Add support for transformation functions without using proj4js, and make

the SphericalMercator mixin register two transformations to/from EPSG:900913,
EPSG:4326. Thanks to Tim for the feedback and review. (Closes #1210)

This allows us to transform points to/from SphericalMercator 
without proj4js support -- and if other projects need similar functionality, 
they can write their own custom transformation functions rather than 
modifying proj4js to support some custom projection.


git-svn-id: http://svn.openlayers.org/trunk/openlayers@5410 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Projection.js

@@ -6,7 +6,7 @@
  * @requires OpenLayers/Util.js
  * 
  * Class: OpenLayers.Projection
- * Class for coordinate transformations between coordinate systems.
+ * Class for coordinate transforms between coordinate systems.
  *     Depends on the proj4js library. If proj4js is not available, 
  *     then this is just an empty stub.
  */
@@ -101,22 +101,69 @@ OpenLayers.Projection = OpenLayers.Class({
     CLASS_NAME: "OpenLayers.Projection" 
 });     
 
+/**
+ * Property: transforms
+ * Transforms is an object, with from properties, each of which may
+ * have a to property. This allows you to define projections without 
+ * requiring support for proj4js to be included.
+ *
+ * This object has keys which correspond to a 'source' projection object.  The
+ * keys should be strings, corresponding to the projection.getCode() value.
+ * Each source projection object should have a set of destination projection
+ * keys included in the object. 
+ * 
+ * Each value in the destination object should be a transformation function,
+ * where the function is expected to be passed an object with a .x and a .y
+ * property.  The function should return the object, with the .x and .y
+ * transformed according to the transformation function.
+ *
+ * Note - Properties on this object should not be set directly.  To add a
+ *     transform method to this object, use the <addTransform> method.  For an
+ *     example of usage, see the OpenLayers.Layer.SphericalMercator file.
+ */
+OpenLayers.Projection.transforms = {};
+
+/**
+ * APIMethod: addTransform
+ * Set a custom transform method between two projections.  Use this method in
+ *     cases where the proj4js lib is not available or where custom projections
+ *     need to be handled.
+ *
+ * Parameters:
+ * from - {String} The code for the source projection
+ * to - {String} the code for the destination projection
+ * method - {Function} A function that takes a point as an argument and
+ *     transforms that point from the source to the destination projection
+ *     in place.  The original point should be modified.
+ */
+OpenLayers.Projection.addTransform = function(from, to, method) {
+    if(!OpenLayers.Projection.transforms[from]) {
+        OpenLayers.Projection.transforms[from] = {};
+    }
+    OpenLayers.Projection.transforms[from][to] = method;
+};
+
 /**
  * APIMethod: transform
- * Read data from a string, and return an object whose type depends on the
- * subclass. 
+ * Transform a point coordinate from one projection to another.  Note that
+ *     the input point is transformed in place.
  * 
  * Parameters:
- * point - {object} input horizontal coodinate
- * sourceProj - {OpenLayers.Projection} source map coordinate system
- * destProj - {OpenLayers.Projection} destination map coordinate system
+ * point - {{OpenLayers.Geometry.Point> | Object} An object with x and y
+ *     properties representing coordinates in those dimensions.
+ * sourceProj - {OpenLayers.Projection} Source map coordinate system
+ * destProj - {OpenLayers.Projection} Destination map coordinate system
  *
  * Returns:
- * point - {object} trasnformed coordinate
+ * point - {object} A transformed coordinate.  The original point is modified.
  */
 OpenLayers.Projection.transform = function(point, source, dest) {
     if (source.proj && dest.proj) {
         point = Proj4js.transform(source.proj, dest.proj, point);
+    } else if (source && dest && 
+               OpenLayers.Projection.transforms[source.getCode()] && 
+               OpenLayers.Projection.transforms[source.getCode()][dest.getCode()]) {
+        OpenLayers.Projection.transforms[source.getCode()][dest.getCode()](point); 
     }
     return point;
 };