[OpenLayers-Users] polymorphic in ol's layer

[maven apache]

Hi:

Through the document in the ol's website,I know that there are so many
layers in ol. And Some of them have the inherit chain.

For example, the

layer.Layer->layer.HttpRequest->layer.Grid->layer.WMS


layer.Grid->layer.TileCache
              ->layer.WMS
              ->layer.XYZ

However, when say inherit,I often think the "polymorphic",But I do not find
it in the source codes.

In my opinion,the class who is inherited should own some not-implemented
method which will be override by its sub class.


Suppose,the layer.Layer have a method "draw(xxxx)" which is not
implementd,then all its sub-class will override this method,when the map
need to re-draw,it call the 'draw()' of each layer.

But in ol's source codes,I did not find anything like this.

So,I wonder what is the inheritance structure in ol?
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