I've struggled a bit to figure out how to convert Uint8List to data that dio client expects.

Here is my code.

Basically you should

http.ByteStream.fromBytes(int8List)

to convert Int8List bytes to Stream<List<int>>.

import 'package:dio/dio.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:http/http.dart' as http;

try {
  // get signed url
  Response response = await dio.get(
      '$apiUrl/s3/sign?objectName=app-test.png&contentType=image%2Fpng&path=dev%2Ftanks%2FDDuMahcPGO%2F');

  final String signedUrl = response.data["signedUrl"];

  // get data stream
  Uint8List int8List = await rootBundle
      .load('assets/fish/1.png')
      .then((value) => value.buffer.asUint8List());

  Response uploadRes = await dio.put(
    signedUrl,
    data: http.ByteStream.fromBytes(int8List),
    onSendProgress: (int sentBytes, int totalBytes) {
      double progressPercent = sentBytes / totalBytes * 100;
      print("upload $progressPercent %");
    },
    options: Options(
      contentType: "image/png",
      headers: {
        "Content-Length": int8List.length,
      },
    ),
  );

  print("uploadRes.statusCode, ${uploadRes.statusCode}");
  // 200 success
} catch (e) {
  print(e);
}