How to add royalty fee to NFT collection

Question

I am currently learning about NFTs and I reached the point where I deployed the contract and I can mint and everything going well. The only missing part is royalty fee on my art work. I've seen two methods:

• Adding royalty fee to the smart contract (which I am not sure how to do it)
• Managing royalty fees from opensea

Can someone please guide me here? Not sure which method is the best and more safe... I am running on Rinkeby testnetwork so I can modify the contract as much as I want in order to test. This is my current contract:

pragma solidity >=0.8.9 <0.9.0;

import 'erc721a/contracts/ERC721A.sol';
import '@openzeppelin/contracts/access/Ownable.sol';
import '@openzeppelin/contracts/utils/cryptography/MerkleProof.sol';
import '@openzeppelin/contracts/security/ReentrancyGuard.sol';

contract FalconContract is ERC721A, Ownable, ReentrancyGuard {

  using Strings for uint256;

  bytes32 public merkleRoot;
  mapping(address => bool) public whitelistClaimed;

  string public uriPrefix = '';
  string public uriSuffix = '.json';
  string public hiddenMetadataUri;
  
  uint256 public cost;
  uint256 public maxSupply;
  uint256 public maxMintAmountPerTx;

  bool public paused = true;
  bool public whitelistMintEnabled = false;
  bool public revealed = false;

  constructor(
    string memory _tokenName,
    string memory _tokenSymbol,
    uint256 _cost,
    uint256 _maxSupply,
    uint256 _maxMintAmountPerTx,
    string memory _hiddenMetadataUri
  ) ERC721A(_tokenName, _tokenSymbol) {
    setCost(_cost);
    maxSupply = _maxSupply;
    setMaxMintAmountPerTx(_maxMintAmountPerTx);
    setHiddenMetadataUri(_hiddenMetadataUri);
  }

  modifier mintCompliance(uint256 _mintAmount) {
    require(_mintAmount > 0 && _mintAmount <= maxMintAmountPerTx, 'Invalid mint amount!');
    require(totalSupply() + _mintAmount <= maxSupply, 'Max supply exceeded!');
    _;
  }

  modifier mintPriceCompliance(uint256 _mintAmount) {
    require(msg.value >= cost * _mintAmount, 'Insufficient funds!');
    _;
  }

  function whitelistMint(uint256 _mintAmount, bytes32[] calldata _merkleProof) public payable mintCompliance(_mintAmount) mintPriceCompliance(_mintAmount) {
    // Verify whitelist requirements
    require(whitelistMintEnabled, 'The whitelist sale is not enabled!');
    require(!whitelistClaimed[_msgSender()], 'Address already claimed!');
    bytes32 leaf = keccak256(abi.encodePacked(_msgSender()));
    require(MerkleProof.verify(_merkleProof, merkleRoot, leaf), 'Invalid proof!');

    whitelistClaimed[_msgSender()] = true;
    _safeMint(_msgSender(), _mintAmount);
  }

  function mint(uint256 _mintAmount) public payable mintCompliance(_mintAmount) mintPriceCompliance(_mintAmount) {
    require(!paused, 'The contract is paused!');

    _safeMint(_msgSender(), _mintAmount);

    payable(owner()).transfer(msg.value);
  }
  
  function mintForAddress(uint256 _mintAmount, address _receiver) public mintCompliance(_mintAmount) onlyOwner {
    _safeMint(_receiver, _mintAmount);
  }

  function walletOfOwner(address _owner) public view returns (uint256[] memory) {
    uint256 ownerTokenCount = balanceOf(_owner);
    uint256[] memory ownedTokenIds = new uint256[](ownerTokenCount);
    uint256 currentTokenId = _startTokenId();
    uint256 ownedTokenIndex = 0;
    address latestOwnerAddress;

    while (ownedTokenIndex < ownerTokenCount && currentTokenId <= maxSupply) {
      TokenOwnership memory ownership = _ownerships[currentTokenId];

      if (!ownership.burned && ownership.addr != address(0)) {
        latestOwnerAddress = ownership.addr;
      }

      if (latestOwnerAddress == _owner) {
        ownedTokenIds[ownedTokenIndex] = currentTokenId;

        ownedTokenIndex++;
      }

      currentTokenId++;
    }

    return ownedTokenIds;
  }

  function _startTokenId() internal view virtual override returns (uint256) {
    return 1;
  }

  function tokenURI(uint256 _tokenId) public view virtual override returns (string memory) {
    require(_exists(_tokenId), 'ERC721Metadata: URI query for nonexistent token');

    if (revealed == false) {
      return hiddenMetadataUri;
    }

    string memory currentBaseURI = _baseURI();
    return bytes(currentBaseURI).length > 0
        ? string(abi.encodePacked(currentBaseURI, _tokenId.toString(), uriSuffix))
        : '';
  }

  function setRevealed(bool _state) public onlyOwner {
    revealed = _state;
  }

  function setCost(uint256 _cost) public onlyOwner {
    cost = _cost;
  }

  function setMaxMintAmountPerTx(uint256 _maxMintAmountPerTx) public onlyOwner {
    maxMintAmountPerTx = _maxMintAmountPerTx;
  }

  function setHiddenMetadataUri(string memory _hiddenMetadataUri) public onlyOwner {
    hiddenMetadataUri = _hiddenMetadataUri;
  }

  function setUriPrefix(string memory _uriPrefix) public onlyOwner {
    uriPrefix = _uriPrefix;
  }

  function setUriSuffix(string memory _uriSuffix) public onlyOwner {
    uriSuffix = _uriSuffix;
  }

  function setPaused(bool _state) public onlyOwner {
    paused = _state;
  }

  function setMerkleRoot(bytes32 _merkleRoot) public onlyOwner {
    merkleRoot = _merkleRoot;
  }

  function setWhitelistMintEnabled(bool _state) public onlyOwner {
    whitelistMintEnabled = _state;
  }

  function withdraw() public onlyOwner nonReentrant {

    (bool hs, ) = payable(0x81C3E1d02c6ad3A1E065193238B40aB0c8E12D9b).call{value: address(this).balance * 50 / 100}('');
    require(hs);

    // This will transfer the remaining contract balance to the owner.
    // Do not remove this otherwise you will not be able to withdraw the funds.
    // =============================================================================
    (bool os, ) = payable(owner()).call{value: address(this).balance}('');
    require(os);
    // =============================================================================
  }

  function _baseURI() internal view virtual override returns (string memory) {
    return uriPrefix;
  }
}

Answer 1

This is a very well known and studied use case. There's an EIP that you can (and should) follow to have the best of the best in terms of security and operability, not to mention that you don't even need to code it. To be fair here, EIP is NOT a an ERC. It is a community request that to date of writting this answer is not approved nor officially backed by Ethereum.

I suggest reading: https://eips.ethereum.org/EIPS/eip-2981

Here you can read an example implementation: https://github.com/dievardump/EIP2981-implementation

Gas costs may be hefty, so be aware of that, maybe you can reduce on some places by sacrificing functionality.