将资产复制到一个临时文件。

  Future<File> copyAsset() async {
    Directory tempDir = await getTemporaryDirectory();
    String tempPath = tempDir.path;
    File tempFile = File('$tempPath/copy.pdf');
    ByteData bd = await rootBundle.load('assets/asset.pdf');
    await tempFile.writeAsBytes(bd.buffer.asUint8List(), flush: true);
    return tempFile;
  }