AWS Client for uploading file to AWS S3 Bucket

AWSClient.dart

import 'dart:convert';
import 'dart:typed_data';
import 'package:http/http.dart' as http;
import 'package:amazon_cognito_identity_dart/sig_v4.dart';

class AWSClient {
  String accessKeyId = 'AKPAAH6EDUOYSGTSJVFB'; // replace with your own access key
  String secretKeyId = 'imLP837nxYarh/DWP+sLskPZqTCFHRS5PVzMRVcP'; // replace with your own secret key
  String region = 'ap-south-1'; // replace with your account's region name
  String bucketname = "your_bucket_name"; // replace with your S3's bucket name
  String s3Endpoint = 'https://your_bucket_name.s3-ap-south-1.amazonaws.com'; // update the endpoint url for your bucket


  dynamic uploadData(String folderName, String fileName, Uint8List data) async {
    final length = data.length;

    final uri = Uri.parse(s3Endpoint);
    final req = http.MultipartRequest("POST", uri);
    final multipartFile = http.MultipartFile('file', http.ByteStream.fromBytes(data), length, filename: fileName);

    final policy = Policy.fromS3PresignedPost(folderName+'/'+fileName, 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);

        return value;
      }
    } catch (e) {
      print(e.toString());

      return e;
    }
  }

}





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}" }
  ]
}
''';
  }
}