What is topics[0] in Event logs?

Question

Here's a snapshot from etherscan.io

Answer 1

topics[0] is the hash of the signature of the event.

Example from Solidity docs:

keccak256("Deposit(address,bytes32,uint256)") is the signature of the event:

event Deposit(
        address indexed _from,
        bytes32 indexed _id,
        uint _value
    );

Answer 2

Note when applying this for structs, the pattern is to nest the structs in a similar way. So:

keccak256("Deposit(address,bytes32,uint256,(bytes32, bytes32))") is the signature of the event:

struct MoreData
{
        bytes32 id1;
        bytes32 id2;
}

event Deposit(
        address indexed _from,
        bytes32 indexed _id,
        uint _value,
        MoreData _moreData
    );