使用fileReader的一个坑及解决

var file=this.$refs.resource.files[0]
var reader = new FileReader();
// 以DataURL的形式读取文件:
reader.readAsDataURL(file);

//读取后的回调js是多线程
reader.onloadend=(e)=>{
}

var fileUpload = function(obj, callback){
            //检测浏览器是否支持FileReader对象
            if(typeof FileReader == "undefined"){
                alert("您的浏览器不支持FileReader对象！");
            }
            var file = obj;
            //判断类型是不是图片
            if(!/image\/\w+/.test(file.type)){
                alert("请确保文件为图像类型");
                return false;
            }
            var reader = new FileReader();
            reader.onload = function (e) {
                var img = new Image,
                    width = 95,    //图片resize宽度
                    quality = 0.2,  //图像质量
                    canvas = document.createElement('canvas'),
                    drawer = canvas.getContext("2d");
                img.src = this.result;
                var scale = parseInt(img.height / img.width);
                img.width=width;
                img.height = width * scale;
                canvas.width = img.width;
                canvas.height = img.height;
                drawer.drawImage(testimgId, 0, 0, canvas.width, canvas.height);
                var base64 = canvas.toDataURL('image/jpeg', 0.2);
                console.log(base64);
                var image_base64 = img.src.replace("data:image/png;base64,","");
                image_base64=encodeURIComponent(image_base64);
                alert("19")
                // 调用回调
                callback&&callback(img.src);
            }
            reader.readAsDataURL(file);
        }

function getBase64(fileObj){
            var file = fileObj,
                cvs = document.getElementById("canvas"),
                ctx = cvs.getContext("2d");
            if(file){
                var url = window.URL.createObjectURL(file);//PS:不兼容IE
                var img = new Image();
                img.src = url;
                img.onload = function(){
                    ctx.clearRect(0,0,cvs.width,cvs.height);
                    cvs.width = img.width;
                    cvs.height = img.height;
                    ctx.drawImage(img,0,0,img.width,img.height);
                    var base64 = cvs.toDataURL("image/png");
                    callback(base64);
                    alert("20")
                }
            }
        }