Events

Events are a way to emit data from a contract. All events must be defined in the Event enum, which must be annotated with the #[event] attribute. An event is defined as a struct that derives the starknet::Event trait. The fields of that struct correspond to the data that will be emitted. An event can be indexed for easy and fast access when querying the data at a later time by adding a #[key] attribute to a field member.

Here's a simple example of a contract that emits an event each time a counter is incremented by the increment function:

#[starknet::contract]
pub mod EventCounter {
    use starknet::{get_caller_address, ContractAddress};
    use starknet::storage::{StoragePointerReadAccess, StoragePointerWriteAccess};
 
    #[storage]
    struct Storage {
        // Counter value
        pub counter: u128,
    }
 
    #[event]
    #[derive(Copy, Drop, Debug, PartialEq, starknet::Event)]
    // The event enum must be annotated with the `#[event]` attribute.
    // It must also derive at least the `Drop` and `starknet::Event` traits.
    pub enum Event {
        CounterIncreased: CounterIncreased,
        UserIncreaseCounter: UserIncreaseCounter
    }
 
    // By deriving the `starknet::Event` trait, we indicate to the compiler that
    // this struct will be used when emitting events.
    #[derive(Copy, Drop, Debug, PartialEq, starknet::Event)]
    pub struct CounterIncreased {
        pub amount: u128
    }
 
    #[derive(Copy, Drop, Debug, PartialEq, starknet::Event)]
    pub struct UserIncreaseCounter {
        // The `#[key]` attribute indicates that this event will be indexed.
        // You can also use `#[flat]` for nested structs.
        #[key]
        pub user: ContractAddress,
        pub new_value: u128,
    }
 
    #[abi(embed_v0)]
    impl EventCounter of super::IEventCounter<ContractState> {
        fn increment(ref self: ContractState, amount: u128) {
            self.counter.write(self.counter.read() + amount);
            // Emit event
            self.emit(Event::CounterIncreased(CounterIncreased { amount }));
            self
                .emit(
                    Event::UserIncreaseCounter(
                        UserIncreaseCounter {
                            user: get_caller_address(), new_value: self.counter.read()
                        }
                    )
                );
        }
    }
}