Add token transfer/redeem events to the EVM token bridge

[kev1n-peters]

As an integrator, I want the EVM token bridge to emit events when a token is transferred with details about the token being transferred. It would be useful for the events to log the recipient, token address, and amount (denormalized) for use in computing statistics like TVL, TVM, transfer volume, token holders, etc. Being able to query these details from an RPC node would be very convenient. I’d like to propose adding the following events to the EVM token bridge contract:

To be emitted when a token is bridged in:

Option 1:

TransferRedeemed already exists, extend it to:

// note that only 3 arguments can be indexed:
event TransferRedeemed(uint16 indexed emitterChainId, bytes32 indexed emitterAddress, uint64 indexed sequence, address recipient, address token, uint256 amount)

Option 2:

// notice that the arguments are indexed compared to option 1 where they are not
event TokenRedeemed(address indexed recipient, address indexed token, uint256 amount)

To be emitted when a token is bridged out:

event TokenTransferred(address indexed sender, address indexed token, uint256 amount)

Thoughts

Would it be useful to include toChain indexed uint16 in Transfer and fromChain indexed uint16 in TokenRedeemed (option 2)?

Please let me know your thoughts, thank you!

[kev1n-peters]

For the time being it's sufficient to grab Transfer and Deposit events by filtering on LogMessagePublished events (where the sender is the token bridge) and TransferRedeemed events. For outbound transfers, the Transfer or Deposit event will occur just before the LogMessagePublished event. For inbound transfers (redeems), the Transfer event will occur just after the TransferRedeemed event. The event logs on EVM are ordered so we can rely on this in most cases.