from django.contrib.gis.gdal import OGRException

from django.contrib.gis.geos import GEOSGeometry, GEOSException

from django.forms.widgets import Textarea

from django.template.loader import render_to_string

class OpenLayersWidget(Textarea):

    """

    Renders an OpenLayers map using the WKT of the geometry.

    """

    def render(self, name, value, attrs=None):

        # Update the template parameters with any attributes passed in.

        if attrs: self.params.update(attrs)

        # Defaulting the WKT value to a blank string -- this

        # will be tested in the JavaScript and the appropriate

        # interfaace will be constructed.

        self.params['wkt'] = ''

        # If a string reaches here (via a validation error on another

        # field) then just reconstruct the Geometry.

        if isinstance(value, basestring):

            try:

                value = GEOSGeometry(value)

            except (GEOSException, ValueError):

                value = None

        if value and value.geom_type.upper() != self.geom_type:

            value = None

        # Constructing the dictionary of the map options.

        self.params['map_options'] = self.map_options()

        # Constructing the JavaScript module name using the ID of

        # the GeometryField (passed in via the `attrs` keyword).

        self.params['module'] = 'geodjango_%s' % self.params['field_name']

        if value:

            # Transforming the geometry to the projection used on the

            # OpenLayers map.

            srid = self.params['srid']

            if value.srid != srid: 

                try:

                    value.transform(srid)

                    wkt = value.wkt

                except OGRException:

                    wkt = ''

            else:

                wkt = value.wkt

            # Setting the parameter WKT with that of the transformed

            # geometry.

            self.params['wkt'] = wkt

        return render_to_string(self.template, self.params)

    def map_options(self):

        "Builds the map options hash for the OpenLayers template."

        # JavaScript construction utilities for the Bounds and Projection.

        def ol_bounds(extent):

            return 'new OpenLayers.Bounds(%s)' % str(extent)

        def ol_projection(srid):

            return 'new OpenLayers.Projection("EPSG:%s")' % srid

        # An array of the parameter name, the name of their OpenLayers

        # counterpart, and the type of variable they are.

        map_types = [('srid', 'projection', 'srid'), 

                     ('display_srid', 'displayProjection', 'srid'), 

                     ('units', 'units', str),

                     ('max_resolution', 'maxResolution', float),

                     ('max_extent', 'maxExtent', 'bounds'),

                     ('num_zoom', 'numZoomLevels', int),

                     ('max_zoom', 'maxZoomLevels', int),

                     ('min_zoom', 'minZoomLevel', int),

                     ]

        # Building the map options hash.

        map_options = {}

        for param_name, js_name, option_type in map_types:

            if self.params.get(param_name, False):

                if option_type == 'srid':

                    value = ol_projection(self.params[param_name])

                elif option_type == 'bounds':

                    value = ol_bounds(self.params[param_name])

                elif option_type in (float, int):

                    value = self.params[param_name]

                elif option_type in (str,):

                    value = '"%s"' % self.params[param_name]

                else:

                    raise TypeError

                map_options[js_name] = value

        return map_options