abstract contract FxBaseChildTunnel is IFxMessageProcessor {
    // MessageTunnel on L1 will get data from this event
    event MessageSent(bytes message);

    // fx child
    address public fxChild;

    // fx root tunnel
    address public fxRootTunnel;

    constructor(address _fxChild) {
        fxChild = _fxChild;
    }

    // Sender must be fxRootTunnel in case of ERC20 tunnel
    modifier validateSender(address sender) {
        require(sender == fxRootTunnel, "FxBaseChildTunnel: INVALID_SENDER_FROM_ROOT");
        _;
    }

    // set fxRootTunnel if not set already
    function setFxRootTunnel(address _fxRootTunnel) external virtual {
        require(fxRootTunnel == address(0x0), "FxBaseChildTunnel: ROOT_TUNNEL_ALREADY_SET");
        fxRootTunnel = _fxRootTunnel;
    }

    function processMessageFromRoot(
        uint256 stateId,
        address rootMessageSender,
        bytes calldata data
    ) external override {
        require(msg.sender == fxChild, "FxBaseChildTunnel: INVALID_SENDER");
        _processMessageFromRoot(stateId, rootMessageSender, data);
    }

    /**
     * @notice Emit message that can be received on Root Tunnel
     * @dev Call the internal function when need to emit message
     * @param message bytes message that will be sent to Root Tunnel
     * some message examples -
     *   abi.encode(tokenId);
     *   abi.encode(tokenId, tokenMetadata);
     *   abi.encode(messageType, messageData);
     */
    function _sendMessageToRoot(bytes memory message) internal {
        emit MessageSent(message);
    }

    /**
     * @notice Process message received from Root Tunnel
     * @dev function needs to be implemented to handle message as per requirement
     * This is called by onStateReceive function.
     * Since it is called via a system call, any event will not be emitted during its execution.
     * @param stateId unique state id
     * @param sender root message sender
     * @param message bytes message that was sent from Root Tunnel
     */
    function _processMessageFromRoot(
        uint256 stateId,
        address sender,
        bytes memory message
    ) internal virtual;
}
/**
 * @notice Mock child tunnel contract to receive and send message from L2
 */
function _sendMessageToRoot(bytes memory message) internal {
    emit MessageSent(message);
}

/**
 * @notice Emit message that can be received on Root Tunnel
 * @dev Call the internal function when need to emit message
 * @param message bytes message that will be sent to Root Tunnel
 * some message examples -
 *   abi.encode(tokenId);
 *   abi.encode(tokenId, tokenMetadata);
 *   abi.encode(messageType, messageData);
 */