How is the Etherscan TokenTracker able to show holder balances?

Question

How is the Etherscan TokenTracker able to show holder addresses? The balances mapping of your average ERC20 token cannot be iterated over by index, nor can the length of the mapping be obtained. So how might holder address information be available to Etherscan?

For example, you can view the Gnosis TokenTracker here and confirm a list of holders.

https://etherscan.io/token/Gnosis#balances

Next, if you view the contract source, you'll see that there's no code that explicitly returns the holder address by way of reference to mapping (address => uint256) balances or anything else. (At least not that I can see.

https://etherscan.io/address/0x6810e776880c02933d47db1b9fc05908e5386b96#code

Many thanks in advance for your help.

Answer 1

I don't think they've released the details of their implementation, but presumably they track Transfer events and update the known balances for each account as those events come in.

Answer 2

I just make a test on my own token and Mint event are not tracked whereas Transfer are. It checks the balance of the sender/recipient after transactions.

Answer 3

Thanks all for your input. They do indeed track Transfer events. Additionally, if you use their token tracker to check a balance, and a balance returns, then they'll add that holder to the list.