[OpenLayers-Dev] feature events

[Ivan Grcic]

Hi Tim,

this feature looks really useful, and would certainly solve the issues
you've mentioned.

I can also add the case when there are several features rendered on
top of each other.
Like this its very easy to access them.

Tnx for introducing this feature,

Cheers

On Sat, Oct 17, 2009 at 1:46 AM, Tim Schaub <tschaub at opengeo.org> wrote:
> Hey-
>
> We've wrestled for some time with browser events being blocked by
> svg/vml elements.  This originally meant that you could only use a
> select feature control with one layer at a time.  Andreas contributed a
> nice solution for selecting features on multiple layers for the last
> release.
>
> I got a number of things wrong in the select feature control (layer gets
> selectedFeatures array [ack] and control maintains it [ug], control has
> select method but layer doesn't, you can't easily deal with
> feature/mouse interaction without getting all the select feature control
> behavior, etc.)  I've long wanted to just have vector layers trigger
> feature related events.
>
> I put together what seems like a potential solution:
> http://dev.openlayers.org/sandbox/tschaub/select/examples/feature-events.html
>
> This introduces a "FeatureAgent" constructor.  If a map has the
> "featureEvents" property set to true, it gets a feature agent that
> manages feature related events (from click and mousemove).  This gives
> us featureclick, nofeatureclick, featureover, and featureout event on
> vector layers.
>
> As described in the example, these events are fired for visible vector
> layers.  The over/out events are similar to mouseover/mouseout for other
> doc elements - except that all features on all layers are essentially
> treated as if they share the same z-index.  This means you get over/out
> for every feature on every layer as you pass over and out.  Similarly,
> the featureclick event "drills" through all features on all layers.  You
> get the event once for each of multiple overlapping features on a single
> layer or the same on multiple layers.  Listeners are triggered for
> features in reverse draw order (top first).  Any listener can return
> false to stop remaining listeners from getting called.
>
> The whole thing relies on document.elementFromPoint [1].  This is a
> handy method that accounts for the currently computed style of all
> elements.  The trick is to set the display style to none for each target
> and call the method again, until all targets on all layers have been
> accounted for.  All this happens in a single browser event listener
> sequence.  Since style display is getting set and reset for all targets
> during the same sequence, we don't trigger a reflow with each display
> change.  It appears that there also isn't a reflow at the end of the
> listener sequence either (though I need some heavier examples to test with).
>
> I've played with the example on Firefox 3.5 and IE 6.  Haven't tested
> other browsers yet.
>
> There are a number of ways this could be implemented:
>
> a) As a control.  Traditional, but we don't need the dom element or
> events object - and this may be a more awkward way in practice.
>
> b) As it is now.  The agent is added/activated if a map property is set.
>  Could also be added/activated after map construction.
>
> c) Crammed into Map.js.  I'd rather not do this.
>
> d) Other (handler, plugin, more).
>
> Thanks for any feedback.
> Tim
>
> [1] https://developer.mozilla.org/en/DOM/document.elementFromPoint
>
> --
> Tim Schaub
> OpenGeo - http://opengeo.org
> Expert service straight from the developers.
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-- 
Ivan Grcic