Events

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 taking action when they happen.

It's best practice to have events emitted before the "Interactions" section of your function (in the CEI model).

• You can have up to 3 indexed parameters (also know as topics) per individual event.

• Event logs can't be accessed by smart contracts, so are only used for off-chain applications such as web apps.

contract SimpleStorage {
    uint256 favoriteNumber;
    event StoredNumber(
        uint256 indexed oldNumber,
        uint256 indexed newNumber,
        uint256 addedNumber,
        address sender
    );

    function store(uint256 _favoriteNumber) public {
        emit StoredNumber(
            favoriteNumber,
            _favoriteNumber,
            _favoriteNumber + favoriteNumber,
            msg.sender
        );
        favoriteNumber = _favoriteNumber;
    }

    function retrieve() public view returns (uint256) {
        return favoriteNumber;
    }
}