[OpenLayers-Dev] Creating custom 3rd party Layers

[Christopher Schmidt]

On Thu, Jan 24, 2008 at 04:44:58PM +0100, Andrea Battisti wrote:
> Hi all,
> 
> I recently very much appreciated the OL's ability to connect to and 
> display different kind of layers on a web page.
> 
> I was wondering how difficult is to extend it to make use of third party 
> geospatial data and protocols.
> I took a quick look at some of the 3rd party implementations (i.e. the 
> WMS and Google classes ...) but what I miss is the general understanding 
> of the whole process of requesting tiles, and the "behind the scenes" 
> working of the Layer class and subclasses.

Note that WMS is not considered "Third Party", because it is not using a
seperate *api* to interact with another HTML element, but instead
talking directly to a server.

Depending on the type of implementation you are looking to work with,
this may or may not apply to your data as well.

If you're working with a service like WMS, where your service can take
arbitrary bounding boxes, you probably want to subclass from WMS, and
override just the 'getURL' function: the getURL function is expected to
be passed an OL.Bounds() object, and return a string which can be put in
<img src="" /> to display an image for that bounds.

If you're working with something that is tiled, you can do something
similar; http://trac.openlayers.org/wiki/UsingCustomTiles has an
explanation of that which may help. 

> I guess it is OpenLayers.Layer.EventPane as it is written in the doc, 
> but is there a simple example to start with?

Event pane is *only* useful if you have a third party API -- one that
requests the images for you.

> Basically for each tile I have to build a formatted request string 
> specifying the spatial bounds, projection and some options, submit the 
> request using http, reading and parsing the reply and extracting from it 
> an url pointing to the actual image data to be displayed.

Yep, you want to subclass WMS, and probably just override the getURL
method: you'll be passed a bounds, and yu'll have 'this' available with
all the properties of your laayer to take advantage of.

Regards,
-- 
Christopher Schmidt
MetaCarta