ERC721Receiver

ERC721Receiver is the standard interface that contracts may implement to receive ERC-721 tokens safely. It is part of the ERC-721 standard on Ethereum and addresses the risk that tokens could be sent to contracts that do not know how to handle them. The interface defines a single function: onERC721Received(address operator, address from, uint256 tokenId, bytes data) external returns (bytes4).

During a safe transfer, when an ERC-721 token is sent to a contract, the token contract checks whether the recipient is a smart contract. If it is, it calls onERC721Received on the recipient. If the recipient contract implements IERC721Receiver and returns the correct magic value, the transfer proceeds; otherwise the transfer is reverted. The required return value is the function selector bytes4(keccak256("onERC721Received(address,address,uint256,bytes)")), which equals 0x150b7a02. In addition to implementing the function, contracts typically expose ERC-165 support by returning true for supportsInterface(0x150b7a02), allowing callers to detect IERC721Receiver support.

Implementers should consider the interaction’s implications, as the onERC721Received call occurs during the transfer and may involve external contract code. This mechanism prevents tokens from being inadvertently locked in contracts that cannot handle ERC-721 tokens, and it is widely used by marketplaces, wallets, and other smart contracts that hold NFTs on behalf of users.

See also: ERC-721, ERC-165, safeTransferFrom.