[OpenLayers-Dev] callbacks versus events
tschaub at opengeo.org
Thu Dec 11 12:25:21 EST 2008
bartvde at osgis.nl wrote:
> Hi list,
> can anybody explain to me when to use a callback and when to use an event.
> If I look at the OL codebase, it seems Handlers always use callbacks, and
> controls use events?
When there is potential for multiple listeners, trigger an event.
When you want to ensure that the the execution sequence branches exactly
once and returns to your control, use a callback.
The idea with handlers is that they are supposed to be composed together
to create specific controls. When you construct a DragPan control, you
know what the behavior with respect to dragging will be. The drag
handler in this case cannot be used to do anything except what the
Handlers are somewhat limited in this way. As enforced by the signature
of the constructor, they are to be used with a single control.
It would be interesting to talk about how things could be designed
differently for 3.0 - perhaps having the map trigger things like
dragstart, drag, dragend, singleclick, etc. Layers, controls, and other
things could register listeners for those events and determine behavior
based on what listeners do and what they return.
> Best regards,
> Dev mailing list
> Dev at openlayers.org
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