Functions in generated dart code don't have correct return value

[Scott1803]

I have created a simple extension of an ERC-721Enumerable token using OpenZeppelin.

The contract includes a function called "addPost", that is supposed to mint the token and return the generated tokenId. The contract code, generated dart code and dart code calling the contract function is added below.

I used the code generation feature to convert the contract to a dart file, which worked fine. However, when I call the function, I could not figure out how to get the expected uint256 return value in dart. Instead, I get a string value starting in "AxB..." that presumably represents an ethereum address.

I'm fairly new to solidity and web3, so I may very well be missing something really obvious.. Any help would be greatly appreciated!

Solidity Contract Code:

// SPDX-License-Identifier: MIT
pragma solidity >=0.4.22 <0.9.0;

//Import contracts to extend upon from open zeppelin project
import "@openzeppelin/contracts/token/ERC721/Extensions/ERC721URIStorage.sol";
import "@openzeppelin/contracts/token/ERC721/Extensions/ERC721Enumerable.sol";

import "@openzeppelin/contracts/utils/Counters.sol";

contract CustomImageToken is ERC721URIStorage {
    using Counters for Counters.Counter;

    //This keeps track and auto increments the id's for the image tokens
    Counters.Counter private _tokenIds;

    //This stores the IPFS image hashes for each image token.
    mapping(string => bool) hashes;

    constructor() ERC721("CustomImageToken", "CIT") {}

    function addPost(
        address _to,
        string memory _ipfsHash,
        string memory _metaLink
    ) public returns (uint256) {
        //Ensure that the image hash doesn't exist yet!
        require(!hashes[_ipfsHash]);

        //Set the new hash to "used"
        hashes[_ipfsHash] = true;

        //Increment the token ID's to create a new one for the new token
        _tokenIds.increment();

        uint256 newItemId = _tokenIds.current();

        //"Mint" (create) the token on the blockchain using its generated ID and assign it to the owner address.
        _mint(_to, newItemId);

        //Set the url to the tokens meta data
        _setTokenURI(newItemId, _metaLink);

        return newItemId;
    }
}

Generated Dart Code:

// Generated code, do not modify. Run `build_runner build` to re-generate!
// @dart=2.12
import 'package:web3dart/web3dart.dart' as _i1;
import 'dart:typed_data' as _i2;

final _contractAbi = _i1.ContractAbi.fromJson(
    '[{"inputs":[],"stateMutability":"nonpayable","type":"constructor"},{"anonymous":false,"inputs":[{"indexed":true,"internalType":"address","name":"owner","type":"address"},{"indexed":true,"internalType":"address","name":"approved","type":"address"},{"indexed":true,"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"Approval","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"internalType":"address","name":"owner","type":"address"},{"indexed":true,"internalType":"address","name":"operator","type":"address"},{"indexed":false,"internalType":"bool","name":"approved","type":"bool"}],"name":"ApprovalForAll","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"internalType":"address","name":"from","type":"address"},{"indexed":true,"internalType":"address","name":"to","type":"address"},{"indexed":true,"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"Transfer","type":"event"},{"inputs":[{"internalType":"address","name":"to","type":"address"},{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"approve","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"address","name":"owner","type":"address"}],"name":"balanceOf","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"getApproved","outputs":[{"internalType":"address","name":"","type":"address"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[{"internalType":"address","name":"owner","type":"address"},{"internalType":"address","name":"operator","type":"address"}],"name":"isApprovedForAll","outputs":[{"internalType":"bool","name":"","type":"bool"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[],"name":"name","outputs":[{"internalType":"string","name":"","type":"string"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"ownerOf","outputs":[{"internalType":"address","name":"","type":"address"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[{"internalType":"address","name":"from","type":"address"},{"internalType":"address","name":"to","type":"address"},{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"safeTransferFrom","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"address","name":"from","type":"address"},{"internalType":"address","name":"to","type":"address"},{"internalType":"uint256","name":"tokenId","type":"uint256"},{"internalType":"bytes","name":"_data","type":"bytes"}],"name":"safeTransferFrom","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"address","name":"operator","type":"address"},{"internalType":"bool","name":"approved","type":"bool"}],"name":"setApprovalForAll","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"bytes4","name":"interfaceId","type":"bytes4"}],"name":"supportsInterface","outputs":[{"internalType":"bool","name":"","type":"bool"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[],"name":"symbol","outputs":[{"internalType":"string","name":"","type":"string"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"tokenURI","outputs":[{"internalType":"string","name":"","type":"string"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[{"internalType":"address","name":"from","type":"address"},{"internalType":"address","name":"to","type":"address"},{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"transferFrom","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"address","name":"_to","type":"address"},{"internalType":"string","name":"_ipfsHash","type":"string"},{"internalType":"string","name":"_metaLink","type":"string"}],"name":"addPost","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"nonpayable","type":"function"}]',
    'CustomImageToken');

class CustomImageToken extends _i1.GeneratedContract {
  CustomImageToken(
      {required _i1.EthereumAddress address,
      required _i1.Web3Client client,
      int? chainId})
      : super(_i1.DeployedContract(_contractAbi, address), client, chainId);

  /// See {IERC721-approve}.
  ///
  /// The optional [transaction] parameter can be used to override parameters
  /// like the gas price, nonce and max gas. The `data` and `to` fields will be
  /// set by the contract.
  Future<String> approve(_i1.EthereumAddress to, BigInt tokenId,
      {required _i1.Credentials credentials,
      _i1.Transaction? transaction}) async {
    final function = self.abi.functions[1];
    assert(checkSignature(function, '095ea7b3'));
    final params = [to, tokenId];
    return write(credentials, transaction, function, params);
  }

  /// See {IERC721-balanceOf}.
  ///
  /// The optional [atBlock] parameter can be used to view historical data. When
  /// set, the function will be evaluated in the specified block. By default, the
  /// latest on-chain block will be used.
  Future<BigInt> balanceOf(_i1.EthereumAddress owner,
      {_i1.BlockNum? atBlock}) async {
    final function = self.abi.functions[2];
    assert(checkSignature(function, '70a08231'));
    final params = [owner];
    final response = await read(function, params, atBlock);
    return (response[0] as BigInt);
  }

  /// See {IERC721-getApproved}.
  ///
  /// The optional [atBlock] parameter can be used to view historical data. When
  /// set, the function will be evaluated in the specified block. By default, the
  /// latest on-chain block will be used.
  Future<_i1.EthereumAddress> getApproved(BigInt tokenId,
      {_i1.BlockNum? atBlock}) async {
    final function = self.abi.functions[3];
    assert(checkSignature(function, '081812fc'));
    final params = [tokenId];
    final response = await read(function, params, atBlock);
    return (response[0] as _i1.EthereumAddress);
  }

  /// See {IERC721-isApprovedForAll}.
  ///
  /// The optional [atBlock] parameter can be used to view historical data. When
  /// set, the function will be evaluated in the specified block. By default, the
  /// latest on-chain block will be used.
  Future<bool> isApprovedForAll(
      _i1.EthereumAddress owner, _i1.EthereumAddress operator,
      {_i1.BlockNum? atBlock}) async {
    final function = self.abi.functions[4];
    assert(checkSignature(function, 'e985e9c5'));
    final params = [owner, operator];
    final response = await read(function, params, atBlock);
    return (response[0] as bool);
  }

  /// See {IERC721Metadata-name}.
  ///
  /// The optional [atBlock] parameter can be used to view historical data. When
  /// set, the function will be evaluated in the specified block. By default, the
  /// latest on-chain block will be used.
  Future<String> name({_i1.BlockNum? atBlock}) async {
    final function = self.abi.functions[5];
    assert(checkSignature(function, '06fdde03'));
    final params = [];
    final response = await read(function, params, atBlock);
    return (response[0] as String);
  }

  /// See {IERC721-ownerOf}.
  ///
  /// The optional [atBlock] parameter can be used to view historical data. When
  /// set, the function will be evaluated in the specified block. By default, the
  /// latest on-chain block will be used.
  Future<_i1.EthereumAddress> ownerOf(BigInt tokenId,
      {_i1.BlockNum? atBlock}) async {
    final function = self.abi.functions[6];
    assert(checkSignature(function, '6352211e'));
    final params = [tokenId];
    final response = await read(function, params, atBlock);
    return (response[0] as _i1.EthereumAddress);
  }

  /// See {IERC721-safeTransferFrom}.
  ///
  /// The optional [transaction] parameter can be used to override parameters
  /// like the gas price, nonce and max gas. The `data` and `to` fields will be
  /// set by the contract.
  Future<String> safeTransferFrom(
      _i1.EthereumAddress from, _i1.EthereumAddress to, BigInt tokenId,
      {required _i1.Credentials credentials,
      _i1.Transaction? transaction}) async {
    final function = self.abi.functions[7];
    assert(checkSignature(function, '42842e0e'));
    final params = [from, to, tokenId];
    return write(credentials, transaction, function, params);
  }

  /// See {IERC721-safeTransferFrom}.
  ///
  /// The optional [transaction] parameter can be used to override parameters
  /// like the gas price, nonce and max gas. The `data` and `to` fields will be
  /// set by the contract.
  Future<String> safeTransferFrom$2(_i1.EthereumAddress from,
      _i1.EthereumAddress to, BigInt tokenId, _i2.Uint8List _data,
      {required _i1.Credentials credentials,
      _i1.Transaction? transaction}) async {
    final function = self.abi.functions[8];
    assert(checkSignature(function, 'b88d4fde'));
    final params = [from, to, tokenId, _data];
    return write(credentials, transaction, function, params);
  }

  /// See {IERC721-setApprovalForAll}.
  ///
  /// The optional [transaction] parameter can be used to override parameters
  /// like the gas price, nonce and max gas. The `data` and `to` fields will be
  /// set by the contract.
  Future<String> setApprovalForAll(_i1.EthereumAddress operator, bool approved,
      {required _i1.Credentials credentials,
      _i1.Transaction? transaction}) async {
    final function = self.abi.functions[9];
    assert(checkSignature(function, 'a22cb465'));
    final params = [operator, approved];
    return write(credentials, transaction, function, params);
  }

  /// See {IERC165-supportsInterface}.
  ///
  /// The optional [atBlock] parameter can be used to view historical data. When
  /// set, the function will be evaluated in the specified block. By default, the
  /// latest on-chain block will be used.
  Future<bool> supportsInterface(_i2.Uint8List interfaceId,
      {_i1.BlockNum? atBlock}) async {
    final function = self.abi.functions[10];
    assert(checkSignature(function, '01ffc9a7'));
    final params = [interfaceId];
    final response = await read(function, params, atBlock);
    return (response[0] as bool);
  }

  /// See {IERC721Metadata-symbol}.
  ///
  /// The optional [atBlock] parameter can be used to view historical data. When
  /// set, the function will be evaluated in the specified block. By default, the
  /// latest on-chain block will be used.
  Future<String> symbol({_i1.BlockNum? atBlock}) async {
    final function = self.abi.functions[11];
    assert(checkSignature(function, '95d89b41'));
    final params = [];
    final response = await read(function, params, atBlock);
    return (response[0] as String);
  }

  /// See {IERC721Metadata-tokenURI}.
  ///
  /// The optional [atBlock] parameter can be used to view historical data. When
  /// set, the function will be evaluated in the specified block. By default, the
  /// latest on-chain block will be used.
  Future<String> tokenURI(BigInt tokenId, {_i1.BlockNum? atBlock}) async {
    final function = self.abi.functions[12];
    assert(checkSignature(function, 'c87b56dd'));
    final params = [tokenId];
    final response = await read(function, params, atBlock);
    return (response[0] as String);
  }

  /// See {IERC721-transferFrom}.
  ///
  /// The optional [transaction] parameter can be used to override parameters
  /// like the gas price, nonce and max gas. The `data` and `to` fields will be
  /// set by the contract.
  Future<String> transferFrom(
      _i1.EthereumAddress from, _i1.EthereumAddress to, BigInt tokenId,
      {required _i1.Credentials credentials,
      _i1.Transaction? transaction}) async {
    final function = self.abi.functions[13];
    assert(checkSignature(function, '23b872dd'));
    final params = [from, to, tokenId];
    return write(credentials, transaction, function, params);
  }

  /// The optional [transaction] parameter can be used to override parameters
  /// like the gas price, nonce and max gas. The `data` and `to` fields will be
  /// set by the contract.
  Future<String> addPost(
      _i1.EthereumAddress _to, String _ipfsHash, String _metaLink,
      {required _i1.Credentials credentials,
      _i1.Transaction? transaction}) async {
    final function = self.abi.functions[14];
    assert(checkSignature(function, 'c264ea43'));
    final params = [_to, _ipfsHash, _metaLink];
    return write(credentials, transaction, function, params);
  }

  /// Returns a live stream of all Approval events emitted by this contract.
  Stream<Approval> approvalEvents(
      {_i1.BlockNum? fromBlock, _i1.BlockNum? toBlock}) {
    final event = self.event('Approval');
    final filter = _i1.FilterOptions.events(
        contract: self, event: event, fromBlock: fromBlock, toBlock: toBlock);
    return client.events(filter).map((_i1.FilterEvent result) {
      final decoded = event.decodeResults(result.topics!, result.data!);
      return Approval(decoded);
    });
  }

  /// Returns a live stream of all ApprovalForAll events emitted by this contract.
  Stream<ApprovalForAll> approvalForAllEvents(
      {_i1.BlockNum? fromBlock, _i1.BlockNum? toBlock}) {
    final event = self.event('ApprovalForAll');
    final filter = _i1.FilterOptions.events(
        contract: self, event: event, fromBlock: fromBlock, toBlock: toBlock);
    return client.events(filter).map((_i1.FilterEvent result) {
      final decoded = event.decodeResults(result.topics!, result.data!);
      return ApprovalForAll(decoded);
    });
  }

  /// Returns a live stream of all Transfer events emitted by this contract.
  Stream<Transfer> transferEvents(
      {_i1.BlockNum? fromBlock, _i1.BlockNum? toBlock}) {
    final event = self.event('Transfer');
    final filter = _i1.FilterOptions.events(
        contract: self, event: event, fromBlock: fromBlock, toBlock: toBlock);
    return client.events(filter).map((_i1.FilterEvent result) {
      final decoded = event.decodeResults(result.topics!, result.data!);
      return Transfer(decoded);
    });
  }
}

class Approval {
  Approval(List<dynamic> response)
      : owner = (response[0] as _i1.EthereumAddress),
        approved = (response[1] as _i1.EthereumAddress),
        tokenId = (response[2] as BigInt);

  final _i1.EthereumAddress owner;

  final _i1.EthereumAddress approved;

  final BigInt tokenId;
}

class ApprovalForAll {
  ApprovalForAll(List<dynamic> response)
      : owner = (response[0] as _i1.EthereumAddress),
        operator = (response[1] as _i1.EthereumAddress),
        approved = (response[2] as bool);

  final _i1.EthereumAddress owner;

  final _i1.EthereumAddress operator;

  final bool approved;
}

class Transfer {
  Transfer(List<dynamic> response)
      : from = (response[0] as _i1.EthereumAddress),
        to = (response[1] as _i1.EthereumAddress),
        tokenId = (response[2] as BigInt);

  final _i1.EthereumAddress from;

  final _i1.EthereumAddress to;

  final BigInt tokenId;
}

Dart Code calling the "addPost" method in the contract:

  Future addImagePost(
      BuildContext context, XFile image, double etherPrice) async {
    final contract =
        CustomImageToken(address: _fanBytesImageAddress, client: _web3Client);

    final ethAddress = await _credentials.extractAddress();
    final pinataResponse = await PinataService.pinFileToIPFS(
        filePath: image.path, creatorAddress: ethAddress.hex);

    if (pinataResponse != null) {
      //TokenID is expected to be a num value for the tokenId, but it's an
      //address string instead.
      final tokenId = await contract.addPost(
        ethAddress,
        pinataResponse.imageHash!,
        pinataResponse.metaHash!,
        credentials: _credentials,
      );

      return tokenId;
    }
    return -1;
  }

[simolus3]

The reason is that addPost is mutable: It changes the contract's state which requires a transaction on the blockchain.

Calling addPost will return the transaction's hash, which can be used to obtain more information after the transaction has been included in a block. AFAIK, there's no good way to return values from a mutable function (see e.g. this) outside of using events.