如何修改Flutter中调用Gmail API的代码以支持发送邮件附件？

要实现带附件的邮件，你需要把原纯文本邮件改成多部分MIME格式（multipart/mixed），以下是修改后的完整代码和关键说明：

核心修改点 #

• 邮件内容改为multipart/mixed类型，拆分出纯文本正文和附件两个MIME部分
• 附件需先读取为字节流，再做Base64编码
• 用唯一边界字符串分隔不同MIME部分，避免内容冲突

修改后的完整代码 #

import 'dart:convert';
import 'dart:io';
import 'package:http/http.dart' as http;

Map<String, String> header = {
  'Authorization': authorizationHeader,
  'X-Goog-AuthUser': xGoogAuthUserHeader,
  'Accept': 'application/json',
  'Content-type': 'application/json',
};

// 生成带附件的MIME格式邮件
Future<String> createEmailWithAttachment(
  String to,
  String from,
  String subject,
  String bodyText,
  File attachmentFile,
) async {
  // 生成唯一边界字符串，用于分隔MIME段落
  final boundary = '----GmailApiBoundary${DateTime.now().millisecondsSinceEpoch}';
  
  // 读取附件并编码为Base64
  final attachmentBytes = await attachmentFile.readAsBytes();
  final attachmentBase64 = base64Encode(attachmentBytes);
  final fileName = attachmentFile.path.split('/').last;

  return '''
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="$boundary"
to: $to
from: $from
subject: ${base64Encode(utf8.encode(subject))}

--$boundary
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit

$bodyText

--$boundary
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="$fileName"

$attachmentBase64
--$boundary--
''';
}

// 发送带附件邮件的调用示例
Future<void> sendEmailWithAttachment() async {
  // 替换为实际收件人、发件人信息
  final to = 'recipient@example.com';
  final from = user.email!;
  final subject = '带附件的测试邮件';
  final bodyText = '这是邮件正文内容';
  // 替换为你的附件文件路径
  final attachmentFile = File('/storage/emulated/0/Documents/test.pdf');

  final formattedEmail = await createEmailWithAttachment(to, from, subject, bodyText, attachmentFile);
  
  // 编码为Gmail API要求的格式
  final bytes = utf8.encode(formattedEmail);
  final base64Email = base64Encode(bytes);
  final body = json.encode({'raw': base64Email});

  final httpsUri = Uri(
    scheme: 'https',
    host: 'www.googleapis.com',
    path: '/gmail/v1/users/${user.email}/messages/send',
  );

  final response = await http.post(httpsUri, headers: header, body: body);
  
  // 可选：处理发送结果
  if (response.statusCode == 200) {
    print('邮件发送成功');
  } else {
    print('发送失败：${response.body}');
  }
}

关键细节说明 #

• 主题编码：Gmail API要求邮件主题遵循RFC 2047标准，需先将主题字符串UTF8编码再做Base64编码
• 附件Content-Type：示例中用application/octet-stream通用类型，可根据附件实际类型修改（如图片用image/jpeg、Excel用application/vnd.openxmlformats-officedocument.spreadsheetml.sheet）
• 文件权限：移动端需确保已申请文件读取权限，否则无法读取附件
• 边界字符串：用时间戳生成唯一值，避免和邮件正文/附件内容冲突

内容的提问来源于stack exchange，提问作者solacking