Bridging

Getting data from L1

The Stackr SDK offers L1 to L2 bridging out of the box through the Bridge Plugin.

index.ts

import { Bridge } from "@stackr/sdk/plugins";
 
Bridge.init(rollup, {
    handlers: {
        // Define the bridging handler here. 
    }
})

How it works?

You can set a bridge contract in your micro-rollup's AppInbox.
The AppInbox has a createTicket function which can only be invoked by the bridge.
When a ticket is created on the L1, the micro-rollup receives the event and processes the ticket.

Building your bridge

1. Write the Bridge contract

Developers can write their custom logic in the Bridge contract and interact with the AppInbox to create the ticket that'll be procsessed on the MRU.

TokenBridge.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.24;
 
interface ITicketFactory {
    function createTicket(
        bytes32 _identifier,
        address _msgSender,
        bytes memory _message
    ) external;
}
 
contract TokenBridge {
    address appInbox;
 
    constructor(address _appInbox) {
        appInbox = _appInbox;
    }
 
    function bridgeETH(address _to) external payable { 
        require(_to != address(0), "bridgeTokens/zero-address");
        require(msg.value > 0, "bridgeTokens/zero-amount");
 
        bytes memory message = abi.encode(_to, msg.value); 
        bytes32 identifier = keccak256("BRIDGE_ETH"); 
 
        ITicketFactory(appInbox).createTicket( 
            identifier, 
            msg.sender, 
            message 
        ); 
    }
}

The bridgeETH function —

Encodes the data to be sent to the micro-rollup.
Computes an identifier for the ticket. This is important as this identifier is used on the micro-rollup to match the ticket to a handler.
Creates the ticket by calling the AppInbox with the ticket identifier, msg.sender, and the message to be attached to the ticket.

2. Set the `Bridge` on the `AppInbox`.

Next up, you need to update the AppInbox to know which Bridge contract to accept ticket creation requests from.

Run the CLI command:

terminal

npx @stackr/cli add bridge

You'll be prompted with the bridge address:

terminal

? Bridge Contract Address

Enter your bridge contract address above and you're set! The CLI will update the Bridge contract address inside the AppInbox.

3. Write the handler to process tickets

You can define a handler to process each different type of ticket.

In our Bridge contract, we created a ticket with the identifier of BRIDGE_ETH. So, we need to write a handler to process the tickets with that specific identifier.

The function defined as a handler has to return —

transitionName - Name of the transition to trigger.
action - Action object that has the inputs to pass to the transition, signature of the MRU's operator on the inputs, and the msgSender.

index.ts

Bridge.init(rollup, {
    handlers: {
        'BRIDGE_ETH': async (args) => {
            const [_to, _amount] = abiCoder.decode(['address', 'uint'], args.data);
            const inputs = {
                address: _to,
                amount: Number(_amount)
            }
            const signature = await signMessage(operator, MintTokenSchema, inputs);
            
            const action = MintTokenSchema.actionFrom({ 
                inputs,
                signature,
                msgSender: operator.address
            })
 
            return {
                transitionName: 'mintToken',
                action: action
            }
        }
    }
})

The function to handle BRIDGE_ETH tickets —

First, decodes the data sent with the ticket.
Generates the operator's signature on the data.
Creates an action to be submitted to the rollup.
Returns the transitionName and the action.

The micro-rollup then takes over and submits the returned action to the correct transition.