Flutter/Dart中使用Dio库下载PDF文件无响应问题排查

我用Dio库编写了下载PDF文件的代码，但点击下载按钮后没有出现预期效果。以下是我的代码及控制台输出：

代码 #

Future download2(Dio dio, String url, String savePath) async {
  String? token = await this.widget.appController.storage.read(key: "token");
  Map<String, String> headers = {
    "Content-Type": "application/json",
    "Accept": "application/json",
    "Authorization": "Bearer " + (token ?? ""),
  };
  try {
    Response response = await dio.get(
      url,
      onReceiveProgress: showDownloadProgress,
      options: Options(
        headers: headers,
        responseType: ResponseType.bytes,
        followRedirects: false,
        validateStatus: (status) {
          return status! < 500;
        }),
    );
    print(response.headers);
    File file = File(savePath);
    var raf = file.openSync(mode: FileMode.write);
    raf.writeFromSync(response.data);
    await raf.close();
  } catch (e) {
    print(e);
  }
}

void showDownloadProgress(received, total) {
  if (total != -1) {
    print((received / total * 100).toStringAsFixed(0) + "%");
  }
}

ElevatedButton(
  child: Text('Download Bill pdf'),
  onPressed: () async {
    var tempDir = await getTemporaryDirectory();
    String fullPath = tempDir.path + "/boo2.pdf'";
    print('full path ${fullPath}');
    download2(dio, pdfUrl, fullPath);
  }
)

控制台输出 #

I/flutter (24548): full path /data/user/0/com.iccs.electromobilityapp/cache/boo2.pdf'
I/flutter (24548): 100%
I/flutter (24548): connection: keep-alive
I/flutter (24548): last-modified: Mon, 17 Oct 2022 10:17:34 GMT
I/flutter (24548): cache-control: no-cache, no-store, max-age=0, must-revalidate
I/flutter (24548): date: Fri, 21 Oct 2022 08:58:48 GMT
I/flutter (24548): vary: Origin
I/flutter (24548): vary: Access-Control-Request-Method
I/flutter (24548): vary: Access-Control-Request-Headers
I/flutter (24548): content-type: text/html;charset=UTF-8
I/flutter (24548): pragma: no-cache
I/flutter (24548): x-xss-protection: 1; mode=block
I/flutter (24548): content-language: en
I/flutter (24548): server: nginx/1.15.12
I/flutter (24548): accept-ranges: bytes
I/flutter (24548): content-length: 6425
I/flutter (24548): x-frame-options: DENY
I/flutter (24548): x-content-type-options: nosniff
I/flutter (24548): expires: 0

问题分析与修复方案 #

1. 文件路径错误 #

控制台显示的路径末尾多了一个单引号：/boo2.pdf'，这会导致系统无法正确识别文件路径，直接修改路径代码：

String fullPath = tempDir.path + "/boo2.pdf";

2. 请求头设置错误 #

当前请求头的Accept是application/json，但我们要下载PDF，应该告诉服务器我们接受PDF格式：

Map<String, String> headers = {
  "Accept": "application/pdf", // 修改为PDF格式
  "Authorization": "Bearer " + (token ?? ""),
};
// 移除Content-Type，GET请求不需要发送JSON请求体，该字段多余

3. 异步调用未等待 #

download2是异步函数，onPressed里没有用await，可能导致异常无法被捕获，修改为：

onPressed: () async {
  var tempDir = await getTemporaryDirectory();
  String fullPath = tempDir.path + "/boo2.pdf";
  print('full path ${fullPath}');
  await download2(dio, pdfUrl, fullPath); // 添加await等待下载完成
}

4. 接口返回非PDF内容 #

控制台里的content-type是text/html，说明接口返回的是HTML页面（可能是权限错误、资源不存在等），需要：

• 确认pdfUrl是否正确指向PDF资源
• 检查token是否有效，是否具备访问该PDF的权限
• 直接在浏览器或Postman中用相同token访问pdfUrl，验证返回内容是否为PDF

可选：验证下载结果 #

在download2的try块末尾添加文件验证逻辑，方便排查问题：

// 下载完成后检查文件状态
if (await file.exists()) {
  print("文件已保存至：${file.path}");
  // 若要直接打开PDF，可引入open_file库调用：await OpenFile.open(file.path);
} else {
  print("文件保存失败");
}

内容的提问来源于stack exchange，提问作者Bill Tertis