# Test

By [BlacksmithTest](https://paragraph.com/@blacksmithtest) · 2022-04-24

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坚固

        function _checkOnERC721Received(
            address from,
            address to,
            uint256 tokenId,
            bytes memory _data
        ) private returns (bool) {
            if (to.isContract()) {
                try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) {
                    return retval == IERC721Receiver.onERC721Received.selector;
                } catch (bytes memory reason) {
                    if (reason.length == 0) {
                        revert("ERC721: transfer to non ERC721Receiver implementer");
                    } else {
                        assembly {
                            revert(add(32, reason), mload(reason))
                        }
                    }
                }
            } else {
                return true;
            }
        }
    

    library Address {
        
        function isContract(address account) internal view returns (bool) {
            // According to EIP-1052, 0x0 is the value returned for not-yet created accounts
            // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned
            // for accounts without code, i.e. `keccak256('')`
            bytes32 codehash;
            bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
            // solhint-disable-next-line no-inline-assembly
            assembly { codehash := extcodehash(account) }
            return (codehash != accountHash && codehash != 0x0);
        }
    }

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*Originally published on [BlacksmithTest](https://paragraph.com/@blacksmithtest/test)*