[ERC1948] NFTに付随するデータを変更できる仕組みを理解しよう！

pragma solidity ^0.5.2;

/**
 * @dev Interface of the ERC1948 contract.
 */
interface IERC1948 {

  /**
   * @dev Emitted when `oldData` is replaced with `newData` in storage of `tokenId`.
   *
   * Note that `oldData` or `newData` may be empty bytes.
   */
  event DataUpdated(uint256 indexed tokenId, bytes32 oldData, bytes32 newData);

  /**
   * @dev Reads the data of a specified token. Returns the current data in
   * storage of `tokenId`.
   *
   * @param tokenId The token to read the data off.
   *
   * @return A bytes32 representing the current data stored in the token.
   */
  function readData(uint256 tokenId) external view returns (bytes32);

  /**
   * @dev Updates the data of a specified token. Writes `newData` into storage
   * of `tokenId`.
   *
   * @param tokenId The token to write data to.
   * @param newData The data to be written to the token.
   *
   * Emits a `DataUpdated` event.
   */
  function writeData(uint256 tokenId, bytes32 newData) external;

}

event DataUpdated(uint256 indexed tokenId, bytes32 oldData, bytes32 newData);

function readData(uint256 tokenId) external view returns (bytes32);

function writeData(uint256 tokenId, bytes32 newData) external;

const ERC1948 = artifacts.require('./ERC1948.sol');

contract('ERC1948', (accounts) => {
  const firstTokenId = 100;
  const empty = '0x0000000000000000000000000000000000000000000000000000000000000000';
  const data = '0x0101010101010101010101010101010101010101010101010101010101010101';
  let dataToken;

  beforeEach(async () => {
    dataToken = await ERC1948.new();
    await dataToken.mint(accounts[0], firstTokenId);
  });

  it('should allow to write and read', async () => {
    let rsp = await dataToken.readData(firstTokenId);
    assert.equal(rsp, empty);
    await dataToken.writeData(firstTokenId, data);
    rsp = await dataToken.readData(firstTokenId);
    assert.equal(rsp, data);
  });

});

/**
 * @dev Implementation of ERC721 token and the `IERC1948` interface.
 *
 * ERC1948 is a non-fungible token (NFT) extended with the ability to store
 * dynamic data. The data is a bytes32 field for each tokenId. If 32 bytes
 * do not suffice to store the data, an authenticated data structure (hash or
 * merkle tree) shall be used.
 */
contract ERC1948 is IERC1948, ERC721 {

  mapping(uint256 => bytes32) data;

  /**
   * @dev See `IERC1948.readData`.
   *
   * Requirements:
   *
   * - `tokenId` needs to exist.
   */
  function readData(uint256 tokenId) external view returns (bytes32) {
    require(_exists(tokenId));
    return data[tokenId];
  }

  /**
   * @dev See `IERC1948.writeData`.
   *
   * Requirements:
   *
   * - `msg.sender` needs to be owner of `tokenId`.
   */
  function writeData(uint256 tokenId, bytes32 newData) external {
    require(msg.sender == ownerOf(tokenId));
    emit DataUpdated(tokenId, data[tokenId], newData);
    data[tokenId] = newData;
  }

}