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前端实现图片（img）预加载

2017-06-02 15:15 274 查看

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
</head>
<style type="text/css">
.box{width: 1000px;height: 7000px;position: relative;}
.igm1,.igm2,.igm3{width: 1000px;height: 1000px;margin-bottom: 500px;}
</style>
<script src="http://libs.baidu.com/jquery/1.11.1/jquery.min.js"></script>
<script type="text/javascript" src="jquery.lazyload.js" ></script>
<body>
<div class="box">
<!--data-original是要加载的图片 src中的是加载动画-->
<img class="igm1" data-original='http://img.8kmm.com/2014/2/201402150857153283.jpg' src="ori.png"/>
<img class="igm2" data-original='http://img.8kmm.com/2014/2/201402150857161214.jpg' src="ori.png"/>
<img class="igm2" data-original='http://img.8kmm.com/2014/2/201402150857172803.jpg' src="ori.png"/>
</div>
<script type="text/javascript">
$("img").lazyload({
effect :"fadeIn",
threshold :200
});
</script>
</body>
</html>

新建jquery.lazyload.js文件导入下面代码即可，当然你也可以去百度上搜

(function($, window, document, undefined) {
var $window = $(window);

$.fn.lazyload = function(options) {
var elements = this;
var $container;
var settings = {
threshold : 0,
failure_limit : 0,
event : "scroll",
effect : "show",
container : window,
data_attribute : "original",
skip_invisible : false,
appear : null,
load : null,
placeholder : "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC"
};

function update() {
var counter = 0;

elements.each(function() {
var $this = $(this);
if (settings.skip_invisible && !$this.is(":visible")) {
return;
}
if ($.abovethetop(this, settings) ||
$.leftofbegin(this, settings)) {
/* Nothing. */
} else if (!$.belowthefold(this, settings) &&
!$.rightoffold(this, settings)) {
$this.trigger("appear");
/* if we found an image we'll load, reset the counter */
counter = 0;
} else {
if (++counter > settings.failure_limit) {
return false;
}
}
});

}

if(options) {
if (undefined !== options.failurelimit) {
options.failure_limit = options.failurelimit;
delete options.failurelimit;
}
if (undefined !== options.effectspeed) {
options.effect_speed = options.effectspeed;
delete options.effectspeed;
}

$.extend(settings, options);
}
$container = (settings.container === undefined ||
settings.container === window) ? $window : $(settings.container);
if (0 === settings.event.indexOf("scroll")) {
$container.on(settings.event, function() {
return update();
});
}

this.each(function() {
var self = this;
var $self = $(self);

self.loaded = false;
if ($self.attr("src") === undefined || $self.attr("src") === false) {
if ($self.is("img")) {
$self.attr("src", settings.placeholder);
}
}
$self.one("appear", function() {
if (!this.loaded) {
if (settings.appear) {
var elements_left = elements.length;
settings.appear.call(self, elements_left, settings);
}
$("<img />")
.one("load", function() {
var original = $self.attr("data-" + settings.data_attribute);
$self.hide();
if ($self.is("img")) {
$self.attr("src", original);
} else {
$self.css("background-image", "url('" + original + "')");
}
$self[settings.effect](settings.effect_speed);

self.loaded = true;
var temp = $.grep(elements, function(element) {
return !element.loaded;
});
elements = $(temp);

if (settings.load) {
var elements_left = elements.length;
settings.load.call(self, elements_left, settings);
}
})
.attr("src", $self.attr("data-" + settings.data_attribute));
}
});
if (0 !== settings.event.indexOf("scroll")) {
$self.on(settings.event, function() {
if (!self.loaded) {
$self.trigger("appear");
}
});
}
});
$window.on("resize", function() {
update();
});
if ((/(?:iphone|ipod|ipad).*os 5/gi).test(navigator.appVersion)) {
$window.on("pageshow", function(event) {
if (event.originalEvent && event.originalEvent.persisted) {
elements.each(function() {
$(this).trigger("appear");
});
}
});
}
$(document).ready(function() {
update();
});

return this;
};
$.belowthefold = function(element, settings) {
var fold;

if (settings.container === undefined || settings.container === window) {
fold = (window.innerHeight ? window.innerHeight : $window.height()) + $window.scrollTop();
} else {
fold = $(settings.container).offset().top + $(settings.container).height();
}

return fold <= $(element).offset().top - settings.threshold;
};

$.rightoffold = function(element, settings) {
var fold;

if (settings.container === undefined || settings.container === window) {
fold = $window.width() + $window.scrollLeft();
} else {
fold = $(settings.container).offset().left + $(settings.container).width();
}

return fold <= $(element).offset().left - settings.threshold;
};

$.abovethetop = function(element, settings) {
var fold;

if (settings.container === undefined || settings.container === window) {
fold = $window.scrollTop();
} else {
fold = $(settings.container).offset().top;
}

return fold >= $(element).offset().top + settings.threshold + $(element).height();
};

$.leftofbegin = function(element, settings) {
var fold;

if (settings.container === undefined || settings.container === window) {
fold = $window.scrollLeft();
} else {
fold = $(settings.container).offset().left;
}

return fold >= $(element).offset().left + settings.threshold + $(element).width();
};

$.inviewport = function(element, settings) {
return !$.rightoffold(element, settings) && !$.leftofbegin(element, settings) &&
!$.belowthefold(element, settings) && !$.abovethetop(element, settings);
};
$.extend($.expr[":"], {
"below-the-fold" : function(a) { return $.belowthefold(a, {threshold : 0}); },
"above-the-top" : function(a) { return !$.belowthefold(a, {threshold : 0}); },
"right-of-screen": function(a) { return $.rightoffold(a, {threshold : 0}); },
"left-of-screen" : function(a) { return !$.rightoffold(a, {threshold : 0}); },
"in-viewport" : function(a) { return $.inviewport(a, {threshold : 0}); },
"above-the-fold" : function(a) { return !$.belowthefold(a, {threshold : 0}); },
"right-of-fold" : function(a) { return $.rightoffold(a, {threshold : 0}); },
"left-of-fold" : function(a) { return !$.rightoffold(a, {threshold : 0}); }
});

})(jQuery, window, document);

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