lazy-load iframes only for current slide, unload when hidden

README.md

@@ -317,7 +317,7 @@ Reveal.configure({
 
 When working on presentation with a lot of media or iframe content it's important to load lazily. Lazy loading means that reveal.js will only load content for the few slides nearest to the current slide. The number of slides that are preloaded is determined by the `viewDistance` configuration option.
 
-To enable lazy loading all you need to do is change your "src" attributes to "data-src" as shown below. This is supported for image, video, audio and iframe elements.
+To enable lazy loading all you need to do is change your "src" attributes to "data-src" as shown below. This is supported for image, video, audio and iframe elements. Lazy loaded iframes will also unload when the containing slide is no longer visible.
 
 ```html
 <section>

js/reveal.js

@@ -2723,7 +2723,7 @@
 		slide.style.display = 'block';
 
 		// Media elements with data-src attributes
-		toArray( slide.querySelectorAll( 'img[data-src], video[data-src], audio[data-src], iframe[data-src]' ) ).forEach( function( element ) {
+		toArray( slide.querySelectorAll( 'img[data-src]', 'video[data-src]', 'audio[data-src]' ) ).forEach( function( element ) {
 			element.setAttribute( 'src', element.getAttribute( 'data-src' ) );
 			element.removeAttribute( 'data-src' );
 		} );
@@ -2909,19 +2909,24 @@
 				}
 			} );
 
-			// iframe embeds
+			// Lazy loading iframes
+			toArray( slide.querySelectorAll( 'iframe[data-src]' ) ).forEach( function( element ) {
+				element.setAttribute( 'src', element.getAttribute( 'data-src' ) );
+			} );
+
+			// Generic postMessage API for non-lazy loaded iframes
 			toArray( slide.querySelectorAll( 'iframe' ) ).forEach( function( el ) {
 				el.contentWindow.postMessage( 'slide:start', '*' );
 			});
 
-			// YouTube embeds
+			// YouTube postMessage API
 			toArray( slide.querySelectorAll( 'iframe[src*="youtube.com/embed/"]' ) ).forEach( function( el ) {
 				if( el.hasAttribute( 'data-autoplay' ) ) {
 					el.contentWindow.postMessage( '{"event":"command","func":"playVideo","args":""}', '*' );
 				}
 			});
 
-			// Vimeo embeds
+			// Vimeo postMessage API
 			toArray( slide.querySelectorAll( 'iframe[src*="player.vimeo.com/"]' ) ).forEach( function( el ) {
 				if( el.hasAttribute( 'data-autoplay' ) ) {
 					el.contentWindow.postMessage( '{"method":"play"}', '*' );
@@ -2945,19 +2950,24 @@
 				}
 			} );
 
-			// iframe embeds
+			// Lazy loading iframes
+			toArray( slide.querySelectorAll( 'iframe[data-src]' ) ).forEach( function( element ) {
+				element.removeAttribute( 'src' );
+			} );
+
+			// Generic postMessage API for non-lazy loaded iframes
 			toArray( slide.querySelectorAll( 'iframe' ) ).forEach( function( el ) {
 				el.contentWindow.postMessage( 'slide:stop', '*' );
 			});
 
-			// YouTube embeds
+			// YouTube postMessage API
 			toArray( slide.querySelectorAll( 'iframe[src*="youtube.com/embed/"]' ) ).forEach( function( el ) {
 				if( !el.hasAttribute( 'data-ignore' ) && typeof el.contentWindow.postMessage === 'function' ) {
 					el.contentWindow.postMessage( '{"event":"command","func":"pauseVideo","args":""}', '*' );
 				}
 			});
 
-			// Vimeo embeds
+			// Vimeo postMessage API
 			toArray( slide.querySelectorAll( 'iframe[src*="player.vimeo.com/"]' ) ).forEach( function( el ) {
 				if( !el.hasAttribute( 'data-ignore' ) && typeof el.contentWindow.postMessage === 'function' ) {
 					el.contentWindow.postMessage( '{"method":"pause"}', '*' );

test/test.html

@@ -44,6 +44,7 @@
 							<li class="fragment">4.2</li>
 							<li class="fragment">4.3</li>
 						</ul>
+						<iframe data-src="http://example.com"></iframe>
 					</section>
 
 					<section>

test/test.js

@@ -495,6 +495,15 @@ Reveal.addEventListener( 'ready', function() {
 		strictEqual( document.querySelectorAll( '.reveal section img[src]' ).length, 1, 'Image source has been set' );
 	});
 
+	test( 'iframe with data-src', function() {
+		Reveal.slide( 0, 0 );
+		strictEqual( document.querySelectorAll( '.reveal section iframe[src]' ).length, 0, 'Iframe source is not set' );
+		Reveal.slide( 2, 0 );
+		strictEqual( document.querySelectorAll( '.reveal section iframe[src]' ).length, 1, 'Iframe source is set' );
+		Reveal.slide( 2, 1 );
+		strictEqual( document.querySelectorAll( '.reveal section iframe[src]' ).length, 0, 'Iframe source is not set' );
+	});
+
 	test( 'background images', function() {
 		var imageSource1 = Reveal.getSlide( 0 ).getAttribute( 'data-background-image' );
 		var imageSource2 = Reveal.getSlide( 1, 0 ).getAttribute( 'data-background' );