How to link my IPFS content to ERC721 contract?

Question

I´m trying to make a simple ERC721 NFT minting contract. I created an image and its corresponding metadata and I´ve uploaded them to the ipfs. When does this image and metadata link with the token created in the smart contract?. I was trying to use this code generated with the Openzeppelin contract wizard:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/utils/Counters.sol";

contract MyToken is ERC721, Ownable {
    using Counters for Counters.Counter;

    Counters.Counter private _tokenIdCounter;

    constructor() ERC721("MyToken", "MTK") {}

    function safeMint(address to) public onlyOwner {
        uint256 tokenId = _tokenIdCounter.current();
        _tokenIdCounter.increment();
        _safeMint(to, tokenId);
    }
}

But I don't see where I can link my nft image and metadata to the contract. I´ve seen that in ERC721 standart by openzeppelin you can set the base URI and tokenURI with the functions tokenURI and _baseURI , but I don't know exactly how to use them. I was planning to create a multiple-item collection (in the ipfs) so I don't know what to use in my case.

Answer 1

The function you wrote is not enough. When you work with NFT, you have to set some state variables. You have to keep track of the listed items, token ids, used token urls. (your contract logic might be different). Also, you have to add some require statements for validation before minting a token. first define your state variables:

 using Counters for Counters.Counter;
  // initially 0
  Counters.Counter private _listedItems;
  Counters.Counter private _tokenIds;
  uint public listingPrice=0.025 ether;

  // mapping is similar to object in javascript
  mapping(string=>bool) private _usedTokenURIs;

then write the minting logic:

function mintToken(string memory tokenURI,uint price) public payable returns (uint){
    // make sure you do not mint same uri
    require( _usedTokenURIs[tokenURI]==false,"Token URI already exists");
    // since this function is payable, the amount that you sent is stored in msg.value by ethereum evm
    require(msg.value==listingPrice,"Price must be equal to listing fee");
    _tokenIds.increment();
    _listedItems.increment();
    uint newTokenId=_tokenIds.current();
    // this is a wrapper for _mint
    _safeMint(msg.sender,newTokenId);
    _setTokenURI(newTokenId, tokenURI);
    _createNftItem(newTokenId,price);
    // update _usedTokenURIs mapping
    _usedTokenURIs[tokenURI]=true;
    return newTokenId;
  }

Now to store the ipfs link in the blockchain you have to call mintToken function on the front end. You create a form, add inputs related to the metadata, and post the json metadata and image to the ipfs. When you post the data you get the nft uri. Store the nft uri in the state. Then you write a createNft function on the front end. Once you successfully call the contract.mintToken function, your ipfs link will be stored on blockchain.