如何使用铸币费创建智能合约

Question

I have an nft market place just like opensea running. I want to charge any artist minting on the site, minting fees of like %5 per item been minted, just like opensea and barkeryswap nft platform.

My Market Place code:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "./NFTCollection.sol";

contract NFTMarketplace {
  uint count;
  uint public offerCount;
  mapping (uint => _Offer) public offers;
  mapping (address => uint) public userFunds;
  mapping(uint => Seller) public sellers;
  NFTCollection nftCollection;
  
  struct _Offer {
    uint offerId;
    uint id;
    address user;
    uint price;
    bool fulfilled;
    bool cancelled;
  }

  struct Seller {
       address userAddres;
       uint balance;
   }

  event Offer(
    uint offerId,
    uint id,
    address user,
    uint price,
    bool fulfilled,
    bool cancelled
  );

  event OfferFilled(uint offerId, uint id, address newOwner);
  event OfferCancelled(uint offerId, uint id, address owner);
  event ClaimFunds(address user, uint amount);

  constructor(address _nftCollection) {
    nftCollection = NFTCollection(_nftCollection);
  }
  
  function makeOffer(uint _id, uint _price) public {
    nftCollection.transferFrom(msg.sender, address(this), _id);
    offerCount ++;
    offers[offerCount] = _Offer(offerCount, _id, msg.sender, _price, false, false);
    emit Offer(offerCount, _id, msg.sender, _price, false, false);
  }

  function fillOffer(uint _offerId) public payable {
    _Offer storage _offer = offers[_offerId];
    require(_offer.offerId == _offerId, 'The offer must exist');
    require(_offer.user != msg.sender, 'The owner of the offer cannot fill it');
    require(!_offer.fulfilled, 'An offer cannot be fulfilled twice');
    require(!_offer.cancelled, 'A cancelled offer cannot be fulfilled');
    require(msg.value == _offer.price, 'The BNB amount should match with the NFT Price');
    nftCollection.transferFrom(address(this), msg.sender, _offer.id);
    _offer.fulfilled = true;
    userFunds[_offer.user] += msg.value;
    sellers[count].userAddres = _offer.user;
    sellers[count].balance = msg.value;
    nftCollection.setTrack(msg.sender, _offer.id);
    count++;
    emit OfferFilled(_offerId, _offer.id, msg.sender);
  }

  function cancelOffer(uint _offerId) public {
    _Offer storage _offer = offers[_offerId];
    require(_offer.offerId == _offerId, 'The offer must exist');
    require(_offer.user == msg.sender, 'The offer can only be canceled by the owner');
    require(_offer.fulfilled == false, 'A fulfilled offer cannot be cancelled');
    require(_offer.cancelled == false, 'An offer cannot be cancelled twice');
    nftCollection.transferFrom(address(this), msg.sender, _offer.id);
    _offer.cancelled = true;
    emit OfferCancelled(_offerId, _offer.id, msg.sender);
  }

  function claimFunds() public {
    require(userFunds[msg.sender] > 0, 'This user has no funds to be claimed');
    payable(msg.sender).transfer(userFunds[msg.sender]);
    emit ClaimFunds(msg.sender, userFunds[msg.sender]);
    userFunds[msg.sender] = 0;    
  }

  function getSellers() public view returns (address[] memory, uint[] memory){
       address[] memory userAddress = new address[](count);
       uint[] memory balances = new uint[](count);

       for(uint i = 0; i < count; i++){
           userAddress[i] = sellers[i].userAddres;
           balances[i] = sellers[i].balance;
       }
       return (userAddress, balances);
   }

  // Fallback: reverts if Ether is sent to this smart-contract by mistake
  fallback () external {
    revert();
  }
}

My question here is, how do I modify the marketplace code to add minting fees let say %5 per item been mint? OR, Let say charge $5 before uploading an item. Can you help with sample code please?

My NFT COLLECTION CODE

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "../client/node_modules/@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "../client/node_modules/@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";

contract NFTCollection is ERC721, ERC721Enumerable {
  string[] public tokenURIs;
  mapping(string => bool) _tokenURIExists;
  mapping(uint => string) _tokenIdToTokenURI;
  mapping(uint => address[]) _itemTrack;

  constructor() 
    ERC721("MTL Collection", "MTL") 
  {
  }

  function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal override(ERC721, ERC721Enumerable) {
    super._beforeTokenTransfer(from, to, tokenId);
  }

  function supportsInterface(bytes4 interfaceId) public view override(ERC721, ERC721Enumerable) returns (bool) {
    return super.supportsInterface(interfaceId);
  }

  function tokenURI(uint256 tokenId) public override view returns (string memory) {
    require(_exists(tokenId), 'ERC721Metadata: URI query for nonexistent token');
    return _tokenIdToTokenURI[tokenId];
  }

  function safeMint(string memory _tokenURI) public {
    require(!_tokenURIExists[_tokenURI], 'The token URI should be unique');
    tokenURIs.push(_tokenURI);    
    uint _id = tokenURIs.length;
    _tokenIdToTokenURI[_id] = _tokenURI;
    setTrack(msg.sender, _id);
    _safeMint(msg.sender, _id);
    _tokenURIExists[_tokenURI] = true;
  }

    function setTrack(address _address, uint _id) public returns(bool){
        _itemTrack[_id].push(_address);
        return true;
    }

    function getTrack(uint _id) public view returns(address[] memory){
        address[] memory users;
        users = _itemTrack[_id];
       return users;

    }
}

My Migration Code:

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

contract Migrations {
  address public owner = msg.sender;
  uint public last_completed_migration;

  modifier restricted() {
    require(
      msg.sender == owner,
      "This function is restricted to the contract's owner"
    );
    _;
  }

  function setCompleted(uint completed) public restricted {
    last_completed_migration = completed;
  }
}

My question here is, how do I modify the marketplace code to add minting fees let say %5 per item been mint? OR, Let say charge $5 before uploading an item. Can you help with sample code please?

Answer 1

In my opinion, you should write it on the Smart Contract. because I think it would be great if everyone can see how much a fee is for now (transparency) and avoid human error to set fees from the code.

Answer 2

You set it on blockchain:

  uint public listingPrice=0.025 ether;

and when you mint token and when buyer wants to put it on sale again, you add a require statement:

require(msg.value==listingPrice,"Price must be equal to listing fee");

You can write a function to change the value in the future

function setListingPrice(uint newPrice) external onlyOwner{
    require(newPrice>0,"Price must be at least 1 wei");
    listingPrice=newPrice;
  }