Customizer Theme Switcher: Lazy load theme screenshots.

props westonruter, ocean90.
fixes #31793.
Built from https://develop.svn.wordpress.org/trunk@32088


git-svn-id: http://core.svn.wordpress.org/trunk@32067 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-controls.js

@@ -552,6 +552,9 @@
 		currentTheme: '',
 		overlay: '',
 		template: '',
+		screenshotQueue: null,
+		$window: $( window ),
+		$customizeSidebar: $( '.wp-full-overlay-sidebar-content:first' ),
 
 		/**
 		 * @since 4.2.0
@@ -582,6 +585,8 @@
 					section.closeDetails();
 				}
 			});
+
+			_.bindAll( this, 'renderScreenshots' );
 		},
 
 		/**
@@ -650,6 +655,7 @@
 				section.closeDetails();
 			});
 
+			var renderScreenshots = _.throttle( _.bind( section.renderScreenshots, this ), 100 );
 			section.container.on( 'input', '#themes-filter', function( event ) {
 				var count,
 					term = event.currentTarget.value.toLowerCase().trim().replace( '-', ' ' ),
@@ -659,10 +665,23 @@
 					control.filter( term );
 				});
 
+				renderScreenshots();
+
 				// Update theme count.
 				count = section.container.find( 'li.customize-control:visible' ).length;
 				section.container.find( '.theme-count' ).text( count );
 			});
+
+			// Pre-load the first 3 theme screenshots.
+			api.bind( 'ready', function () {
+				_.each( section.controls().slice( 0, 3 ), function ( control ) {
+					var img, src = control.params.theme.screenshot[0];
+					if ( src ) {
+						img = new Image();
+						img.src = src;
+					}
+				});
+			});
 		},
 
 		/**
@@ -706,9 +725,7 @@
 					}
 				});
 				api.panel.each( function ( otherPanel ) {
-					if ( panel !== otherPanel ) {
 					otherPanel.collapse( { duration: 0 } );
-					}
 				});
 
 				content.show( 0, function() {
@@ -718,6 +735,8 @@
 					section.addClass( 'current-panel' );
 					overlay.addClass( 'in-themes-panel' );
 					container.scrollTop( 0 );
+					_.delay( panel.renderScreenshots, 10 ); // Wait for the controls
+					panel.$customizeSidebar.on( 'scroll.customize-themes-section', _.throttle( panel.renderScreenshots, 300 ) );
 					if ( args.completeCallback ) {
 						args.completeCallback();
 					}
@@ -729,6 +748,7 @@
 				siblings.removeClass( 'open' );
 				section.removeClass( 'current-panel' );
 				overlay.removeClass( 'in-themes-panel' );
+				panel.$customizeSidebar.off( 'scroll.customize-themes-section' );
 				content.delay( 180 ).hide( 0, function() {
 					content.css( 'margin-top', 'inherit' ); // Reset
 					if ( args.completeCallback ) {
@@ -742,6 +762,54 @@
 			}
 		},
 
+		/**
+		 * Render control's screenshot if the control comes into view.
+		 *
+		 * @since 4.2.0
+		 */
+		renderScreenshots: function( ) {
+			var section = this;
+
+			// Fill queue initially.
+			if ( section.screenshotQueue === null ) {
+				section.screenshotQueue = section.controls();
+			}
+
+			// Are all screenshots rendered?
+			if ( ! section.screenshotQueue.length ) {
+				return;
+			}
+
+			section.screenshotQueue = _.filter( section.screenshotQueue, function( control ) {
+				var $imageWrapper = control.container.find( '.theme-screenshot' ),
+					$image = $imageWrapper.find( 'img' );
+
+				if ( ! $image.length ) {
+					return false;
+				}
+
+				if ( $image.is( ':hidden' ) ) {
+					return true;
+				}
+
+				// Based on unveil.js.
+				var wt = section.$window.scrollTop(),
+					wb = wt + section.$window.height(),
+					et = $image.offset().top,
+					ih = $imageWrapper.height(),
+					eb = et + ih,
+					threshold = ih * 3,
+					inView = eb >= wt - threshold && et <= wb + threshold;
+
+				if ( inView ) {
+					control.container.trigger( 'render-screenshot' );
+				}
+
+				// If the image is in view return false so it's cleared from the queue.
+				return ! inView;
+			} );
+		},
+
 		/**
 		 * Advance the modal to the next theme.
 		 *
@@ -1916,6 +1984,15 @@
 
 				api.section( control.section() ).showDetails( control.params.theme );
 			});
+
+			control.container.on( 'render-screenshot', function() {
+				var $screenshot = $( this ).find( 'img' ),
+					source = $screenshot.data( 'src' );
+
+				if ( source ) {
+					$screenshot.attr( 'src', source );
+				}
+			});
 		},
 
 		/**

wp-includes/class-wp-customize-control.php

@@ -1248,7 +1248,7 @@ class WP_Customize_Theme_Control extends WP_Customize_Control {
 		<div class="theme" tabindex="0" data-preview-url="<?php echo esc_attr( $preview_url ); ?>" aria-describedby="{{ data.theme.id }}-action {{ data.theme.id }}-name">
 			<# if ( data.theme.screenshot[0] ) { #>
 				<div class="theme-screenshot">
-					<img src="{{ data.theme.screenshot[0] }}" alt="" />
+					<img data-src="{{ data.theme.screenshot[0] }}" alt="" />
 				</div>
 			<# } else { #>
 				<div class="theme-screenshot blank"></div>

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-beta4-32087';
+$wp_version = '4.2-beta4-32088';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.