[Users] Layer Tree Tutorial questions

[Gary White]

Can someone please explain the "Sub-Layers" section of this tutorial
(http://www.geoext.org/tutorials/layertree-tutorial.html#sub-layers)?
Specifically I'm wondering how the param property in the loader works.  I
understand that the loader is a LayerParamLoader but the underlying
mechanics are unclear.  I was originally under the assumption that this was
used as a read-only mechanism to grab data from an already
configured/retrieved layer as in using "LAYERS" in order to retrieve the
following layers from groupLayer:

 

        layers: [

            "topp:tasmania_state_boundaries",

            "topp:tasmania_water_bodies",

            "topp:tasmania_cities",

            "topp:tasmania_roads"

        ],

 

After reading the note about using the "CQL_FILTER" param I became confused.
Here are some questions that can hopefully clear things up for me (and
possibly others):

 

1 - Is this used to create a new "CQL_FILTER" and/or modify an existing one
or just to read one that already exists?  

2 - What would be the use case for reading a "CQL_FILTER" that already
exists?  

3 - If creating/modifying a "CQL_FILTER", does this reinitiate a WMS etc.
request in order to fulfill the new filter?  

4 - If this is not used for just read-only purposes, can you also
create/modify a layer list using "LAYERS"?

5 - What is the lifecycle of the LayerParamLoader and the underlying (in
this case) OpenLayers.Layer.WMS?   i.e. Does the LayerParamLoader trigger
the WMS request from the server or is that request initiated separately?

 

Is there a working example showing the use of "CQL_FILTER" in this manner?
(I couldn't find one)

 

BTW, I was even more confused when playing around with a value other than
"LAYERS" for the param property.  For instance, when I changed it as
follows:

 

        loader: {

            param: "FORMAT"

        }

 

the group layer node was populated with a single node displaying the text
"image/gif" but the map displayed all 4 layers when I clicked the checkbox.
Can anyone explain this behavior?

 

Thanks!

Gary

 

 

 

 

 

 

 

 

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