Implementing your callback

Now, we’ll write the callback function on Chiado using the onMessage implementation you provided. This contract will handle the message sent from Sepolia.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract ChiadoReceiver {
    /// @dev Yaru contract on the Chiado w.r.t the Sepolia
    address public YARU = 0xBA9165973963a6E5608f03b9648c34A737E48f68;  
    /// @dev Source chain ID of Sepolia
    uint256 public SOURCE_CHAIN_ID = 11155111;
    /// @dev Expected threshold to be met by the adapters
    uint256 public EXPECTED_THRESHOLD;
    /// @dev Sender SepoliaSender contract address
    address public SOURCE_SENDER_ADDRESS;
    /// @dev hash of adapters address 
    bytes32 public ADAPTERS_HASH;
    /// @dev message that is last received from sender contract's sendMessageToTarget function
    string public lastReceivedMessage;
    

    constructor(uint256 expectedThreshold, address sourceSender, address[] memory adapters){
        EXPECTED_THRESHOLD = expectedThreshold;
        SOURCE_SENDER_ADDRESS = sourceSender;
        ADAPTERS_HASH = keccak256(abi.encodePacked(adapters));
    }

    /// @dev Function that gets triggered when the message is relayed, called by Yaru contract
    /// @param chainId chainId of the chain where message is sending from
    /// @param sender sender contract
    /// @param threshold threshold of the message that should be met by adapters
    /// @param adapters an array of adapters to check the threshold with
    /// @param data abi-encoded message
    /// @return 
    function onMessage(
        uint256, /*messageId*/
        uint256 chainId,
        address sender,
        uint256 threshold,
        address[] memory adapters,
        bytes memory data
    ) external returns (bytes memory) {
        require(msg.sender == YARU, "only called by Yaru");
        require(chainId == SOURCE_CHAIN_ID, "invalid source chain ID");
        require(threshold == EXPECTED_THRESHOLD, "invalid number of threshold");
        require(sender == SOURCE_SENDER_ADDRESS, "invalid sender address from source chain");
        require( keccak256(abi.encodePacked(adapters)) == ADAPTERS_HASH, "invalid adapters hash");
    
        // Decode the message and store it
        (string memory message) = abi.decode(data, (string));
        lastReceivedMessage = message;

        return "";
    }
}

Explanation:

• The onMessage function implements the callback, which is triggered when the Yaru contract on Chiado relays the message.

• The function checks that the Yaru contract on Chiado is the sender, that the chainId, sender address, and threshold match what was expected, and that the list of adapters matches the expected hash.

• After the validations pass, the message is decoded and stored in the contract.

Deployment:

• Deploy this contract on Chiado. Ensure that the values like SOURCE_SENDER_ADDRESS (Sepolia sender contract), EXPECTED_THRESHOLD, and ADAPTERS are correctly configured.

• Yaru address with respect to the source chain can be found in Yaho & Yaru

• A list of adapters on the target chain can be found in Oracles