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README.md

Non Fungible Cats

Today's quest objective is to master Non Fungible Tokens, NFTs.

To represent a simple token, we used a smart contract with with each blockchain address amount of tokens. We talk about fungible token because each token has the same value. A Non-Fungible Token is a token with a unique identifier, usually part of a collection. NFTs are used to represent playing cards, works of art, financial securities and even physical objects.

Internally fungible token that we have seen in the prior quest are represented with a mapping from addresses to an amount:

Address    ---> Amount
0x2FE34         20

Non-Fungible Tokens are represented with a mapping of unique identifiers to an owner

Identifier ---> Address
123455          0x2FE34

In addition, each token is linked to an Uniform Resource Identifier (URI) where additional information about the NFT can be found, such as metadata, an image...

Identifier ---> Address
123455          0x2FE34
           ---> URI
                bafkreiajlq3

Content

Mandatory

  1. Napping Cats
  2. Showcase

Optional

  1. Automated Reveal

Exercise 1: Napping Cats

In this exercise we will create a basic NFT smart contract, and use it to instantiate an NFT collection with some custom functions

Instructions

Basic NFT

  • Create a smart contract basicNFT
  • Create a constructor function that takes a string that will be used as a name for the NFT.
  • Create a public function ownerOf() that associate a token to the address of its owner
  • Create a public function TokenURI() that associate a token to a resource
  • Create a function transfer(uint, address) that takes as parameters an identifier (a positive integer) and an address to allow the owner of a NFT to transfer it.

Optionally you can implement or reuse the full ERC721 standard

Napping cats

  • Create a smart contract Napping Cats that inherits from basicNFT
  • Its constructor function takes a new parameter, initial price, that fixes the initial sale price of the NFT collection
  • Instantiate the first three cats inside the smart contract with for each a json file (0.json, 1.json, 2.json) that contain a field name and a field image that points to a local or online image. NFTs initially don't have an owner

Trading functions

  • Add to Napping Cats a function listToken(uint256 id, uint256 price) that lists the token for sale at the proposed price
  • Add to Napping Cats a function buyToken(uint256 id), payable, that allows an user to buy a token
    • For the initial sale price defined by the constructor for new tokens
    • For the listed price by the function listToken

Optionally, the price for new tokens is an auction. It starts at 100x of the initial sale price or the last paid price for a new token and decreases over time to reach 0 after 24 hours.

Resources

Exercise 2: Showcase

We will now be able to display our NFT collection on a website and allow user to use the interface to buy and sell tokens.

Instructions

  • Reuse the Napping Cats smart contract from the previous exercise
  • Create a webpage index.html that connects to the napping Cats smart contract and displays the corresponding URIs as a table
  • Add a button to connect a wallet.
  • Offer a buy button connected to the buyToken
  • optionally : the buy button display the current price of this specific token
  • Once the wallet is connected, offer a sell token under the tokens owned by the user
    • The button should open a modal or include an input field to give a listing price.
  • provide a script and a guide to deploy the smart contract and serve the webpage

Resources

Exercise 3: Automated Reveal (Optional)

In some auctions, NFT images are only revealed once purchased.

For this we will create a node server that will serve the images only once they are purchased.

Instructions

  • Create a node server file index.js
  • Listen on port 3030 and connect to a local node 8545
  • Add the nappingCats exercise, modifying tokenURI to match the local node server
  • The server is launched with a smart contract address as parameter
  • For each request for an image X.jpg, only return it if the token has an owner.
  • Optionnally, using IPFS
    • The Json file contain the futur hash of tokens on IPFS
    • The server periodically monitor the smart contract for new owners
    • Once a new owner is found, the image is published on a local or remote IPFS server

Resources