Added extend method to Interface

lib/interface.js

@@ -58,6 +58,7 @@ function extend()
 
     util.propCopy( props, prototype );
 
+    attach_extend( new_interface );
     new_interface.prototype   = prototype;
     new_interface.constructor = new_interface;
 
@@ -90,3 +91,29 @@ function prop_check( props )
     return props;
 }
 
+
+/**
+ * Attaches extend method to the given function (interface)
+ *
+ * @param  {Function}  func  function (interface) to attach method to
+ *
+ * @return  {undefined}
+ */
+function attach_extend( func )
+{
+    /**
+     * Shorthand for extending interfaces
+     *
+     * This method can be invoked on the object, rather than having to call
+     * Interface.extend( this ).
+     *
+     * @param  {Object}  props  properties to add to extended interface
+     *
+     * @return  {Object}  extended interface
+     */
+    util.defineSecureProp( func, 'extend', function( props )
+    {
+        return extend( this, props );
+    });
+}
+

test/test-interface-extend.js

@@ -91,3 +91,26 @@ assert.ok(
     "Interfaces can be extended with additional abstract methods"
 );
 
+
+assert.ok(
+    ( BaseType.extend instanceof Function ),
+    "Interface contains extend method"
+);
+
+
+var SubType2 = BaseType.extend(
+{
+    second: abstractMethod(),
+});
+
+assert.ok(
+    ( new SubType2 instanceof BaseType ),
+    "Interface extend method can extend interfaces"
+);
+
+assert.ok(
+    ( SubType2.prototype.second instanceof Function ),
+    "Interfaces can be extended with additional abstract methods using " +
+        "shorthand extend method"
+);
+