An example Solidity contract for "buyer pays" minting at the time of first sale.
This directory contains example code written for the [Lazy Minting guide on Solidity]. See the full article for context and details on how everything works!
The basic gist is that the gas cost to mint an NFT is pushed onto the buyer as part of the first sale transaction. This works by having the NFT creator sign a "voucher" containing the data that will be recorded into the blockchain when the NFT is minted.
The Solidity contract is in contracts/LazyNFT.sol. It defines an ERC721 smart contract that allows an NFT buyer to redeem a signed data structure called an NFTVoucher that represents an NFT.
In JavaScript, we have a LazyMinter class, defined in lib/LazyMinter.js. The LazyMinter is initialized with the address of the deployed contract and an ethers.js Signer that holds the NFT creator's private signing key. You can use the LazyMinter's createVoucher method to get an NFTVoucher object and signature that can be given to a buyer. The buyer can then present those to the redeem function to mint and purchase the NFT.
To turn an NFTVoucher into a real NFT, the buyer needs to present a valid signature that was produced by an account authorized to mint NFTs.
Signatures are produced according to EIP-712, the draft standard for creating signatures of typed, structured data. This allows wallets like MetaMask to present a "human readable" view of the data being signed, and it prevents some nasty classes of replay attack by "tying" signatures to a specific instance of a specific smart contract.
See the createVoucher method and its helpers in lib/LazyMinter.js to see how signature creation works. The validation in Solidity is done in the _hash and _verify private functions of the LazyNFT contract.
There is currently no "user interface", but you can run the unit tests:
npm install
npm run test> lazy-mint-example@1.0.0 test
> npx hardhat test
Compiling 15 files with 0.8.4
Compilation finished successfully
LazyNFT
✓ Should deploy (747ms)
✓ Should redeem an NFT from a signed voucher (163ms)
✓ Should fail to redeem an NFT that's already been claimed (138ms)
✓ Should fail to redeem an NFT voucher that's signed by an unauthorized account (90ms)
✓ Should fail to redeem an NFT voucher that's been modified (82ms)
✓ Should redeem if payment is >= minPrice (116ms)
✓ Should fail to redeem if payment is < minPrice (75ms)
✓ Should make payments available to minter for withdrawal (110ms)
8 passing (2s)