Index

Chapter 11: Using an Interface

Continuing our previous example with NumberInterface, once we've defined the interface as:

contract NumberInterface {
function getNum(address _myAddress) public view returns (uint);
}

We can use it in a contract as follows:

contract MyContract {
  address NumberInterfaceAddress = 0xab38...
  // ^ The address of the FavoriteNumber contract on Ethereum
  NumberInterface numberContract = NumberInterface(NumberInterfaceAddress);
  // Now `numberContract` is pointing to the other contract

  function someFunction() public {
    // Now we can call `getNum` from that contract:
    uint num = numberContract.getNum(msg.sender);
    // ...and do something with `num` here
  }
}

In this way, your contract can interact with any other contract on the Ethereum blockchain, as long they expose those functions as public or external.

Put it to the test

Let's set up our contract to read from the CryptoKitties smart contract!
1. I've saved the address of the CryptoKitties contract in the code for you, under a variable named ckAddress. In the next line, create a KittyInterface named kittyContract, and initialize it with ckAddress — just like we did with numberContractabove.

address ckAddress = 0x06012c8cf97BEaD5deAe237070F9587f8E7A266d;

KittyInterface kittyContract = KittyInterface(ckAddress);

pragma solidity ^0.4.25;

import "./zombiefactory.sol";

contract KittyInterface {
  function getKitty(uint256 _id) external view returns (
    bool isGestating,
    bool isReady,
    uint256 cooldownIndex,
    uint256 nextActionAt,
    uint256 siringWithId,
    uint256 birthTime,
    uint256 matronId,
    uint256 sireId,
    uint256 generation,
    uint256 genes
  );
}

contract ZombieFeeding is ZombieFactory {

  address ckAddress = 0x06012c8cf97BEaD5deAe237070F9587f8E7A266d;

  KittyInterface kittyContract = KittyInterface(ckAddress);

  function feedAndMultiply(uint _zombieId, uint _targetDna) public {
    require(msg.sender == zombieToOwner[_zombieId]);
    Zombie storage myZombie = zombies[_zombieId];
    _targetDna = _targetDna % dnaModulus;
    uint newDna = (myZombie.dna + _targetDna) / 2;
    _createZombie("NoName", newDna);
  }

}