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'

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

Events

Example

Etherscan

Links