erc721.sol

pragma solidity >=0.5.0 <0.6.0;

contract ERC721 {
  event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);
  event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

  //Using indexed

  /*
In order to filter events and only listen for changes related to the current user, our Solidity contract would have to use the indexed keyword, like we did in the Transfer event of our ERC721 implementation:

event Transfer(address indexed _from, address indexed _to, uint256 _tokenId);
In this case, because _from and _to are indexed, that means we can filter for them in our event listener in our front end:

// Use `filter` to only fire this code when `_to` equals `userAccount`
cryptoZombies.events.Transfer({ filter: { _to: userAccount } })
.on("data", function(event) {
  let data = event.returnValues;
  // The current user just received a zombie!
  // Do something here to update the UI to show it
}).on("error", console.error);
As you can see, using events and indexed fields can be quite a useful practice for listening to changes to your contract and reflecting them in your app's front-end.

*/

  function balanceOf(address _owner) external view returns (uint256);
  function ownerOf(uint256 _tokenId) external view returns (address);
  function transferFrom(address _from, address _to, uint256 _tokenId) external payable;
  function approve(address _approved, uint256 _tokenId) external payable;
}