图片延迟加载 - 前端开发技能必备系列

(function( $ ){
$.fn.imglazyload = function( options ){
var o = $.extend({
attr : 'lazy-src',
container : window,
event : 'scroll',
fadeIn : false,
threshold : 0,
vertical : true
}, options ),

event = o.event,
vertical = o.vertical,
container = $( o.container ),
threshold = o.threshold,
// 将jQuery对象转换成DOM数组便于操作
elems = $.makeArray( $(this) ),
dataName = 'imglazyload_offset',
OFFSET = vertical ? 'top' : 'left',
SCROLL = vertical ? 'scrollTop' : 'scrollLeft',
winSize = vertical ? container.height() : container.width(),
scrollCoord = container[ SCROLL ](),
docSize = winSize + scrollCoord;

// 延迟加载的触发器
var trigger = {

init : function( coord ){
return coord >= scrollCoord &&
coord <= ( docSize + threshold );
},

scroll : function( coord ){
var scrollCoord = container[ SCROLL ]();
return coord >= scrollCoord &&
coord <= ( winSize + scrollCoord + threshold );
},

resize : function( coord ){
var scrollCoord = container[ SCROLL ](),
winSize = vertical ?
container.height() :
container.width();
return coord >= scrollCoord &&
coord <= ( winSize + scrollCoord + threshold );
}
};

var loader = function( triggerElem, event ){
var i = 0,
isCustom = false,
isTrigger, coord, elem, $elem, lazySrc;

// 自定义事件只要触发即可，无需再判断
if( event ){
if( event !== 'scroll' && event !== 'resize' ){
isCustom = true;
}
}
else{
event = 'init';
}

for( ; i < elems.length; i++ ){
isTrigger = false;
elem = elems[i];
$elem = $( elem );
lazySrc = $elem.attr( o.attr );

if( !lazySrc || elem.src === lazySrc ){
continue;
}
// 先从缓存获取offset值，缓存中没有才获取计算值,
// 将计算值缓存，避免重复获取引起的reflow
coord = $elem.data( dataName );

if( coord === undefined ){
coord = $elem.offset()[ OFFSET ];
$elem.data( dataName, coord );
}

isTrigger = isCustom || trigger[ event ]( coord );

if( isTrigger ){
// 加载图片
elem.src = lazySrc;
if( o.fadeIn ){
$elem.hide().fadeIn();
}
// 移除缓存
$elem.removeData( dataName );
// 从DOM数组中移除该DOM
elems.splice( i--, 1 );
}
}

// 所有的图片加载完后卸载触发事件
if( !elems.length ){
if( triggerElem ){
triggerElem.unbind( event, fire );
}
else{
container.unbind( o.event, fire );
}
$( window ).unbind( 'resize', fire );
elems = null;
}

};

var fire = function( e ){
loader( $(this), e.type );
};

// 绑定事件
container = event === 'scroll' ? container : $( this );
container.bind( event, fire );
$( window ).bind( 'resize', fire );

// 初始化
loader();

return this;
};

})( jQuery );