js 本地预览的简单实现方法

// JavaScript Document
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var ImagePreview = function(file, img, options) {

 this.file = $(file);//文件对象
 this.img = $(img);//预览图片对象

 this._preload = null;//预载图片对象
 this._data = "";//图像数据
 this._upload = null;//remote模式使用的上传文件对象

 var opt = this._setOptions(options);

 this.action = opt.action;
 this.timeout = opt.timeout;
 this.ratio = opt.ratio;
 this.maxWidth = opt.maxWidth;
 this.maxHeight = opt.maxHeight;

 this.onCheck = opt.onCheck;
 this.onShow = opt.onShow;
 this.onErr = opt.onErr;

 //设置数据获取程序
 this._getData = this._getDataFun(opt.mode);
 //设置预览显示程序
 this._show = opt.mode !== "filter" ? this._simpleShow : this._filterShow;
};
//根据浏览器获取模式
ImagePreview.MODE = $B.ie7 || $B.ie8 ? "filter" :
 $B.firefox ? "domfile" :
 $B.opera || $B.chrome || $B.safari ? "remote" : "simple";
//透明图片
ImagePreview.TRANSPARENT = $B.ie7 || $B.ie6 ?
 "mhtml:" + document.scripts[document.scripts.length - 1].getAttribute("src", 4) + "!blankImage" :
 "data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==";

ImagePreview.prototype = {
  //设置默认属性
  _setOptions: function(options) {
    this.options = {//默认值
  mode:  ImagePreview.MODE,//预览模式
  ratio:  0,//自定义比例
  maxWidth: 0,//缩略图宽度
  maxHeight: 0,//缩略图高度
  onCheck: function(){},//预览检测时执行
  onShow:  function(){},//预览图片时执行
  onErr:  function(){},//预览错误时执行
  //以下在remote模式时有效
  action:  undefined,//设置action
  timeout: 0//设置超时(0为不设置)
    };
    return $.extend(this.options, options || {});
  },
  //开始预览
  preview: function() {
 if ( this.file && false !== this.onCheck() ) {
  this._preview( this._getData() );
 }
  },

  //根据mode返回数据获取程序
  _getDataFun: function(mode) {
 switch (mode) {
  case "filter" :
   return this._filterData;
  case "domfile" :
   return this._domfileData;
  case "remote" :
   return this._remoteData;
  case "simple" :
  default :
   return this._simpleData;
 }
  },
  //滤镜数据获取程序
  _filterData: function() {
 this.file.select();
 try{
  return document.selection.createRange().text;
 } finally { document.selection.empty(); }
  },
  //domfile数据获取程序
  _domfileData: function() {
 return this.file.files[0].getAsDataURL();
  },
  //远程数据获取程序
  _remoteData: function() {
 this._setUpload();
 this._upload && this._upload.upload();
  },
  //一般数据获取程序
  _simpleData: function() {
 return this.file.value;
  },

  //设置remote模式的上传文件对象
  _setUpload: function() {
 if ( !this._upload && this.action !== undefined && typeof QuickUpload === "function" ) {
  var oThis = this;
  this._upload = new QuickUpload(this.file, {
   onReady: function(){
    this.action = oThis.action; this.timeout = oThis.timeout;
    var parameter = this.parameter;
    parameter.ratio = oThis.ratio;
    parameter.width = oThis.maxWidth;
    parameter.height = oThis.maxHeight;
   },
   onFinish: function(iframe){
    try{
     oThis._preview( iframe.contentWindow.document.body.innerHTML );
    }catch(e){ oThis._error("remote error"); }
   },
   onTimeout: function(){ oThis._error("timeout error"); }
  });
 }
  },

  //预览程序
  _preview: function(data) {
 //空值或相同的值不执行显示
 if ( !!data && data !== this._data ) {
  this._data = data; this._show();
 }
  },

  //设置一般预载图片对象
  _simplePreload: function() {
 if ( !this._preload ) {
  var preload = this._preload = new Image(), oThis = this,
   onload = function(){ oThis._imgShow( oThis._data, this.width, this.height ); };
  this._onload = function(){ this.onload = null; onload.call(this); }
  preload.onload = $B.ie ? this._onload : onload;
  preload.onerror = function(){ oThis._error(); };
 } else if ( $B.ie ) {
  this._preload.onload = this._onload;
 }
  },
  //一般显示
  _simpleShow: function() {
 this._simplePreload();
 this._preload.src = this._data;
  },

  //设置滤镜预载图片对象
  _filterPreload: function() {
 if ( !this._preload ) {
  var preload = this._preload = document.createElement("div");
  //隐藏并设置滤镜
  $D.setStyle( preload, {
   width: "1px", height: "1px",
   visibility: "hidden", position: "absolute", left: "-9999px", top: "-9999px",
   filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod='image')"
  });
  //插入body
  var body = document.body; body.insertBefore( preload, body.childNodes[0] );
 }
  },
  //滤镜显示
  _filterShow: function() {
 this._filterPreload();
 var preload = this._preload,
  data = this._data.replace(/[)'"%]/g, function(s){ return escape(escape(s)); });
 try{
  preload.filters.item("DXImageTransform.Microsoft.AlphaImageLoader").src = data;
 }catch(e){ this._error("filter error"); return; }
 //设置滤镜并显示
 this.img.style.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod='scale',src="" + data + "")";
 this._imgShow( ImagePreview.TRANSPARENT, preload.offsetWidth, preload.offsetHeight );
  },

  //显示预览
  _imgShow: function(src, width, height) {
 var img = this.img, style = img.style,
  ratio = Math.max( 0, this.ratio ) || Math.min( 1,
    Math.max( 0, this.maxWidth ) / width  || 1,
    Math.max( 0, this.maxHeight ) / height || 1
   );
 //设置预览尺寸
 style.width = Math.round( width * ratio ) + "px";
 style.height = Math.round( height * ratio ) + "px";
 //设置src
 img.src = src;
 this.onShow();
  },

  //销毁程序
  dispose: function() {
 //销毁上传文件对象
 if ( this._upload ) {
  this._upload.dispose(); this._upload = null;
 }
 //销毁预载图片对象
 if ( this._preload ) {
  var preload = this._preload, parent = preload.parentNode;
  this._preload = preload.onload = preload.onerror = null;
  parent && parent.removeChild(preload);
 }
 //销毁相关对象
 this.file = this.img = null;
  },
  //出错
  _error: function(err) {
 this.onErr(err);
  }
}

<input id="idFile" type="file" name="pic" /><img id="idImg" alt="" src="http://www.uoften.com/" />
<script>
var ip = new ImagePreview( $$("idFile"), $$("idImg"), {
 maxWidth: 200, maxHeight: 2000, action: "ImagePreview.ashx"
});
ip.img.src = ImagePreview.TRANSPARENT;
ip.file.onchange = function(){ ip.preview(); };
</script>