uni.getFileSystemManager().readFile({
      filePath: url, //选择图片返回的相对路径
      encoding: "base64", //编码格式
      success: (res) => {
        //成功的回调
        const base64 = res.data; 
      },
    });

uni.chooseImage

          await my.getImageInfo({
            src: imagePath,
            success: async (res) => {
              this.imgWidth = res.width / 2.5;
              this.imgHeight = res.height / 2.5;
              let canvas = my.createCanvasContext("canvas");
              canvas.drawImage(imagePath, 0, 0, this.imgWidth, this.imgHeight); // 1. 绘制图片至canvas
              // 绘制完成后执行回调
              canvas.draw(false, async () => {
                let base64 = await canvas.toDataURL({
                  width: this.imgWidth,
                  height: this.imgHeight,
                  quality: 1,
                });
                this.base64 = base64;

                base64 = base64.replace("data:image/png;base64,", "");
                await this.uploadImageRequest(base64);
              });
            },
          });