Composable Bridging

Use Across to bridge + execute a transaction

What is Composable Bridging:?

You can instruct Across to execute a transaction upon filling your deposit on the destination chain. This transaction would be executed atomically with the fill transaction.

NOTE: The transaction that gets executed on the destination chain must be non-reverting otherwise user deposits may risk getting locked.

How does it work?

When a relayer fills your deposit by calling fillRelay() on the destination SpokePool, if the deposit has a message attached, then the SpokePool will attempt to call handleAcrossMessage() on your recipient address and pass in the following params:

• handleAcrossMessage(address tokenSent, uint256 amount, bool fillCompleted, address relayer, bytes memory message)

Requirements

• The deposit message is not empty

• The recipient address is a contract on the destinationChainId that implements a public handleAcrossMessage(address,uint256,bool,address,bytes) function, and this function must be non-reverting

• The additional gas cost to execute the above function is compensated for in the deposit's relayerFeePct

Detailed instructions

• Construct your message

• Use the Across API to get an estimate of the relayerFeePct you should set for your message and recipient combination

• Call deposit() passing in your message

• Once the relayer calls fillRelay() on the destination, your recipient's handleAcrossMessage will be executed

Example: Implementing a Bridge and Unwrap

• Imagine that I want to bridge ETH from Ethereum to Optimism and receive ETH, not wrapped WETH on Optimism

• I will deploy the following contract on Optimism which unwraps received WETH into ETH and sends to a designated owner EOA

contract MyUnwrapper {
    WETHInterface weth;
    addess payable owner;
    constructor() public {
        owner = msg.sender;
    }
    function handleAcrossMessage(
        address tokenSent, 
        uint256 amount, 
        bool, // fillCompleted unused
        address, // relayer is unused 
        bytes memory // message is unused
    ) external { 
        require(tokenSent == address(weth), "received token not WETH");
        weth.withdraw(amount);
        (bool sent, bytes memory data) = owner.call{value: amount}("");
        require(sent, "Failed to send Ether");
    } 
}

• Call Across-API’s /suggested-fees endpoint with params ?token=0xWETH-on-ethereum-address&destinationChainId=10&amount= x&originChainId=1&recipient=MyUnwrapper.address&message=0x1234
• Here we set message to something useless but not-zero so that the destination SpokePool ultimately calls handleAcrossMessage on MyUnwrapper