How to write a purchase function for an ERC721 using an ERC20?

Question

I am trying to write a function in our erc721 contract to purchase an nft token with an erc20 token. As I understand it, it is not possible to import erc20 into an erc721. Does that mean the purchase function goes into a separate contract by itself? In that case, how would we access the mint function from the erc721 contract?

I have searched all over google and stack exchange with "solidity purchase nft with erc20 token", and can't find anything. Which means i am probably missing something obvious

Answer 1

It is possible, but the flow/ux will be slightly different than buying from eth (where user send eth along the mint() transaction):

-user will have to approve (ie your frontend need to handle it) your erc721 contract as spender of his (the users') erc20

-your mint function will then call the erc20.transferFrom(user, you/your contract, the amount) and mint if successful.

function mint(uint256 quantity) external returns (bool) {
    require(erc20.transferFrom(msg.sender, address(this), price*quantity), "transfer error");
    _mint(quantity, user);
} //this is a mock and need further logic/control, esp if you accept third-party erc20