Reading emitted events

Starknet events

A contract may emit events throughout its execution. Each event contains the following fields:

from_address: address of the contract emitting the events
keys: a list of field elements
data: a list of field elements

The events are stored in a block on the blockchain.

Events in the Cairo code

You have to analyze the Cairo code of your smart contract, to recover the list of data emitted by the event. An example in Cairo 0:

@event
func log_data(d1: felt, d2: felt, d3: felt) {
}

@external
func my_func{syscall_ptr: felt*, pedersen_ptr: HashBuiltin*, range_check_ptr}() {
    ...
    log_data.emit(start_field, high_range, status_prog);
    ...
    return ();
}

Here, we can see that the event will store 3 felts.

Once compiled, this code will generate an abi file containing:

{
    "data": [
        {"name": "d1", "type": "felt"},
        {"name": "d2", "type": "felt"},
        {"name": "d3", "type": "felt"},
    ],
    "keys": [],
    "name": "log_data",
    "type": "event",
}

Recover the event data

Once the my_func is invoked, the event is stored in the blockchain and you get in return the transaction hash.

import { InvokeTransactionReceiptResponse } from 'starknet';

const resu = await myTestContract.my_func();
const txReceiptDeployTest: InvokeTransactionReceiptResponse = await provider.waitForTransaction(
  resu.transaction_hash
);
console.log('events =', txReceiptDeployTest.events);

Now, you have all the events of the block. Here, we have 2 events - the last one contains our data:

[
  [Object: null prototype] {
    data: [
      '0x2345b8cdd1eb333ac0959f7d908394b6540234345590e83367ae2a6cfbd4107'
    ],
    from_address: '0x465e68294995849bd00ac9f6ad4ee12be3cec963d8fe27172a1eadda608c110',
    keys: [
      '0x28f911b08eb032a94e35f766f1310b2df2267eb9d25bb069a1e3a6754e4206d'
    ]
  },
  [Object: null prototype] {
    data: [
      '0x7e00d496e324876bbc8531f2d9a82bf154d1a04a50218ee74cdd372f75a551a',
      '0x3711666a3506c99c9d78c4d4013409a87a962b7a0880a1c24af9fe193dafc01',
      '0x1d3d81545c000'
    ],
    from_address: '0x49d36570d4e46f48e99674bd3fcc84644ddd6b96f7c741b1562b82f9e004dc7',
    keys: [
      '0x99cd8bde557814842a3121e8ddfd433a539b8c9f14bf31ebf108d12e6196e9'
    ]
  }
]

Use the contract deployment address testContractAddress, to filter the events and read the data from your smart contract:

const event = txReceiptDeployTest.events.find(
  (it) => num.cleanHex(it.from_address) === num.cleanHex(testContractAddress)
) || { data: [] };

const eventD1 = event.data[0];
const eventD2 = event.data[1];
const eventD3 = event.data[2];

If you do not have the transaction hash, you have to search in the blocks of Starknet. See an example here.