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The smart contracts that are executed by the EVM for our application were writ- ten using Solidity, the official programming language used to modify the Ethereum state. [31] For testing and user feedback purposes the smart contracts were deployed on an Ethereum testnet. This mimics a deployment on the main Ethereum network and allows us to measure the performance of the application and test its features. We have used the ERC-721 standard for interacting with NFT tokens on the Ethereum blockchain and we expect any ERC-721 tokens to be compatible with our application. 4.1.1 ERC-721 standard and OpenZeppelin We have covered what the ERC-721 standard means in the background section of the report. In this subsection we build on those explanations and discuss how ERC-721 tokens are implemented, as well as our usage of this type of token. ERC-721 is an open standard for smart contracts that is implemented by Non- Fungible Tokens (NFTs). Each ERC-721 token is a unique, scarce item and its interface specifies what functions should be implemented to create, interact and manage NFT tokens. They have different characteristics and each has a unique tokenId that identifies it in the list of NFTs created by the ERC-721 smart contract which was used for minting. [32] For the project we are using the OpenZeppelin contracts library. They have interfacesforERC-721smartcontractsandwewillfocusontwoofthem: IERC721 and . [34] is an interface with function definitions for building ERC-721 compli- ant smart contracts and we will use it when interacting with the addresses of the smart contracts of the tokens that we will be receiving. ERC-721 tokens have an owner address and a list of approved addresses that can initiate transfers. This is relevant in our case because we need our smart contracts to be able to transfer, receive and hold those tokens. We will be calling the approve function pointing to the address of our smart contract to become approved by the ERC-721 token smart contract. In case we need to transfer it we will call the safe transfer sending as parameters the to and from address, as well as the id of the NFT. In our case the address that the token will be sent to is a smart contract. There- fore, the receiving smart contract has to implement the IERC721Receiver interface. IERC721Receiver will be implemented by our smart contracts for security rea- sons and will indicate that we support handling of ERC-721 tokens, reassuring users that their assets will not be lost by being sent to an non-compliant smart contract. The function that we need to implement is onERC721Received which has to return a selector to acknowledge the receipt of the token, as shown in Listing 4.1. The selector is will acknowledge that our smart contract can handle receipt of ERC-721 tokens and is equivalent to the constant value or the value of IERC721Receiver IERC721 0x150b7a02 bytes4(keccak256("onERC721Received(address,address,uint256,bytes)")) . 1 function onERC721Received(address operator, address from, 35PDF Image | NFT finance Leveraging Non-Fungible Tokens
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