Index

Chapter 13: Events

Our contract is almost finished! Now let's add an event.
Events are a way for your contract to communicate that something happened on the blockchain to your app front-end, which can be 'listening' for certain events and take action when they happen.
Example:

// declare the event
event IntegersAdded(uint x, uint y, uint result);

function add(uint _x, uint _y) public {
  uint result = _x + _y;
  // fire an event to let the app know the function was called:
  emit IntegersAdded(_x, _y, result);
  return result;
}

Your app front-end could then listen for the event. A javascript implementation would look something like:

YourContract.IntegersAdded(function(error, result) {
// do something with result
}

Put it to the test

We want an event to let our front-end know every time a new zombie was created, so the app can display it.
1. Declare an event called NewZombie. It should pass zombieId (a uint), name (a string), and dna (a uint).

2. Modify the _createZombie function to fire the NewZombie event after adding the new Zombie to our zombies array.

3. You're going to need the zombie's id. array.push() returns a uint of the new length of the array - and since the first item in an array has index 0, array.push() - 1 will be the index of the zombie we just added. Store the result of zombies.push() - 1 in a uint called id, so you can use this in the NewZombie event in the next line.

pragma solidity ^0.4.25;

contract ZombieFactory {

    event NewZombie(uint zombieId, string name, uint dna);

    uint dnaDigits = 16;
    uint dnaModulus = 10 ** dnaDigits;

    struct Zombie {
        string name;
        uint dna;
    }

    Zombie[] public zombies;

    function _createZombie(string _name, uint _dna) private {
        uint id = zombies.push(Zombie(_name, _dna)) - 1;
        emit NewZombie(id, _name, _dna);
    }

    function _generateRandomDna(string _str) private view returns (uint) {
        uint rand = uint(keccak256(abi.encodePacked(_str)));
        return rand % dnaModulus;
    }

    function createRandomZombie(string _name) public {
        uint randDna = _generateRandomDna(_name);
        _createZombie(_name, randDna);
    }

}