实现页面图片延迟载入方法jquery.lazyload.js(1/2)

试页面的html代码中将所有要载入的图片都放到了textarea标签中，其目的就是防止页面载入时这些img标签向服务器发出请求，从而实现减少不必要的请求数，实际应用中也可以将不需第一次载入的html代码放到textarea中。
(function($) {
$.fn.lazyload = function(options) {
var settings = {
threshold : 0, //阀值
failurelimit : 0,
event : "scroll",
effect : "show",
container : window
};
if(options) {
$.extend(settings, options);
}
/* fire one scroll event per scroll. not one scroll event per image. */
var elements = this;
if ("scroll" == settings.event) {
$(settings.container).bind("scroll", function(event) {
var counter = 0;
elements.each(function() {
if ($.abovethetop($(this).parent(), settings) || $.leftofbegin($(this).parent(), settings)) {
/* nothing. */
} else if (!$.belowthefold($(this).parent(), settings) && !$.rightoffold($(this).parent(), settings)) {
$(this).trigger("appear");
} else {
if (counter++ > settings.failurelimit) {
return false;
}
}
});
/* remove image from array so it is not looped next time. */
var temp = $.grep(elements, function(element) {
return !element.loaded;
});
elements = $(temp);
});
}
this.each(function() {
var self = this;
/* when appear is triggered load original image. */
$(self).one("appear", function() {
if (!this.loaded) {
//alert($(self).parent().html($(self).html()));
var tmp_str = $(self).html();
tmp_str = tmp_str.replace("<", "<");
tmp_str = tmp_str.replace(">", ">");
$(self).parent().append(tmp_str);
self.loaded = true;
}
});
/* when wanted event is triggered load original image */
/* by triggering appear. */
if ("scroll" != settings.event) {
$(self).bind(settings.event, function(event) {
if (!self.loaded) {
$(self).trigger("appear");
}
});
}
});
/* force initial check if images should appear. */
$(settings.container).trigger(settings.event);
return this;
};
/* convenience methods in jquery namespace. */
/* use as $.belowthefold(element, {threshold : 100, container : window}) */
$.belowthefold = function(element, settings) {
if (settings.container === undefined || settings.container === window) {
var fold = $(window).height() + $(window).scrolltop();
} else {
var fold = $(settings.container).offset().top + $(settings.container).height();
}
return fold <= $(element).offset().top - settings.threshold;
};
$.rightoffold = function(element, settings) {
if (settings.container === undefined || settings.container === window) {
var fold = $(window).width() + $(window).scrollleft();
} else {
var fold = $(settings.container).offset().left + $(settings.container).width();
}
return fold <= $(element).offset().left - settings.threshold;
};
$.abovethetop = function(element, settings) {
if (settings.container === undefined || settings.container === window) {
var fold = $(window).scrolltop();
} else {
var fold = $(settings.container).offset().top;
}
return fold >= $(element).offset().top + settings.threshold + $(element).height();
};
$.leftofbegin = function(element, settings) {
if (settings.container === undefined || settings.container === window) {
var fold = $(window).scrollleft();
} else {
var fold = $(settings.container).offset().left;
}
return fold >= $(element).offset().left + settings.threshold + $(element).width();
};
/* custom selectors for your convenience. */
/* use as $("img:below-the-fold").something() */
$.extend($.expr[':'], {
"below-the-fold" : "$.belowthefold(a, {threshold : 0, container: window})",
"above-the-fold" : "!$.belowthefold(a, {threshold : 0, container: window})",
"right-of-fold" : "$.rightoffold(a, {threshold : 0, container: window})",
"left-of-fold" : "!$.rightoffold(a, {threshold : 0, container: window})"
});
})(jquery);


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