Patch that adds support for arbitrary OpenLayers constructors to olwidget

olwidget.patch

diff --git a/doc-src/olwidget.js.rst b/doc-src/olwidget.js.rst
index 4a5ff97..dd8504b 100644
--- a/doc-src/olwidget.js.rst
+++ b/doc-src/olwidget.js.rst
@@ -306,7 +306,30 @@ General map display
     ``'ve.shaded'``, ``'ve.aerial'``, ``'ve.hybrid'``, ``'wms.map'``,
     ``'wms.nasa'``, ``'yahoo.map'``, and ``'cloudmade.<num>'`` (where ``<num>``
     is the number for a cloudmade style).  A blank map can be obtained using
-    ``'wms.blank'``.  Additional providers or options can be manually added
+    ``'wms.blank'``.
+
+    Other providers can be added by choosing ``custom.<name>`` where
+    ``<name>`` corresponds to a property in a ``customBaseLayers``
+    object that you would need to create. It must have a ``class``
+    property corresponding to an OpenLayers layer subclass, and an
+    ``args`` property that is a list of arguments to pass to that
+    constructor.  For example::
+
+    var customBaseLayers = {'opengeo_osm':  # to use this, your olwidget layers would include ['custom.opengeo_osm']
+        {"class": "WMS",  # The OpenLayers.Layer subclass to use.
+         "args": [  # These are passed as arguments to the constructor.
+            "OpenStreetMap (OpenGeo)",
+            "http://maps.opengeo.org/geowebcache/service/wms",
+            {"layers": "openstreetmap",
+             "format": "image/png",
+             "bgcolor": "#A1BDC4",
+             },
+            {"wrapDateLine": True
+             },
+            ],
+         }
+
+    Additional options and layers can also be manually added
     using the normal OpenLayers apis (see `this provider example
     <examples/other_providers.html>`_).
 
diff --git a/js/olwidget.js b/js/olwidget.js
index d530525..bd221a2 100644
--- a/js/olwidget.js
+++ b/js/olwidget.js
@@ -144,6 +144,32 @@ var olwidget = {
             });
         }
     },
+    custom: {
+        // Support for arbitrary OpenLayers base layers.
+        // To use this, ensure that customBaseLayers[type] exists, and
+        // has both a 'class' string (name of the OL constructor) and
+        // an 'args' array to pass to that constructor.
+        map: function(type) {
+            var classname = customBaseLayers[type]['class'];
+            var class_ = OpenLayers.Layer[classname];
+            var args = customBaseLayers[type].args;
+            // Can't use .apply() directly with an OL constructor,
+            // because we don't have a suitable `this` argument.
+            // Instead make a constructor function with a .prototype
+            // property the same as our class.prototype.
+            // The `new` keyword creates an instance from that prototype
+            // which will be used as `this` in the constructor call.
+            // `new` is also the only way to get the new instance to have
+            // the correct actual prototype, which is *not* the same as
+            // class.prototype. Thanks Tim Schaub for explaining this bit of
+            // javascript OOP to me.
+            var constructor = function() {
+                class_.prototype.initialize.apply(this, args);
+            };
+            constructor.prototype = class_.prototype;
+            return new constructor();
+        }
+    },
     /*
      * Utilities
      */