Fixed issue #101: swap callback

Based on enix's patch, cleaned it up and fixed a couple minor issues

Author: dfunckt

lib/sammy.js

@@ -1181,18 +1181,26 @@
     //      var app = $.sammy(function() {
     //
     //        // implements a 'fade out'/'fade in'
-    //        this.swap = function(content) {
-    //          this.$element().hide('slow').html(content).show('slow');
-    //        }
-    //
-    //        get('#/', function() {
-    //          this.partial('index.html.erb') // will fade out and in
-    //        });
+    //        this.swap = function(content, callback) {
+    //          var context = this;
+    //          context.$element().fadeOut('slow', function() {
+    //            context.$element().html(content);
+    //            context.$element().fadeIn('slow', function() {
+    //              if (callback) {
+    //                callback.apply();
+    //              }
+    //            });
+    //          });
+    //        };
     //
     //      });
     //
-    swap: function(content) {
-      return this.$element().html(content);
+    swap: function(content, callback) {
+      var $el = this.$element().html(content);
+      if (_isFunction(callback)) {
+        callback.apply();
+      }
+      return $el;
     },
 
     // a simple global cache for templates. Uses the same semantics as
@@ -1587,8 +1595,28 @@
 
     // `render()` the `location` with `data` and then `swap()` the
     // app's `$element` with the rendered content.
-    partial: function(location, data) {
-      return this.render(location, data).swap();
+    partial: function(location, data, callback) {
+      
+      // invoked as partial(location, data, callback)
+      if (location && _isFunction(callback)) {
+        return this.render(location, data, function() {
+          return callback;
+        }).swap();
+      }
+
+      // invoked as partial(location, callback)
+      if (!callback && _isFunction(data)) {
+        return this.render(location, null, function() {
+          return data;
+        }).swap();
+      }
+
+      // invoked as partial(location)
+      if (location && !callback && !data) {
+        return this.render(location, null, function() {
+          return function() {};
+        }).swap();
+      }
     },
 
     // defers the call of function to occur in order of the render queue.
@@ -1859,8 +1887,8 @@
 
     // `render()` the `location` with `data` and then `swap()` the
     // app's `$element` with the rendered content.
-    partial: function(location, data) {
-      return new Sammy.RenderContext(this).partial(location, data);
+    partial: function(location, data, callback) {
+      return new Sammy.RenderContext(this).partial(location, data, callback);
     },
 
     // create a new `Sammy.RenderContext` calling `send()` with an arbitrary
@@ -1925,8 +1953,8 @@
     },
 
     // A shortcut to app's `swap()`
-    swap: function(contents) {
-      return this.app.swap(contents);
+    swap: function(contents, callback) {
+      return this.app.swap(contents, callback);
     },
 
     // Raises a possible `notFound()` error for the current path.