IGeoAware interface

The IGeoAware interface defines four methods objects must implement in order to be usable with PrimaGIS:

• getGeometry
• getGeoTitle
• getGeoDescription
• getGeoURL

Each method is documented in the IGeoAware interface defined in interfaces.py file within the PrimaGIS product. The most important method is getGeometry, which must return a PCL geometry object (which are defined in the cartography.spatial module) representing the spatial data. Depending on your needs the geometry object may represent either POINT, LINESTRING or POLYGON geometries. The other three methods should return metadata about the object that will be shown and used in the popup windows.

GeoAwareMixin

The most straight-forward method for spatially extending existing AT objects would be to make your own product extend the AT object and then implement the four methods mentioned above by hand. In order to help developers implement the IGeoAware interface, the PrimaGIS.GeoAwareMixin module defines mixin classes and schemas for each type of geometry that your own products may inherit. This will reduce the amount of code you have to write unless your needs are very specific.

GeoATImage - a spatially enabled ATImage

GeoATImage from the GeoAwareObjects product is a simple content type that extends the ATImage object from ATContentTypes making it usable with PrimaGIS. This allows you to show images within the popup windows on the map. The source code for GeoATImage is shown below

from Products.PrimaGIS.interfaces import IGeoAware
from Products.PrimaGIS.GeoAwareMixin import PointMixinSchema, PointMixin
from Products.ATContentTypes.types import ATImage
from Products.Archetypes.public import registerType
from AccessControl import ClassSecurityInfo
from Products.GeoAwareObjects.config import PROJECTNAME

class GeoATImage(ATImage.ATImage, PointMixin):
    """
    This class extends the ATImage from ATContentTypes with methods
    that implement the IGeoAware interface which allows the images
    to rendered on the map as ECMAScript popups.
    """

    __implements__ = IGeoAware,

    schema = ATImage.ATImage.schema.copy() + PointMixinSchema

    security = ClassSecurityInfo()

    meta_type = 'GeoATImage'
    archetype_name = 'Geo-aware AT Image'

    ## IGeoAware implementation

    security.declarePublic('getGeoDescription')
    def getGeoDescription(self):
        """
        Return the XHTML image tag to render the image in the
        popup window.

        This method overrides the PointMixin.getGeoDescription() method.
        """
        return self.tag()

registerType(GeoATImage, PROJECTNAME)

The implementation is quite trivial. The important part is that our new content type inherits both the existing AT content type and a mixin class from PrimaGIS.GeoAwareMixin. In this particular case it is natural to use the POINT geometry and thus we inherit from GeoAwareMixin.PointMixin. In the same way we set the schema for our new content type to contain both the original ATImage schema and a GeoAwareMixin.PointMixinSchema.

At this point we already have a working, spatially enabled derivate of ATImage. Inheriting PointMixin automatically implements the IGeoAware interface making GeoATImage to be usable with PrimaGIS. Because we wish to have the image represented by GeoATImage to rendered in the popup window, we must override the getGeoDescription() method in order to achieve this. Having the new getGeoDescription() method return the image tag will accomplish this.

At this point we have a new derived content type that allows us to render images on PrimaGIS maps and we have achieved this with only few lines of code.

Discussion

This article showed us how to spatially extend existing AT objects. It may not be always feasible to create a new content type only to have it made usable with PrimaGIS. The GeoArchetypesProxy allows using arbitrary AT objects as data for the maps without having to modify them or create new content types. The GeoArchetypesProxy objects implement the IGeoAware interface and store the spatial data within them and simply refer to the corresponding AT objects.

The proxy approach is somewhat limited and currently supports only POINT geometries and by default renders the Dublin Core Description field in the popup window. In this particular case we needed to modify that behaviour to render the image instead and opted to create a new derived content type.

The mixin classes and schemas defined in GeoAwareMixin module aim to be very general and if you have very specific or complex needs then implementing IGeoAware by hand is recommended.