有几种方法可以做到。一种方法是将sign your request与Signature V4和POST一起将您的文件发送到S3。
首先，创建一个策略帮助器：

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，此方法需要您提供访问密钥和密钥。如果您使用Cognito等服务，建议您获取临时访问密钥和密钥。使用临时访问的示例在这里找到。

• 免责声明：我是Signature V4 package的原作者。*

您可以使用包amazon_s3_cognito上传和删除图片到amazon s3。
我是插件的作者，我们正在使用这个插件成功地在我们的许多项目。

import 'package:amazon_s3_cognito/amazon_s3_cognito.dart';
        import 'package:amazon_s3_cognito/aws_region.dart';

        String uploadedImageUrl = await AmazonS3Cognito.uploadImage(
                  _image.path, BUCKET_NAME, IDENTITY_POOL_ID);

        //Use the below code to upload an image to amazon s3 server
        //I advise using this method for image upload.
        String uploadedImageUrl = await AmazonS3Cognito.upload(
                    _image.path,
                    BUCKET_NAME,
                    IDENTITY_POOL_ID,
                    IMAGE_NAME,
                    AwsRegion.US_EAST_1,
                    AwsRegion.AP_SOUTHEAST_1)

_image. path =要上传的图像的路径（flutter中的file. path方法）
IMAGE_NAME =此图像以您在此给出的名称上传到s3服务器。

//use below code to delete an image
         String result = AmazonS3Cognito.delete(
                    BUCKET_NAME,
                    IDENTITY_POOL_ID,
                    IMAGE_NAME,
                    AwsRegion.US_EAST_1,
                    AwsRegion.AP_SOUTHEAST_1)

对于获取图像，您可以使用cached_network_image包
CachedNetworkImage可以直接使用或通过ImageProvider使用。

CachedNetworkImage(
            imageUrl: "http://via.placeholder.com/350x150",
            placeholder: (context, url) => new CircularProgressIndicator(),
            errorWidget: (context, url, error) => new Icon(Icons.error),
         ),`enter code here`

    Future<String> _getFilePath(Asset asset,ListingImage listingImage) async{
       try {
         if (!isUploadCancelled) {
           // getting a directory path for saving
           final directory = await getTemporaryDirectory();
           String path = directory.path;
           File file = File(path + "/temp_" + listingImage.index.toString() + "_"+DateTime.now().microsecondsSinceEpoch.toString());
           listingImage.file = file;
           file = await file.writeAsBytes( asset.imageData.buffer.asUint8List(asset.imageData.offsetInBytes, asset.imageData.lengthInBytes));

           return file.path;
         } else {
           return null;
         }
       } catch(exceptioon) {
         return null;
       }

      }

您可以使用Flutter Multipart，类似于这样

// open a bytestream
    var stream = new http.ByteStream(DelegatingStream.typed(_image.openRead()));
    // get file length
    var length = await _image.length();
    // string to uri
    var uri = Uri.parse(apiUrl);
    // create multipart request
    var request = new http.MultipartRequest("POST", uri);
    NetworkUtils.addAuthHeaders(request);
    // multipart that takes file
    var multipartFile = new http.MultipartFile('file', stream, length,
        filename: basename(_image.path),
        contentType: new MediaType("image", "jpg"));
    // add file to multipart
    request.files.add(multipartFile);
    request.fields.addAll(body);
    // send
    var response = await request.send();
    print(response.statusCode);
    // listen for response
    response.stream.transform(utf8.decoder).listen((value) {
      print(value);
    });
  }

您可以使用包amazon_s3_cognito上传图像。但它在iOS端有问题，你可以试试我的叉子。
把这行写进你的pubspec.yaml：

amazon_s3_cognito:
  git:
    URL: https://github.com/zhihaozhang/amazon_s3_cognito.git