ERC721 – Smart Contract Transfer Tokens as the Contract Owners

erc-721

This is the hypothetic use case I came up to learn about smart contract programming.

I am trying to create a ERC721 Smart Contract that would mint tokens for photographers who want to sell their work. The photographer uploads a picture and I put it somewhere and include that link as the tokenURI for my UniquePhotoNFT.

When I mint a new token of UniquePhotoNFT, it has an ID (1,2,3,4, etc) with all different token URIs. All those tokens belong to me as I am holding all those tokens for my users (so they don't need to deal with any of the blockchain staff).

When someone comes to buy a UniquePhotoNFT with token id 3, I can easily transfer that token 3 to the buyer's address. However, if the new owner decides to sell the same token to someone else, how can I make the transfer on their behalf? It won't let me do it because as the contract owner, I am not the token owner.

Best Answer

The EIP-721 provides a function isApprovedForAll(address owner, address operator)

Query if an address is an authorized operator for another address

That function can be used enable a special address to always be authorized to operate on the NFTs.

Related Solutions

Solidity – Proper Way to Implement ‘Buyable’ ERC721 Tokens

So I ended up going with a solution that used my contract as an intermediate third party. The flow goes as follows:

The seller approves the contract address to act on their behalf.
Then, when the buyer wants to buy the token, the contract transfers the token from the buyer to the seller, and transfers the payment as well

Something along the lines of:

function setForSale(uint256 _tokenId) external {
    address owner = ownerOf(_tokenId);

    require(isValidToken(_tokenId));
    require(owner == msg.sender || authorized[owner][msg.sender]);

    allowance[_tokenId] = address(this);
    tokensForSale.push(_tokenId);
    // set the sale price etc

    emit Approval(owner, address(this), _tokenId);
}

function buy(uint256 _tokenId) external payable {
    address buyer = msg.sender;
    uint payedPrice = msg.value;

    require(isValidToken(_tokenId));
    require(getApproved(_tokenId) == address(this));
    // require payedPrice >= salePrice

    // pay the seller
    // remove token from tokensForSale

    transferFrom(ownerOf(_tokenId), buyer, _tokenId);
}

Obviously you can do this with a third-party contract, but I found this easy and fairly straightforward using the given interface.

[Ethereum] How to import files into remix (ERC721 contract)

import "github.com/OpenZeppelin/openzeppelin-solidity/contracts/token/ERC721/IERC721.sol";
import "github.com/OpenZeppelin/openzeppelin-solidity/contracts/token/ERC721/IERC721Receiver.sol";
import "github.com/OpenZeppelin/openzeppelin-solidity/contracts/math/SafeMath.sol";
import "github.com/OpenZeppelin/openzeppelin-solidity/contracts/utils/Address.sol";
import "github.com/OpenZeppelin/openzeppelin-solidity/contracts/drafts/Counters.sol";
import "github.com/OpenZeppelin/openzeppelin-solidity/contracts/introspection/ERC165.sol";

This compiles. I am not sure about the v2.0.0 on counter. I replaced that with master.

Best Answer

Related Solutions

Solidity – Proper Way to Implement ‘Buyable’ ERC721 Tokens

[Ethereum] How to import files into remix (ERC721 contract)

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