Index

Chapter 3: onlyOwner Function Modifier

Now that our base contract ZombieFactory inherits from Ownable, we can use the onlyOwner function modifier in ZombieFeeding as well.
This is because of how contract inheritance works. Remember:

ZombieFeeding is ZombieFactory
ZombieFactory is Ownable

Thus ZombieFeeding is also Ownable, and can access the functions / events / modifiers from the Ownable contract. This applies to any contracts that inherit from ZombieFeeding in the future as well.

Function Modifiers

A function modifier looks just like a function, but uses the keyword modifier instead of the keyword function. And it can't be called directly like a function can — instead we can attach the modifier's name at the end of a function definition to change that function's behavior.
Let's take a closer look by examining onlyOwner:

pragma solidity ^0.4.25;

/**
* @title Ownable
* @dev The Ownable contract has an owner address, and provides basic authorization control
* functions, this simplifies the implementation of "user permissions".
*/
contract Ownable {
  address private _owner;

  event OwnershipTransferred(
    address indexed previousOwner,
    address indexed newOwner
  );

  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   */
  constructor() internal {
    _owner = msg.sender;
    emit OwnershipTransferred(address(0), _owner);
  }

  /**
   * @return the address of the owner.
   */
  function owner() public view returns(address) {
    return _owner;
  }

  /**
   * @dev Throws if called by any account other than the owner.
   */
  modifier onlyOwner() {
    require(isOwner());
    _;
  }

  /**
   * @return true if `msg.sender` is the owner of the contract.
   */
  function isOwner() public view returns(bool) {
    return msg.sender == _owner;
  }

  /**
   * @dev Allows the current owner to relinquish control of the contract.
   * @notice Renouncing to ownership will leave the contract without an owner.
   * It will not be possible to call the functions with the `onlyOwner`
   * modifier anymore.
   */
  function renounceOwnership() public onlyOwner {
    emit OwnershipTransferred(_owner, address(0));
    _owner = address(0);
  }

  /**
   * @dev Allows the current owner to transfer control of the contract to a newOwner.
   * @param newOwner The address to transfer ownership to.
   */
  function transferOwnership(address newOwner) public onlyOwner {
    _transferOwnership(newOwner);
  }

  /**
   * @dev Transfers control of the contract to a newOwner.
   * @param newOwner The address to transfer ownership to.
   */
  function _transferOwnership(address newOwner) internal {
    require(newOwner != address(0));
    emit OwnershipTransferred(_owner, newOwner);
    _owner = newOwner;
  }
}

Notice the onlyOwner modifier on the renounceOwnership function. When you call renounceOwnership, the code inside onlyOwner executes first. Then when it hits the _;statement in onlyOwner, it goes back and executes the code inside renounceOwnership.
So while there are other ways you can use modifiers, one of the most common use-cases is to add quick require check before a function executes.
In the case of onlyOwner, adding this modifier to a function makes it so only the owner of the contract (you, if you deployed it) can call that function.
Note: Giving the owner special powers over the contract like this is often necessary, but it could also be used maliciously. For example, the owner could add a backdoor function that would allow him to transfer anyone's zombies to himself!
So it's important to remember that just because a DApp is on Ethereum does not automatically mean it's decentralized — you have to actually read the full source code to make sure it's free of special controls by the owner that you need to potentially worry about. There's a careful balance as a developer between maintaining control over a DApp such that you can fix potential bugs, and building an owner-less platform that your users can trust to secure their data.

Put it to the test

Now we can restrict access to setKittyContractAddress so that no one but us can modify it in the future.
1. Add the onlyOwner modifier to setKittyContractAddress.

Old code:

  // Modify this function:
  function setKittyContractAddress(address _address) external {
    kittyContract = KittyInterface(_address);
  }

New code:

  // Modify this function:
  function setKittyContractAddress(address _address) external onlyOwner {
    kittyContract = KittyInterface(_address);
  }

Total code:

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 {

  KittyInterface kittyContract;

  // Modify this function:
  function setKittyContractAddress(address _address) external onlyOwner {
    kittyContract = KittyInterface(_address);
  }

  function feedAndMultiply(uint _zombieId, uint _targetDna, string _species) public {
    require(msg.sender == zombieToOwner[_zombieId]);
    Zombie storage myZombie = zombies[_zombieId];
    _targetDna = _targetDna % dnaModulus;
    uint newDna = (myZombie.dna + _targetDna) / 2;
    if (keccak256(abi.encodePacked(_species)) == keccak256(abi.encodePacked("kitty"))) {
      newDna = newDna - newDna % 100 + 99;
    }
    _createZombie("NoName", newDna);
  }

  function feedOnKitty(uint _zombieId, uint _kittyId) public {
    uint kittyDna;
    (,,,,,,,,,kittyDna) = kittyContract.getKitty(_kittyId);
    feedAndMultiply(_zombieId, kittyDna, "kitty");
  }

}