我的 flutter 应用程序使用 firebase 作为后端，但我需要将媒体文件(照片和视频)存储在我的 s3 存储桶中。任务是将从图像选择器检索到的媒体上传到 s3 并取回 url，然后可以将其作为字符串存储在我的 firebase 数据库中。

问题是 dart 2 缺少 aws 库或 api。我在 pub 中找到了 3 个，但其中 2 个与 dart 2 不兼容，并且 1 正在开发中。有没有人使用 dart 2 在 flutter 中实现这一点？欢迎任何建议。谢谢。

我找到的包是 (pub.dartlang.org):aws_client、aws_interop、amazon_s3

最佳答案

有几种方法可以做到这一点。一种方法是sign your request与 Signature V4并将您的文件 POST 到 S3。

首先，创建一个策略助手:

import 'dart:convert';
import 'package:amazon_cognito_identity_dart/sig_v4.dart';

class Policy {
  String expiration;
  String region;
  String bucket;
  String key;
  String credential;
  String datetime;
  int maxFileSize;

  Policy(this.key, this.bucket, this.datetime, this.expiration, this.credential,
      this.maxFileSize,
      {this.region = 'us-east-1'});

  factory Policy.fromS3PresignedPost(
    String key,
    String bucket,
    String accessKeyId,
    int expiryMinutes,
    int maxFileSize, {
    String region,
  }) {
    final datetime = SigV4.generateDatetime();
    final expiration = (DateTime.now())
        .add(Duration(minutes: expiryMinutes))
        .toUtc()
        .toString()
        .split(' ')
        .join('T');
    final cred =
        '$accessKeyId/${SigV4.buildCredentialScope(datetime, region, 's3')}';
    final p = Policy(key, bucket, datetime, expiration, cred, maxFileSize,
        region: region);
    return p;
  }

  String encode() {
    final bytes = utf8.encode(toString());
    return base64.encode(bytes);
  }

  @override
  String toString() {
    return '''
{ "expiration": "${this.expiration}",
  "conditions": [
    {"bucket": "${this.bucket}"},
    ["starts-with", "\$key", "${this.key}"],
    {"acl": "public-read"},
    ["content-length-range", 1, ${this.maxFileSize}],
    {"x-amz-credential": "${this.credential}"},
    {"x-amz-algorithm": "AWS4-HMAC-SHA256"},
    {"x-amz-date": "${this.datetime}" }
  ]
}
''';
  }
}

然后，使用您的策略助手签署您的请求并通过 http.MultipartRequest 上传:

import 'dart:convert';
import 'dart:io';
import 'package:path/path.dart' as path;
import 'package:async/async.dart';
import 'package:http/http.dart' as http;
import 'package:test/test.dart';
import 'package:amazon_cognito_identity_dart/sig_v4.dart';
import './policy.dart';

void main() {
  const _accessKeyId = 'AKXXXXXXXXXXXXXXXXXX';
  const _secretKeyId = 'xxxxxxxxxxxxxxxxxxxxxxxx/xxxxxxxxxxxxxxx';
  const _region = 'ap-southeast-1';
  const _s3Endpoint =
      'https://bucketname.s3-ap-southeast-1.amazonaws.com';

  final file = File(path.join('/path/to/file', 'square-cinnamon.jpg'));
  final stream = http.ByteStream(DelegatingStream.typed(file.openRead()));
  final length = await file.length();

  final uri = Uri.parse(_s3Endpoint);
  final req = http.MultipartRequest("POST", uri);
  final multipartFile = http.MultipartFile('file', stream, length,
      filename: path.basename(file.path));

  final policy = Policy.fromS3PresignedPost('uploaded/square-cinnamon.jpg',
      'bucketname', _accessKeyId, 15, length,
      region: _region);
  final key =
      SigV4.calculateSigningKey(_secretKeyId, policy.datetime, _region, 's3');
  final signature = SigV4.calculateSignature(key, policy.encode());

  req.files.add(multipartFile);
  req.fields['key'] = policy.key;
  req.fields['acl'] = 'public-read';
  req.fields['X-Amz-Credential'] = policy.credential;
  req.fields['X-Amz-Algorithm'] = 'AWS4-HMAC-SHA256';
  req.fields['X-Amz-Date'] = policy.datetime;
  req.fields['Policy'] = policy.encode();
  req.fields['X-Amz-Signature'] = signature;

  try {
    final res = await req.send();
    await for (var value in res.stream.transform(utf8.decoder)) {
      print(value);
    }
  } catch (e) {
    print(e.toString());
  }
}

请注意，此方法需要您提供您的访问 key 和 secret key 。如果您使用像 Cognito 这样的服务，建议您获取一个临时的 Access Key 和 Secret Key。找到使用临时访问的示例 here .

免责声明:我是 Signature V4 package 的原作者.

关于amazon-web-services - 在 Flutter 中从 AWS S3 上传和获取媒体文件，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/53811487/