elementUI+Upload 上传图片 先进行压缩

  beforeUpload(file) {
            let _this = this;
            return new Promise((resolve, reject) => {
                // 判定图片大小是否小于10MB
                //let isLt10M = file.size / 1024 / 1024 < 10;
                //if (!isLt10M) {
                    //不管是不是小于10MB都会压缩
                    let image = new Image(),
                        resultBlob = "";
                    image.src = URL.createObjectURL(file);
                    image.onload = () => {
                        // 调用方法获取blob格式
                        resultBlob = _this.compressupload(image, file);
                        resolve(resultBlob);
                    };
                    image.onerror = () => {
                        reject();
                    };
               // }
            });
        },
        /* 图片压缩方法-canvas压缩 */
        compressupload(image, file) {
            let canvas = document.createElement("canvas");
            let ctx = canvas.getContext("2d");
            let { width } = image,
                { height } = image;
            canvas.width = width;
            canvas.height = height;
            ctx.fillRect(0, 0, canvas.width, canvas.height);
            ctx.drawImage(image, 0, 0, width, height);
            // 进行压缩 
            let compressData = canvas.toDataURL(file.type || "image/jpeg", 0.5);
            // 压缩后调用方法进行base64转Blob
            let blobImg = this.dataURItoBlob(compressData);
            return blobImg;
        },

        /* base64转Blob对象 */
        dataURItoBlob(data) {
            let byteString;
            if (data.split(",")[0].indexOf("base64") >= 0) {
                byteString = atob(data.split(",")[1]);
            } else {
                byteString = unescape(data.split(",")[1]);
            }
            let mimeString = data
                .split(",")[0]
                .split(":")[1]
                .split(";")[0];
            let ia = new Uint8Array(byteString.length);
            for (let i = 0; i < byteString.length; i += 1) {
                ia[i] = byteString.charCodeAt(i);
            }
            return new Blob([ia], { type: mimeString });
        },