[OpenLayers-Dev] feature events

[Roald de Wit]

Tim,

This is great stuff! Does this also pave the way for other layers (like 
WMS) to receive mouse events? I had a chat with someone on IRC a number 
weeks ago. He needed to have mouse-over and out events for tiles. This 
worked until a vector layer was placed 'above' his WMS layer and blocked 
all mouse events.

Regards, Roald

Regards, Roald

Tim Schaub 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
>
>