature using layer.featureAt(0) but for some reason
not all providers return a record at this index.  Writing a patch now.

>>Question 2: how does it handle empty fields (null values)? would string
concatenation still work or do we need to >>coalesce() like in PostreSQL?

Yeah null values when used with || are just converted to empty strings.

>> Remark:
>> wouldn't it be cleverer to use your great new query builder and introduce
the concept of a "virtual column" in the >>data provider?

That would be a cool idea and something I have thought about, just don't
have the skills or time at present.  I plan to redo the field calculator to
use my new expression builder widget at some stage so we can look at the
virtual column thing then.

- Nathan

On Thu, Oct 27, 2011 at 5:46 PM, Andreas Neumann <a.neumann at carto.net>wrote:

> Hi Nathan and others,
>
> Thanks for developing the expression based labeling. Wow - it really looks
> great.
>
> I have two questions and a remark:
>
> Question 1: the output preview of the label does not seem to work when I
> use a field - is there something I need to do to enable it? I would expect
> it to pull in the first record or some random record to retrieve values?
>
> Question 2: how does it handle empty fields (null values)? would string
> concatenation still work or do we need to coalesce() like in PostreSQL?
>
> Remark:
> wouldn't it be cleverer to use your great new query builder and introduce
> the concept of a "virtual column" in the data provider? This way we could
> use your query builder for labeling, for tooltips, for read only display in
> tables and forms and in the identify tool. Potentially in the future in
> other scenarios as well? The query builder would then extend the data
> provider and introduce a new virtual column which could then be treated like
> all other columns, but read only.
>
> Thanks a lot for your cool work and feature enhancement towards labeling!
>
> Andreas
>
> --
> --
> Andreas Neumann
> Böschacherstrasse 10A
> 8624 Grüt (Gossau ZH)
> Switzerland
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