add design assertions to the doc/ol3.txt doc

doc/ol3.txt

@@ -205,3 +205,42 @@ Question: who determines "best" resolution?  (static function?)
 
 
 Todo: if tile layer extent stored in TileLayer rather than TileGrid then extent will occasionally need to be passed to TileGrid functions for cropping
+
+DESIGN ASSERTIONS
+=================
+
+- A map has a renderer (the map renderer).
+- A map has a camera.
+- Multiple maps can share the same camera.
+- A map has a layer list.
+
+- A layer view stores view-related states for a layer.
+- View-related states include visibility, opacity, saturation, hue, etc.
+- A layer view has a layer.
+- Multiple layer views can share the same layer.
+- In other words a layer can be viewed in different manners.
+
+- The map renderer responds to events.
+- The map renderer receives events from the camera.
+- The map renderer creates layer renderers.
+
+- A layer can have multiple projections (the supported projections).
+- A layer advertizes the projections it supports.
+- A layer returns no data if asked data for an unsupported projection.
+
+- A control may listen to map events.
+- A control may listen to camera events.
+- A map navigation control acts on the camera.
+
+- Types can be described in MVC terms.
+- Models don't know what rendering means.
+- Maps are models.
+- Layers are models.
+- Layer views are models (sorry!).
+- Cameras are models.
+- Layer lists are collections.
+- Renderers are views.
+- Controls are views or controllers or both.
+- An attribution control is a view.
+- A map navigation control is a controller.
+- A zoom slider control is both a view and a controller.