[mapserver-dev] Questions on MapServer architecture and attribute conversions
Seth G
sethg at geographika.co.uk
Sat Mar 16 16:48:07 PDT 2019
Hi,
I implemented a simple addition to MapScript to retrieve the gmlItemObj for a layer item (field) and added to the pull request.
This currently works for the Python bindings but is throwing an error for C# I'll look into.
To use gmlItemObj I had to include mapows.h in mapscript.i. When I initially did this there were 28 errors similar to below:
mapscriptPYTHON_wrap.obj : error LNK2019: unresolved external symbol msWMSDispatch referenced in function _wrap_msWMSDispatch
The relevant commit is at: https://github.com/mapserver/mapserver/pull/5765/commits/0260296eecfb8b9bad979fc20ba5f9034ca37ffb (Ill see what happened to the whitespace in this commit and revert with just the relevant changes).
I simply added the macro MS_DLL_EXPORT to export all these functions and was able to build and test the MapScript bindings.
All the other header files included in SWIG have #ifndef SWIG guards everywhere. Is anyone able to explain to me the purpose of these and provide examples of where these should be added to mapows.h?
Seth
--
web:http://geographika.co.uk
twitter: @geographika
On Sat, Mar 16, 2019, at 11:58 AM, Lime, Steve D (MNIT) wrote:
> I wish we could make it less GMLish and more generic. For example, it feels odd in MVT code to be relying on what feels like format-specific metadata. The various service specs can use a generic “ows” or service specific metadata namespaces. Could do the same for output... “out” as the generic namespace with “gml”, “mvt”, “ogr”, “kml” as specific namespaces.
>
> gmlItemObj and gmlItemListObj could also be renamed more generically and the latter could become iteminfo as part of a layer. A cast function for type conversations would also be helpful for more standard use.
>
> Just thinking out loud - I know that’s way more than you want to get into,
>
> —Steve
>
>
> *From:* mapserver-dev <mapserver-dev-bounces at lists.osgeo.org> on behalf of Seth G <sethg at geographika.co.uk>
> *Sent:* Friday, March 15, 2019 6:58:41 PM
> *To:* mapserver-dev at lists.osgeo.org
> *Subject:* Re: [mapserver-dev] Questions on MapServer architecture and attribute conversions
>
> Hi Steve, Even,
>
> Thanks for your replies. Thinking back I think I've run into the issue of strings being used for int fields from a database, and CLASS expressions.
>
> I may make an attempt at the following based on both of your comments:
>
> - add gmlItemObj (and also gmlItemListObj) as classes to MapScript
> - allow msGMLGetItems to be called on a layerObj to populate this list
> - allow this list to be associated/passed to a method on a shapeObj, and use the GML types to set the output attribute types
> - if there are no field definitions supplied then attribute names will simply be indexes, and all values strings
>
> Seth
> --
> web:http://geographika.co.uk
> twitter: @geographika
>
> On Thu, Mar 14, 2019, at 10:14 PM, Lime, Steve D (MNIT) wrote:
> > Couple of questions here...
> >
> > On your pull request. I don't think there's a need for a RFC given the
> > scope of the change. it's not breaking anything and seems like a nice
> > addition - I certainly defer to your expertise as the Python MapScript
> > maintainer. I guess it's never a bad idea to alert the -dev list and
> > take your email as that notice - so +1 from me.
> >
> > On your assumptions, both are correct.
> >
> > On item types, it's funny this hasn't come up more over the years. Type
> > conversions seem to happen either for output using the metadata
> > referenced or in the expression handling based on context. You're
> > correct the that MapScript, most of the time you'd get the column/item
> > names from layer but not always since you can work with shapefiles
> > directly and independently of a layerObj or mapObj.
> >
> > Each layer has an attribute has something called iteminfo and it holds
> > the index value associated with corresponding value in the items array.
> > So you'd look up the item someone wanted for labeling purposes (or
> > whatever) and would know what value to grab from a shapes values array.
> > Those item index values are stored all over the place (e.g. labelitem
> > and labelitemindex). I think with a scheme like Even describes
> > items/iteminto could be replaced by one property (items) that would
> > equate to an array of field definitions and then specific things like a
> > labelitem becomes a struct consisting of a string and a pointer to one
> > of the items. For some reason I thought Dan made a run at that at one
> > point but I might be mistaken.
> >
> > -----Original Message-----
> > From: mapserver-dev [mailto:mapserver-dev-bounces at lists.osgeo.org] On
> > Behalf Of Even Rouault
> > Sent: Thursday, March 14, 2019 11:39 AM
> > To: mapserver-dev at lists.osgeo.org
> > Subject: Re: [mapserver-dev] Questions on MapServer architecture and
> > attribute conversions
> >
> > Seth,
> >
> > >
> > > I've created a pull request to add a __geo_interface__ to the Python
> > > MapScript PointObj, LineObj, and ShapeObjs. This allows Python scripts to
> > > easily share MapScript objects with other Python geospatial libraries such
> > > as Shapely, QGIS, and ArcPy. It also completes a nice cycle as Sean Gillies
> > > was a core developer of the MapScript Python bindings and created
> > > __geo_interface__. This only affects the Python MapScript bindings - do I
> > > need to create a RFC or ask for a vote to merge this for a 7.4 release?
> > >
> > > I also have a few questions on the internal architecture of MapServer
> > > classes. Could other devs confirm (or correct) the following assumptions?
> > >
> > > - the shapeObj has no link back to a layerObj - they are completely
> > > independent with no way to get a reference to a layer from a shape - all
> > > shapeObj attribute values are stored internally as strings
> >
> > Yes that's my understanding too.
> >
> > >
> > > To get attribute names for a shapeObj I had to get retrieve them from the
> > > relevant layerObj and store these in a new property on the shapeObj in
> > > Python MapScript. I can then output something like the following:
> > >
> > > "properties": {
> > > "guid": "954BADBF-2891-48ED-A132-AED39C60E4C9",
> > > "featureid": "203529",
> > > "classid": "12"
> > > }
> > >
> > > It would be nice to be able to convert the property values to their relevant
> > > types e.g. integers and floats. Looking through the MapServer source they
> > > seem to only be converted to types using METADATA blocks and
> > > "gml_FIELD_NAME_type" "datatype" and in certain cases e.g. in
> > > mapogroutput.c Are these conversions all handled differently for different
> > > data inputs? Is there an easy way to reuse these conversions in MapScript?
> >
> > Yes this lack of proper typing on layer attributes is a bit inconvenient and
> > causes lot of adhoc coding in various areas of the code base: ogroutput, OGC
> > filtering, WFS, GML output, MVT output, etc.
> > The fundamental issue is that layerObj::items is just an array of field names.
> >
> > For brainstorming... A potential solution could take inspiration from what is
> > done in GDAL where:
> > OGRLayer (~ equivalent of layerObj) -> contains a OGRFeatureDefn
> > OGRFeature (~ equivalent of shapeObj) -> points to a OGRFeatureDefn, and has
> > an array of field values
> >
> > A OGRFeatureDefn contains (among other things) an array of OGRFieldDefn
> > A OGRFieldDefn is a set of attribute: field name, field type, etc...
> >
> > Or if not doing a majorhaul, have indeed a pointer from shapeObj back to
> > layerObj (which interesting lifetime consideration, but I'm not sure MapScript
> > really shines on maintaining object ownership), and use information returned
> > by msGMLGetItems().
> >
> >
> > Even
> >
> >
> > --
> > Spatialys - Geospatial professional services
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