Events

Events

• Events allow us to “print” information on the blockchain in a way that is more searchable and gas efficient than just saving to storage variables

• Events are not stored in a contract like regular storage, but rather in a special logs section of a transaction.

• Logs are gas efficient way to check for specific data that's not required for the contract itself. Cannot be accessed by smart contracts

• Applications (outside of blockchain) can subscribe and listen to these events through the RPC interface of an event client.

To put it simply, events are ways to communicate with a client application or front-end website that something has happened on the blockchain.

Event arguments marked as indexed, can be searched

// Format:
// can take up to 3 indexed parameters
event MyEvent(uint _arg1, address indexed _arg2)

• Up to 3 parameters can be indexed; these are referred to as topics.

• if you declare an argument to be indexed it will be hashed, and can look them up

• Logs and their events data cannot be retrievable from within contracts.

• Events are inheritable members of a contract.

• Events are cheap.

Example

// emit event when name is registered or relinquished
event NameRegistered(address indexed owner, string indexed name);
event NameRelinquished(address indexed owner, string indexed name);

// register an available name
function registerName(string memory _name) public {
    require(nameOwner[_name] == address(0), "Already registered");
    nameOwner[_name] = msg.sender;
    emit NameRegistered(msg.sender, _name);
}

Etherscan

• Address: address of contract that emitted the event

• Topics: indexed parameters of events

• Data: abi-encoded non-indexed parameters of events

  • have to decode using the abi of the contract

  • if contract is verified on etherscan, can view in decoded mode 'dec'

https://kovan.etherscan.io/address/0x136607f1f09312d041b2e60396105f58bd063d66#events

topic[0] always refers to the hash of the event itself keccak256(NameRegistered(<address>, <name>))

Indexing strings: Fundamentally, it can be done.

• Indexed event arguments are stored in topics, and must be of fixed size;

• Solidity hashes the string and stored it in the topic. Filtering by the hash value of string would work.

• However, hash functions are one-way. We cannot decode the hashes to obtain the string.

• https://ethereum.stackexchange.com/questions/6840/indexed-event-with-string-not-getting-logged

Links

• https://blog.chain.link/events-and-logging-in-solidity/

• https://www.youtube.com/watch?v=w18c9HLEuBs