[OpenLayers-Users] WFS.fromWMSLayer selection question

[Christopher Schmidt]

On Wed, Nov 18, 2009 at 12:56:34PM -0800, Dragan Podvezanec wrote:
> 
> Hi all.
> 
> I have 10 WMS layers (polygons) which do not overlap. I'm trying to panZoom
> to the center of the polygon which user clicked. I registered
> "featureselected" events on all WMS layers, like this:
> 
> for(var i in map.layers[layer]) {		
> 						mapa.events.layers[layer] = new OpenLayers.Control.GetFeature({
> 						protocol: OpenLayers.Protocol.WFS.fromWMSLayer(map.layers[layer],{
> 							geometryName: 'geometry',
> 							srsName: "EPSG:900913"
> 							}),
> 							hover: false
> 						});
> 
> 						mapa.events.layers[layer][i].events.register("featureselected", this,
> function(e) {
> 
> 						var objekt = e.feature;
> 						var bbox = (objekt.geometry.bounds);
> 						var newcenter = (bbox.getCenterLonLat());
> 						mapa.panTo(newcenter);
> }
> 
> And this works. Map gets centered to the object that I have clicked. But as
> I see, there are 10 post request to Geoserver, featureselected is triggered
> on every WMS layer. Is this not an overhead? Why is event featureselected
> triggered, even I have clicked outside a layer?

The featureselected event *isn't* triggered -- that event just 
centers the map. The event that requests the datai s the GetFeature control.

The GetFeature Control has no way of knowing that there is a feature under
your cursor without asking the server.  What you have downloaded is just 
images -- OL knows nothing about the geometries.

As such, it has to ask the server for each of the layers whether there is an
intersecting feature.

> 1. Is this normal behavior?
> 2. What would happen if I have 100 layers?

Probably what you would expect.

> 3. Should I use different approach? (To make invisible WFS layers on top of
> WMS layers, and then register featureselected to WFS layers?)

You could, assuming 'invisible' layers actually get select events (I don't
know if they do.)

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-- 
Christopher Schmidt
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