PingPong Example

This tutorial demonstrates how to send a cross-chain message using Router Protocol's CrossTalk.

You'll learn how to:

• Deploy Router-compatible contracts
• Approve the feePayer for your contracts
• Prepare metadata for cross-chain calls
• Send cross-chain messages

We recommend using Remix for an easy-to-follow experience. The only prerequisite is a set-up Metamask account.

info

If you're new to Remix, follow our basic guide for using Remix here.

Overview PingPong

In this example, you'll deploy the same contract on two different chains. You'll then send a ping from chain A to chain B, facilitated by Router Protocol's CrossTalk. The contract on chain B will receive the ping and respond back to Router Protocol. Finally, Router Protocol will send an acknowledgment message back to the contract on chain A.

Contract Setup

1. Open Remix and create a new file called PingPong.sol
2. Paste the following contract into it:

PingPong.sol Contract

PingPong.sol

//SPDX-License-Identifier: UNLICENSED
pragma solidity >=0.8.0 <0.9.0;

import "@routerprotocol/evm-gateway-contracts/contracts/IGateway.sol";

/// @title PingPong
/// @author Yashika Goyal
/// @notice This is a cross-chain ping pong smart contract to demonstrate how one can
/// utilise Router CrossTalk for cross-chain transactions.
contract PingPong {
    address public owner;
    uint64 public currentRequestId;

    // srcChainId + requestId => pingFromSource
    mapping(string => mapping(uint64 => string)) public pingFromSource;
    // requestId => ackMessage
    mapping(uint64 => string) public ackFromDestination;

    // instance of the Router's gateway contract
    IGateway public gatewayContract;

    // custom error so that we can emit a custom error message
    error CustomError(string message);

    // event we will emit while sending a ping to destination chain
    event PingFromSource(
        string indexed srcChainId,
        uint64 indexed requestId,
        string message
    );
    event NewPing(uint64 indexed requestId);

    // events we will emit while handling acknowledgement
    event ExecutionStatus(uint256 indexed eventIdentifier, bool isSuccess);
    event AckFromDestination(uint64 indexed requestId, string ackMessage);

    constructor(address payable gatewayAddress, string memory feePayerAddress) {
        owner = msg.sender;

        gatewayContract = IGateway(gatewayAddress);

        gatewayContract.setDappMetadata(feePayerAddress);
    }

    /// @notice function to set the fee payer address on Router Chain.
    /// @param feePayerAddress address of the fee payer on Router Chain.
    function setDappMetadata(string memory feePayerAddress) external {
        require(msg.sender == owner, "only owner");
        gatewayContract.setDappMetadata(feePayerAddress);
    }

    /// @notice function to set the Router Gateway Contract.
    /// @param gateway address of the gateway contract.
    function setGateway(address gateway) external {
        require(msg.sender == owner, "only owner");
        gatewayContract = IGateway(gateway);
    }

    /// @notice function to generate a cross-chain request to ping a destination chain contract.
    /// @param destChainId chain ID of the destination chain in string.
    /// @param destinationContractAddress contract address of the contract that will handle this
    /// @param str string to be pinged to destination
    /// @param requestMetadata abi-encoded metadata according to source and destination chains
    function iPing(
        string calldata destChainId,
        string calldata destinationContractAddress,
        string calldata str,
        bytes calldata requestMetadata
    ) public payable {
        currentRequestId++;

        bytes memory packet = abi.encode(currentRequestId, str);
        bytes memory requestPacket = abi.encode(destinationContractAddress, packet);
        gatewayContract.iSend{ value: msg.value }(
        1,
        0,
        string(""),
        destChainId,
        requestMetadata,
        requestPacket
        );
        emit NewPing(currentRequestId);
    }

    /// @notice function to get the request metadata to be used while initiating cross-chain request
    /// @return requestMetadata abi-encoded metadata according to source and destination chains
    function getRequestMetadata(
        uint64 destGasLimit,
        uint64 destGasPrice,
        uint64 ackGasLimit,
        uint64 ackGasPrice,
        uint128 relayerFees,
        uint8 ackType,
        bool isReadCall,
        string memory asmAddress
    ) public pure returns (bytes memory) {
        bytes memory requestMetadata = abi.encodePacked(
        destGasLimit,
        destGasPrice,
        ackGasLimit,
        ackGasPrice,
        relayerFees,
        ackType,
        isReadCall,
        asmAddress
        );
        return requestMetadata;
    }

    /// @notice function to handle the cross-chain request received from some other chain.
    /// @param packet the payload sent by the source chain contract when the request was created.
    /// @param srcChainId chain ID of the source chain in string.
    function iReceive(
        string memory, //requestSender,
        bytes memory packet,
        string memory srcChainId
    ) external returns (uint64, string memory) {
        require(msg.sender == address(gatewayContract), "only gateway");
        (uint64 requestId, string memory sampleStr) = abi.decode(
        packet,
        (uint64, string)
        );
        if (
        keccak256(abi.encodePacked(sampleStr)) == keccak256(abi.encodePacked(""))
        ) {
        revert CustomError("String should not be empty");
        }
        pingFromSource[srcChainId][requestId] = sampleStr;

        emit PingFromSource(srcChainId, requestId, sampleStr);

        return (requestId, sampleStr);
    }

    /// @notice function to handle the acknowledgement received from the destination chain
    /// back on the source chain.
    /// @param requestIdentifier event nonce which is received when we create a cross-chain request
    /// We can use it to keep a mapping of which nonces have been executed and which did not.
    /// @param execFlag a boolean value suggesting whether the call was successfully
    /// executed on the destination chain.
    /// @param execData returning the data returned from the handleRequestFromSource
    /// function of the destination chain.
    function iAck(
        uint256 requestIdentifier,
        bool execFlag,
        bytes memory execData
    ) external {
        (uint64 requestId, string memory ackMessage) = abi.decode(
        execData,
        (uint64, string)
        );

        ackFromDestination[requestId] = ackMessage;

        emit ExecutionStatus(requestIdentifier, execFlag);
        emit AckFromDestination(requestId, ackMessage);
    }
}

Key Contract Functions

• iPing: Initiates the cross-chain message by calling Router's IGateway.iSend.
• iReceive: Serves as the entry point on the destination contract.
• iAck: Handles the acknowledgment in a bidirectional cross-chain message on the source contract.

Compiling the Contract

For compatibility with Sapphire, compile the contract using compiler version 0.8.24 and evm version paris (under advanced configuration).

Deploying the Contract

Deploy the PingPong contract on two different chains: Sapphire Testnet and Polygon Amoy.

Deploying on Sapphire Testnet

1. Obtain TEST tokens for Sapphire Testnet from the Oasis faucet.
2. In Metamask, switch to the Sapphire Testnet network and select Injected Provider - MetaMask as the environment in Remix
3. Fill in the deployment parameters:

• gatewayAddress: 0xfbe6d1e711cc2bc241dfa682cbbff6d68bf62e67 (current Sapphire Testnet Gateway)
• feePayerAddress: Your current account address (copy from MetaMask or Remix)

4. Deploy the contract on Sapphire Testnet

Remix Example

Deploying on Polygon Amoy

1. Obtain POL tokens for Polygon Amoy Testnet from the Polygon faucet.
2. Switch to the Polygon Amoy network in Metamask.
3. Fill in the deployment parameters:

• gatewayAddress: 0x778a1f43459a05accd8b57007119f103c249f929 (current Polygon Amoy Gateway)
• feePayerAddress: Your current account address (copy from MetaMask or Remix)

4. Deploy the contract on Polygon Amoy

Remix Example

Approving the Fee Payer

After deploying the contracts, approve the fee payer on the Router Chain:

1. Obtain Router test tokens from the Router faucet.
2. Approve the contracts on the Router Explorer.

For detailed instructions on fee payer approval, see our approval guide.

Executing PingPong

Now that you've deployed the contracts and approved the fee payer, you can play PingPong. This process involves two steps:

1. Obtaining the Request Metadata
2. Executing the iPing function

Step 1: Obtaining Request Metadata

Call the getRequestMetadata() function with the following parameters:

Argument	Example Value	Description
destGasLimit	300000	Gas limit on destination chain
destGasPrice	100000000000	Gas price on destination chain
ackGasLimit	300000	Gas limit on source chain for ack
ackGasPrice	100000000000	Gas price on source chain for ack
relayerFees	10000000000	Relayer fees on Router chain
ackType	3	Acknowledge type
isReadCall	false	Weather the call is read-only
asmAddress	"0x"	Address for the additional security module

Remix Example

info

You will need the bytes answer in the next step, so copy it!

For more information about request metadata, see the Router documentation.

Step 2: Executing iPing()

To initiate the cross-chain message, call iPing with these parameters:

Argument	Value	Description
destChainId	23295	Destination Chain ID (e.g. Sapphire)
destinationContractAddress	0x<your-contract>	Contract address on the destination chain
str	"Hello"	Message to include in the ping
requestMetadata	<bytes string>	Bytes response from the getRequestMetada call

After sending the transaction, you can monitor its status on the Router Explorer.

Remix Example

This completes the PingPong example, demonstrating cross-chain messaging using Router Protocol's CrossTalk framework.