Take a percentage of msg.value on ERC721 transfer

Question

I want to transfer part of the value paid to an address whenever an NFT is traded. If the value is 0, then nothing is transferred.

My original idea was to override the transfer function in oppenzeppelin/ERC721, but its impossible to override a nonpayable function to payable.

Any way of doing this? The idea is for a DAO to make passive income every card trade.

Answer 1

You can make all the transfer functions private and add a fallback function which is payable. In the fallback function use msg.sig to determine which function to call. You can calculate the signatures like this.

bytes4 sigOne = bytes4(keccak256('safeTransferFrom(address,address,uint256)'))
bytes4 sigTwo = bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)'))

then you can implement the fallback function that calls the private transfer functions with arguments decoded. like so.

pragma solidity 0.6.6;

contract Test { 

    bytes4 sigOne = bytes4(keccak256('safeTransferFrom(address,address,uint256)'));
    bytes4 sigTwo = bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)'));

    fallback () external payable {
        require(msg.value > 0);
        // transfer amount to the address

        bytes memory payload = abi.encodePacked(bytes28(0), msg.data);
        bytes4 sig = msg.sig;

        if (sig == sigOne) {
            (
                bytes32 _,
                address from,
                address to,
                uint256 value,
            ) = abi.decode(payload, (bytes32, address, address, uint256));

            safeTransferFrom(from, to, value);
        } else if (sig == sigTwo) {
            (
                bytes32 _,
                address from,
                address to,
                uint256 value,
                bytes memory data,
            ) = abi.decode(payload, (bytes32, address, address, uint256, bytes));

            safeTransferFrom(from, to, value, data);
        } else {
            revert("signature did not match");
        }
    }
}

Hope it helps!