touchweb手机网站图片加载方法（canvas加载和延迟加载）

一、canvas图片加载

关于canvas加载，我的方法是，将文章中所有用到图片的地方，都用canvas代替，给canvas一个data-src，里面存放img的路径，通过canvas方法渲染图片。因为图片渲染需要时间，一般会给canvas一个背景，背景可以用gif图片。图片渲染好了之后把背景去掉。

我的canvas代码如下：

canvasload: function () {
            //canvas加载图片
            var imglength = $("#您的id").find("canvas").length;
            if (imglength > ) {
                $("#您的id").find("canvas").each(function () {
                    var imgSrc = $(this).data("src");
                    var imageObj = new Image();
                    imageObj.canvs = $(this)[];
                    var cvs = imageObj.canvs.getContext('2d');
                    if (cvs) {
                        imageObj.onload = function () {
                            imageObj.canvs.width = this.width;
                            imageObj.canvs.height = this.height;
                            cvs.drawImage(this, , );
                            $(imageObj.canvs).css("background-image", "none");
                        }
                        imageObj.src = imgSrc;
                    }
                })
            }
},

用法很简单，直接在页面中引用这个函数就可以了，注意，所有canvas外层要包裹一个ID

二、图片延迟加载

图片延迟加载方法有很多，下面简单介绍一个，可以引入一个jquery插件。这个插件是一个网友写的，试了一下能用，代码如下：

(function ($) {
    $.fn.lazyloading = function (options) {
        var defaults = {
            preyimg: "/load.gif",
            picpath: "data-original",
            container: $(window),
            loadfirst: false, //进入页面后是否加载当前页面的图片
            defaultHeightID: "lazyloadingHeight"//页面上默认高度的input标签id
            //imgPaddingID: "lazyloadingPadding"//img的padding值
        };
        var params = $.extend({}, defaults, options || {});
        params.cache = [];
        $(this).each(function () {
            var node = this.nodeName.toLowerCase(), url = $(this).attr(params["picpath"]), preyimg = params["preyimg"];
            var defaultheight = $("#" + params["defaultHeightID"]).val(); //, padding = $("#" + params["imgPaddingID"]).val(); //
            //重组
            var data = {
                obj: $(this),
                tag: node,
                url: url,
                preyimg: preyimg,
                defaultheight: defaultheight
            };
            params.cache.push(data);
        });  

        var init = function () {
            $.each(params.cache, function (i, data) {
                var thisImg = data.obj, tag = data.tag, url = data.url, preyimg = data.preyimg;
                if (typeof (url) != "undefined")// 判断是否需要延迟加载
                {
                    var parent1 = thisImg.parent(); //a
                    var Inner = null; //
                    if (parent1.is("a") == true) {//img wrap by a
                        Inner = parent1;
                    }
                    else {
                        Inner = thisImg;
                    }
                    var width = thisImg.attr("width") || thisImg.css("width");
                    var height = data.defaultheight || thisImg.css("height");
                    //if (i == 0) alert(data.defaultheight);
                    var attrheight = thisImg.attr("height");
                    if (attrheight != null) height = attrheight;
                    if (width != null && width.indexOf("px") > -) width.replace("px", "");
                    if (height != null && height.indexOf("px") > -) height.replace("px", "");
                    var divstr = "<div class='.loading' style='text-align: left;position:relative;float:left;width:" + width + "px;";
                    var HasHeight = true; //图片是否指定了高度
                    divstr = divstr + "height:" + height + "px;";
                    if (attrheight == null || attrheight == "") {
                        HasHeight = false;
                    }  

                    thisImg.css("position", "relative");  

                    divstr = divstr + "' ></div>"
                    //修正外层div：text-align的影响
                    Inner.wrap(divstr);
                    //修正img外面不是a标签时parent()已经改变的问题
                    parent1 = thisImg.parent();
                    if (HasHeight == true) { parent1.attr("lazyloading_hasheight", ""); } //是否指定了高度
                    else { { parent1.attr("lazyloading_hasheight", ""); } }
                    parent1.append("<img class='loadhiddenimg' width='0' height='0' style='display:none;' src='' />");
                    thisImg.attr("src", preyimg);
                    thisImg.removeAttr("width").removeAttr("height");
                    thisImg.attr("width1", width).attr("height1", attrheight);  

                    ////thisImg.attr("width", "50px").attr("height", "50px"); //loading图大小
                    //thisImg.css("margin", "0 auto");
                    thisImg.css("margin", ((height / ) - ) + "px auto auto " + ((width / ) - ) + "px");
                    $(".lazyloading").css("display", "table"); //.css("position", "relative");
                }
            });
        }
        //动态显示数据
        var loading1 = function () { };
        var loading = function () {
            //窗口的高度+看不见的顶部的高度=屏幕低部距离最顶部的高度
            var thisButtomTop = parseInt($(window).height()) + parseInt($(window).scrollTop());
            var thisTop = parseInt($(window).scrollTop()); //屏幕顶部距离最顶部的高度  

            $.each(params.cache, function (i, data) {
                var thisImg = data.obj, tag = data.tag, url = data.url, post, posb;  

                if (thisImg) {//对象不为空
                    if (typeof (url) != "undefined") {// 判断是否需要延迟加载
                        var PictureTop = parseInt(thisImg.offset().top);
                        //如果处理可见范围内，并且原图地址data-original不等于src,则加载图片
                        if (PictureTop >= thisTop && PictureTop <= thisButtomTop && thisImg.attr("data-original") != thisImg.attr("src")) {
                            var hiddenImg = thisImg.siblings("img.loadhiddenimg");  

                            hiddenImg.load(function () { //隐藏图片加载完之后的回调函数
                                var width = thisImg.attr("width1");
                                var height = thisImg.attr("height1");
                                thisImg.attr("width", width).attr("height", height).removeAttr("width1").removeAttr("height1");
                                thisImg.css("margin", "0 auto");
                                if (thisImg.parent().attr("lazyloading_hasheight") == "") {//没有指定高度时，加载图片后去掉div高度自适应
                                    if (thisImg.parent().is("a") == true) {
                                        thisImg.parent().parent().css("height", "");
                                    }
                                    else {
                                        thisImg.parent().css("height", "");
                                    }
                                }
                                thisImg.load(function () {
                                    if (thisImg.parent().is("a") == true) {
                                        thisImg.parent().parent().css("height", thisImg.height());
                                    }
                                    else {
                                        thisImg.parent().css("height", thisImg.height());
                                    }
                                });
                                thisImg.css('opacity', '0.2');
                                thisImg.attr("src", hiddenImg.attr("src"));
                                thisImg.animate({ opacity: 1.0 });
                                if (thisImg.attr("alt") != "") {
                                    thisImg.attr("title", thisImg.attr("alt"));
                                    thisImg.attr("alt", "");
                                }
                            }).error(function () {
                                thisImg.error(function () {
                                    thisImg.css("margin", "0 auto auto 0");
                                    if (thisImg.parent().attr("lazyloading_hasheight") == "") {//没有指定高度时，加载图片后去掉div高度自适应
                                        if (thisImg.parent().is("a") == true) {
                                            thisImg.parent().parent().css("height", "");
                                        }
                                        else {
                                            thisImg.parent().css("height", "");
                                        }
                                    }
                                });
                                thisImg.attr("src", hiddenImg.attr("src")); //alert("error");
                                if (thisImg.attr("alt") != "") {
                                    thisImg.attr("title", thisImg.attr("alt"));
                                    thisImg.attr("alt", "");
                                }
                            });
                            hiddenImg.attr("src", url);
                        }
                    }
                }
            });
        };
        //初始化
        init();
        //事件触发
        //加载完毕即执行
        if (params["loadfirst"] == true) loading();
        //滚动执行
        params.container.bind("scroll", loading).bind("resize", loading);
    };
})(jQuery); 

用法很简单：

<a href="#"><img  alt="haorooms博客" src="" data-original="http://www.haorooms.com/uploads/images/table.gif" class="lazyloading" /></a>
<script type="text/javascript">
$(function () {
    $("img.lazyloading").lazyloading({ loadfirst: true });
})

</script>