[OpenLayers-Users] HOWTO: Using the script protocol and modifying
the filter. A solution for usb/disk-based map distribution
Phil Scadden
p.scadden at gns.cri.nz
Wed Jun 8 20:07:51 EDT 2011
WFS queries are usually (and best) done through ajax mechanisms.
However, if you have a map application on standalone disk or similar
accessing remote map servers, you have a problem with browser security
which will block you.
The solution is use the script protocol instead, which uses script
insertion, GET and JSONP to achieve similar thing. And the emphasis is
similar. I have more or less got this working but it was quite a slog.
Here is how which might somebody some time and if anyone can come up
with better solution, i'd be all ears. Note that I have modded some bits
of OL2.11 to achieve this as well.
The protocol setup looks something like this:
wfsProtocol = new OpenLayers.Protocol.Script({
url: mywfsURL,
callbackKey: "format_options",
callbackPrefix: "callback:",
params: {
service: "WFS",
version: "1.1.0",
srsName: mySRS,
request: "GetFeature",
featurePrefix: myFeaturePrefix,
typeName: myfeatureType,
outputFormat: "json"
},
filterToParams: function(filter, params) {
// filter to params code
}
});
}
You use the protocol with same code that you would use for wfs protocol eg:
wfsProtocol.read({
filter: new OpenLayers.Filter.Spatial({
type:
OpenLayers.Filter.Spatial.INTERSECTS,
value: e.feature.geometry
}),
callback: mycallback,
scope: strategy
});
The fun comes with the filtertoParams function in the protocol setup
(which I left as blank in the above code. This has to take the filter
and convert into set of parameters to use for a HTTP GET. By default (if
you dont specify anything, it uses a QueryStringFilter. As far as I can
see, this only works properly for a LOGICAL AND filter. Not at all for
OR and spatial has too many complications for its simple approach.
For everything except AND filters, the filter needs to be converted to a
cql Filter with all of cql filters limitations. The biggies are:
1/ (E)CQL Filters work exclusively in the native SRS. Returned GML
however will be converted into the requested SRS.
2/ Lat/long native projections expect the geometry string to be in
lat/long order, not x,y. This is big problem if you code is used where
users can add own layers because you have to figure out what the kind of
SRS it is.
3/ Like is supported but not ilike. Implicit AND clauses can be put in
as separate parameters however. (Have form propertyname__ilike=value).
Here is my filterToParams function. Its not comprehensive but covers
filters I use (mostly).
filterToParams: function(filter, params) {
// example to demonstrate BBOX serialization
delete params.cql_filter;
if (filter.type ===
OpenLayers.Filter.Spatial.BBOX) {
params.bbox = filter.value.toArray();
if (filter.projection) {
params.bbox.push(filter.projection.getCode());
}
} else if (filter.type ===
OpenLayers.Filter.Spatial.DWITHIN) {
var geom = filter.value;
transformGeometry(geom);
if (geom.x>=-180 && geom.x<=360
&& geom.y>=-90 && geom.y<=90) { // is this a lat long?
var xx = geom.x;
geom.x = geom.y;
geom.y = xx;
}
params.cql_filter =
'DWITHIN(SHAPE,' + geom.toString() + ',' + filter.distance + ',meters)';
} else if (filter.type ===
OpenLayers.Filter.Spatial.INTERSECTS) {
var geom = filter.value;
transformGeometry(geom);
geom.components[0].swapXY();
params.cql_filter =
'INTERSECTS(SHAPE,' + geom.toString() + ')';
} else if
(OpenLayers.Format.QueryStringFilter) {
var format = new
OpenLayers.Format.QueryStringFilter({
srsInBBOX: this.srsInBBOX
});
params = format.writecql(filter, params);
}
return params;
}
Notes about it.
transformGeometry is application-specific procedure to transform the
geometry into the native SRS of the layer. There is quite a lot to this
Firstly, when the layer is added to map, then the SRS has to be looked
at. If not already supported then you need use script injection to bring
in the appropriate projection from proj4. The layer native SRS has to be
tracked so it can be compared with current map projection. If there are
different, then transformGeometry needs to do a geom.transform to get it
into native.
The next step is to determine whether this is lat/long SRS. If it is,
then need to swapXY. I have extended the linearRing with a swapXY method.
swapXY: function() {
for (var i=0, len=this.components.length; i<len - 1; i++) {
var component = this.components[i];
var xx = component.x;
component.x = component.y;
component.y = xx;
}
this.bounds = null;
return this;
},
Note on DWITHIN and Geoserver. Geoserver just completely ignores the
distanceUnits parameter. You need to make sure the distance units are in
the native SRS as well.
For none spatial filters, I fell back on QueryStringFilter. It does not
support OR however, so I extended with a writeccql method.
It looks like:
writecql: function(filter, params) {
params = params || {};
params.cql_filter = params.cql_filter || [];
var className = filter.CLASS_NAME;
var filterType =
className.substring(className.lastIndexOf(".") + 1);
switch (filterType) {
case "Comparison":
var op = cmpToStr[filter.type];
if (op !== undefined) {
var value = filter.value;
if (filter.type ==
OpenLayers.Filter.Comparison.LIKE) {
op = 'like';
value = "'" +
value.toLowerCase().replace(/\*/g, "\%") + "'";
filter.property = 'strToLowerCase(' +
filter.property + ')';
}
params.cql_filter = params.cql_filter +
filter.property + " " + op + " " + value;
} else {
OpenLayers.Console.warn(
"Unknown comparison filter type " +
filter.type);
}
break;
case "Logical":
if (filter.type === OpenLayers.Filter.Logical.AND) {
for (var i=0,len=filter.filters.length; i<len;
i++) {
if (params.cql_filter !="")
params.cql_filter = params.cql_filter + ' AND ';
params = this.writecql(filter.filters[i],
params); ;
}
} else if (filter.type ===
OpenLayers.Filter.Logical.OR) {
for (var i=0,len=filter.filters.length; i<len;
i++) {
if (params.cql_filter !="")
params.cql_filter = params.cql_filter + ' OR ';
params = this.writecql(filter.filters[i],
params);
}
} else {
OpenLayers.Console.warn(
"Unsupported logical filter type " +
filter.type);
}
break;
default:
OpenLayers.Console.warn("Unknown filter type " +
filterType);
}
return params;
},
Now it should be possible to combine this into ordinary write and
support ilike AND parameters but I didnt figure that out. Note also that
this version actually creates an ECQL filter for geoserver for Like
comparison operators. This will only work with geoserver2.1 and others
that support ECQL. For more general use, remove the strToLowerCase()
function on the like. Unfortunately, this make LIKE case-sensitive.
I would love someone to tidy this up, but if not, I hope this saves
someone a lot of work.
--
Phil Scadden, Senior Scientist GNS Science Ltd 764 Cumberland St,
Private Bag 1930, Dunedin, New Zealand Ph +64 3 4799663, fax +64 3 477 5232
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