Events

Events in Solidity are a way for a 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.

Event Declaration

Events are declared with the event keyword, followed by the name of the event, and then a list of parameters.

event MyEvent(bool _myParam);

Emitting Events

Events are emitted with the emit keyword, followed by the name of the event, and then a list of arguments.

emit MyEvent(true);

Indexing events

Events can be indexed, which allows you to filter events based on the indexed parameters.

event MyEvent(uint256 indexed _myParam);

💰 Cost of indexing

Indexing events costs more gas than non-indexed events. You should only index events that you plan to filter on, otherwise you are unnecessarily increasing the cost on users of your contract.