arcgis js 加载动态层图片

define([
    "dojo/_base/declare",
    "dojo/_base/lang",
    "esri/layers/DynamicMapServiceLayer",
    "esri/geometry/Extent"
], function(
    declare,
    lang,
    DynamicMapServiceLayer,
    Extent
) {
    return declare("DynamicMapSuperPost", DynamicMapServiceLayer, { // create DynamicMapLayer by extending esri.layers.TiledMapServiceLayer
        constructor:function(){
            this.spatialReference = new esri.SpatialReference({wkid:4490});
            this.initialExtent = this.fullExtent = new Extent({"xmin":109.3868865977334,"ymin":29.938091498799054,"xmax":117.6127404019528,"ymax":37.81307950094276,"spatialReference": this.spatialReference});
            this.loaded = true;
            this.onLoad(this);
        },
        getImageUrl: function(extent, width, height, callback) {
            var params = {
                //request: "GetMap",
                //transparent: true,
                format: "png",//"image/png",
                //bgcolor: "ffffff",
                //version: "1.1.1",
                layers: 'id1,id2,id3,id4',
                //styles: "default,default",
                //exceptions: "application/vnd.ogc.se_xml",

                //changing values
                bbox: extent.xmin + "," + extent.ymin + "," + extent.xmax + "," + extent.ymax,
                //bbox:'113.68330445912024,34.75990865047065,113.71526537935056,34.77527045072429',
                MAPID: 110,
                //GRAY:0,
                //LAYERHIDEOID:'`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^',
                //LAYERHIDEOID:'`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^`^',
                //srs: "EPSG:" + extent.spatialReference.wkid,
                 width,
                height: height
            };
            var curParams = dojo.objectToQuery(params);

            var _self = this;
            var curentImgId = this._img_loading.id;
　　　　　　　//这个地方是使用了servlet返回动态层的png图片,原因是有些动态层是定制的，要符合他们要求
            var url = '../proxy';

            var xhr = new XMLHttpRequest();
            xhr.responseType = 'blob';
            xhr.open('POST', url, true);//POST请求，请求地址，是否异步
            xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded')
            xhr.onload = function (ev) {
                if (this.status == 200) {
                    var data = this.response;
                    var blob = new Blob();
                    blob = data;
                    var blobImage = window.URL.createObjectURL(blob);
                    if (_self._img_loading && _self._img_loading.id == curentImgId) {
                        callback(blobImage);
                    } else {//缩放或移动中img被arcgis删除了,丢失dom节点
                        window.URL.revokeObjectURL(blobImage);//释放
                    }
                }
            }
            xhr.send(curParams);

        },
        _imgSrcFunc: function (a) {
            this._img_loading.src = a
            this._img_loading.onload = function () {
                window.URL.revokeObjectURL(this.src);//释放
            }
        }
    });
});