根据图片路径生成二进制流，下载图片

let url = this.codes[0].qrImgUrl
this.getBase64(url).then(function(base64){
　　let save_link = document.createElementNS('http://www.w3.org/1999/xhtml', 'a');
　　save_link.href = base64;
　　if(that.pageType=='mediaIds'){
　　　　save_link.download = that.data[0].mediaNo+'.jpg';
　　}else{
　　　　save_link.download = that.data[0].salesmanName+that.data[0].accNo+'.jpg';
　　}
　　let event = document.createEvent('MouseEvents');
　　event.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
　　save_link.dispatchEvent(event);
},function(err){
});

getBase64(img){
　　let that = this
　　let image = new Image();
　　image.crossOrigin = '';
　　image.src = img;
　　let deferred=$.Deferred();
　　if(img){
　　　　image.onload =function (){
　　　　deferred.resolve(that.getBase64Image(image));//将base64传给done上传处理
　　}
　　return deferred.promise();//问题要让onload完成后再return sessionStorage['imgTest']
　　}
},

getBase64Image(img,width,height) {//width、height调用时传入具体像素值，控制大小 ,不传则默认图像大小
let canvas = document.createElement("canvas");
canvas.width = width ? width : img.width;
canvas.height = height ? height : img.height;

let ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
let dataURL = canvas.toDataURL();
return dataURL;
},