lazyload.js

/*!
 * An jQuery | zepto plugin for lazy loading images.
 * author -> jieyou
 * see https://github.com/jieyou/lazyload
 * use some tuupola's code https://github.com/tuupola/jquery_lazyload (BSD)
 * use component's throttle https://github.com/component/throttle (MIT)
 */
;(function(factory){
    if(typeof define === 'function' && define.amd){ // AMD
        // you may need to change `define([------>'jquery'<------], factory)` 
        // if you use zepto, change it rely name, such as `define(['zepto'], factory)`
        define(['jquery'], factory)
        // define(['zepto'], factory)
    }else{ // Global
        factory(window.jQuery || window.Zepto)
    }
})(function($,undefined){
    var w = window,
        $window = $(w),
        defaultOptions = {
            threshold                   : 0,
            failure_limit               : 0,
            event                       : 'scroll',
            effect                      : 'show',
            effect_params               : null,
            container                   : w,
            data_attribute              : 'original',
            data_srcset_attribute       : 'original-srcset',
            skip_invisible              : true,
            appear                      : emptyFn,
            load                        : emptyFn,
            vertical_only               : false,
            check_appear_throttle_time  : 300,
            url_rewriter_fn             : emptyFn,
            no_fake_img_loader          : false,
            placeholder_data_img        : 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC',
            // for IE6\7 that does not support data image
            placeholder_real_img        : 'http://ditu.baidu.cn/yyfm/lazyload/0.0.1/img/placeholder.png'
            // todo : 将某些属性用global来配置，而不是每次在$(selector).lazyload({})内配置
        },
        type // function

    function emptyFn(){}

    type = (function(){
        var object_prototype_toString = Object.prototype.toString
        return function(obj){
            // todo: compare the speeds of replace string twice or replace a regExp
            return object_prototype_toString.call(obj).replace('[object ','').replace(']','')
        }
    })()

    function belowthefold($element, options){
        var fold
        if(options._$container == $window){
            fold = ('innerHeight' in w ? w.innerHeight : $window.height()) + $window.scrollTop()
        }else{
            fold = options._$container.offset().top + options._$container.height()
        }
        return fold <= $element.offset().top - options.threshold
    }

    function rightoffold($element, options){
        var fold
        if(options._$container == $window){
            // Zepto do not support `$window.scrollLeft()` yet.
            fold = $window.width() + ($.fn.scrollLeft?$window.scrollLeft():w.pageXOffset)
        }else{
            fold = options._$container.offset().left + options._$container.width()
        }
        return fold <= $element.offset().left - options.threshold
    }

    function abovethetop($element, options){
        var fold
        if(options._$container == $window){
            fold = $window.scrollTop()
        }else{
            fold = options._$container.offset().top
        }
        // console.log('abovethetop fold '+ fold)
        // console.log('abovethetop $element.height() '+ $element.height())
        return fold >= $element.offset().top + options.threshold  + $element.height()
    }

    function leftofbegin($element, options){
        var fold
        if(options._$container == $window){
            // Zepto do not support `$window.scrollLeft()` yet.
            fold = $.fn.scrollLeft?$window.scrollLeft():w.pageXOffset
        }else{
            fold = options._$container.offset().left
        }
        return fold >= $element.offset().left + options.threshold + $element.width()
    }

    function checkAppear($elements, options){
        var counter = 0
        $elements.each(function(i,e){
            var $element = $elements.eq(i)
            if(($element.width() <= 0 && $element.height() <= 0) || $element.css('display') === 'none'){
                return
            }
            function appear(){
                $element.trigger('_lazyload_appear')
                // if we found an image we'll load, reset the counter 
                counter = 0
            }
            // If vertical_only is set to true, only check the vertical to decide appear or not
            // In most situations, page can only scroll vertically, set vertical_only to true will improve performance
            if(options.vertical_only){
                if(abovethetop($element, options)){
                    // Nothing. 
                }else if(!belowthefold($element, options)){
                    appear()
                }else{
                    if(++counter > options.failure_limit){
                        return false
                    }
                }
            }else{
                if(abovethetop($element, options) || leftofbegin($element, options)){
                    // Nothing. 
                }else if(!belowthefold($element, options) && !rightoffold($element, options)){
                    appear()
                }else{
                    if(++counter > options.failure_limit){
                        return false
                    }
                }
            }
        })
    }

    // Remove image from array so it is not looped next time. 
    function getUnloadElements($elements){
        return $elements.filter(function(i){
            return !$elements.eq(i).data('_lazyload_loadStarted')
        })
    }

    // throttle : https://github.com/component/throttle , MIT License
    function throttle (func, wait) {
        var ctx, args, rtn, timeoutID // caching
        var last = 0

        return function throttled () {
            ctx = this
            args = arguments
            var delta = new Date() - last
            if (!timeoutID)
                if (delta >= wait) call()
                else timeoutID = setTimeout(call, wait - delta)
            return rtn
        }

        function call () {
            timeoutID = 0
            last = +new Date()
            rtn = func.apply(ctx, args)
            ctx = null
            args = null
        }
    }

    if(!$.fn.hasOwnProperty('lazyload')){

        $.fn.lazyload = function(options){
            var $elements = this,
                isScrollEvent,
                isScrollTypeEvent,
                throttleCheckAppear

            if(!$.isPlainObject(options)){
                options = {}
            }

            $.each(defaultOptions,function(k,v){
                var typeK = type(options[k])
                if($.inArray(k,['threshold','failure_limit','check_appear_throttle_time']) != -1){ // these params can be a string
                    if(typeK == 'String'){
                        options[k] = parseInt(options[k],10)
                    }else if(typeK != 'Number'){
                        options[k] = v
                    }
                }else if(k == 'container'){ // options.container can be a seletor string \ dom \ jQuery object
                    if(options.hasOwnProperty(k)){   
                        if(options[k] == w || options[k] == document){
                            options._$container = $window
                        }else{
                            options._$container = $(options[k])
                        }
                    }else{
                        options._$container = $window
                    }
                    delete options.container
                }else if(defaultOptions.hasOwnProperty(k) && (!options.hasOwnProperty(k) || (typeK != type(defaultOptions[k])))){
                    options[k] = v
                }
            })

            isScrollEvent = options.event == 'scroll'
            throttleCheckAppear = options.check_appear_throttle_time == 0?
                checkAppear
                :throttle(checkAppear,options.check_appear_throttle_time)

            // isScrollTypeEvent cantains custom scrollEvent . Such as 'scrollstart' & 'scrollstop'
            // https://github.com/search?utf8=%E2%9C%93&q=scrollstart
            isScrollTypeEvent = isScrollEvent || options.event == 'scrollstart' || options.event == 'scrollstop'

            $elements.each(function(i,e){
                var element = this,
                    $element = $elements.eq(i),
                    placeholderSrc = $element.attr('src'),
                    originalSrcInAttr = $element.attr('data-'+options.data_attribute), // `data-original` attribute value
                    originalSrc = options.url_rewriter_fn == emptyFn?
                        originalSrcInAttr:
                        options.url_rewriter_fn.call(element,$element,originalSrcInAttr),
                    originalSrcset = $element.attr('data-'+options.data_srcset_attribute),
                    isImg = $element.is('img')

                if($element.data('_lazyload_loadStarted') || placeholderSrc == originalSrc){
                    $element.data('_lazyload_loadStarted',true)
                    $elements = getUnloadElements($elements)
                    return
                }

                $element.data('_lazyload_loadStarted',false)

                // If element is an img and no src attribute given, use placeholder. 
                if(isImg && !placeholderSrc){
                    // For browsers that do not support data image.
                    $element.one('error',function(){ // `on` -> `one` : IE6 triggered twice error event sometimes
                        $element.attr('src',options.placeholder_real_img)
                    }).attr('src',options.placeholder_data_img)
                }
                
                // When appear is triggered load original image. 
                $element.one('_lazyload_appear',function(){
                    var effectParamsIsArray = $.isArray(options.effect_params),
                        effectIsNotImmediacyShow
                    function loadFunc(){
                        // In most situations, the effect is immediacy show, at this time there is no need to hide element first
                        // Hide this element may cause css reflow, call it as less as possible
                        if(effectIsNotImmediacyShow){
                            // todo: opacity:0 for fadeIn effect
                            $element.hide()
                        }
                        if(isImg){
                            // attr srcset first
                            if(originalSrcset){
                                $element.attr('srcset', originalSrcset)
                            }
                            if(originalSrc){
                                $element.attr('src', originalSrc)
                            }
                        }else{
                            $element.css('background-image','url("' + originalSrc + '")')
                        }
                        if(effectIsNotImmediacyShow){
                            $element[options.effect].apply($element,effectParamsIsArray?options.effect_params:[])
                        }
                        $elements = getUnloadElements($elements)
                    }
                    if(!$element.data('_lazyload_loadStarted')){
                        effectIsNotImmediacyShow = (options.effect != 'show' && $.fn[options.effect] && (!options.effect_params || (effectParamsIsArray && options.effect_params.length == 0)))
                        if(options.appear != emptyFn){
                            options.appear.call(element, $element, $elements.length, options)
                        }
                        $element.data('_lazyload_loadStarted',true)
                        if(options.no_fake_img_loader || originalSrcset){
                            if(options.load != emptyFn){
                                $element.one('load',function(){
                                    options.load.call(element, $element, $elements.length, options)
                                })
                            }
                            loadFunc()
                        }else{
                            $('<img />').one('load', function(){ // `on` -> `one` : IE6 triggered twice load event sometimes
                                loadFunc()
                                if(options.load != emptyFn){
                                    options.load.call(element, $element, $elements.length, options)
                                }
                            }).attr('src',originalSrc)
                        }
                    }
                })

                // When wanted event is triggered load original image 
                // by triggering appear.                              
                if (!isScrollTypeEvent){
                    $element.on(options.event, function(){
                        if (!$element.data('_lazyload_loadStarted')){
                            $element.trigger('_lazyload_appear')
                        }
                    })
                }
            })

            // Fire one scroll event per scroll. Not one scroll event per image. 
            if(isScrollTypeEvent){
                options._$container.on(options.event, function(){
                    throttleCheckAppear($elements, options)
                })
            }

            // Check if something appears when window is resized. 
            // Force initial check if images should appear when window is onload. 
            $window.on('resize load', function(){
                throttleCheckAppear($elements, options)
            })

            // Force initial check if images should appear. 
            $(function(){
                throttleCheckAppear($elements, options)
            })
            
            return this
        }
    }
})