the postMessage API now works for getter methods

README.md

@@ -1065,6 +1065,8 @@ The framework has a built-in postMessage API that can be used when communicating
 <window>.postMessage( JSON.stringify({ method: 'slide', args: [ 2 ] }), '*' );
 ```
 
+#### postMessage Events
+
 When reveal.js runs inside of an iframe it can optionally bubble all of its events to the parent. Bubbled events are stringified JSON with three fields: namespace, eventName and state. Here's how you subscribe to them from the parent window:
 
 ```javascript
@@ -1076,7 +1078,25 @@ window.addEventListener( 'message', function( event ) {
 } );
 ```
 
-This cross-window messaging can be toggled on or off using configuration flags.
+#### postMessage Callbacks
+
+When you call any method via the postMessage API, reveal.js will dispatch a message with the return value. This is done so that you can call a getter method and see what the result is. Check out this example:
+
+```javascript
+<revealWindow>.postMessage( JSON.stringify({ method: 'getTotalSlides' }), '*' );
+
+window.addEventListener( 'message', function( event ) {
+	var data = JSON.parse( event.data );
+	// `data.method`` is the method that we invoked
+	if( data.namespace === 'reveal' && data.eventName === 'callback' && data.method === 'getTotalSlides' ) {
+		data.result // = the total number of slides
+	}
+} );
+```
+
+#### Turning postMessage on/off
+
+This cross-window messaging can be toggled on or off using configuration flags. These are the default values.
 
 ```javascript
 Reveal.initialize({

js/reveal.js

@@ -1276,7 +1276,11 @@
 
 					// Check if the requested method can be found
 					if( data.method && typeof Reveal[data.method] === 'function' ) {
-						Reveal[data.method].apply( Reveal, data.args );
+						var result = Reveal[data.method].apply( Reveal, data.args );
+
+						// Dispatch a postMessage event with the returned value from
+						// our method invocation for getter functions
+						dispatchPostMessage( 'callback', { method: data.method, result: result } );
 					}
 				}
 			}, false );
@@ -1981,8 +1985,25 @@
 
 		// If we're in an iframe, post each reveal.js event to the
 		// parent window. Used by the notes plugin
+		dispatchPostMessage( type );
+
+	}
+
+	/**
+	 * Dispatched a postMessage of the given type from our window.
+	 */
+	function dispatchPostMessage( type, data ) {
+
 		if( config.postMessageEvents && window.parent !== window.self ) {
-			window.parent.postMessage( JSON.stringify({ namespace: 'reveal', eventName: type, state: getState() }), '*' );
+			var message = {
+				namespace: 'reveal',
+				eventName: type,
+				state: getState()
+			};
+
+			extend( message, data );
+
+			window.parent.postMessage( JSON.stringify( message ), '*' );
 		}
 
 	}