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Omnichain Contracts

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First Omnichain Contract

First Omnichain Contract

In this tutorial you will create a simple omnnichain contract, deploy it on ZetaChain and make a contract call from a connected chain.

Prerequisites

• Node.js v12 or later
• Yarn v1 or later
• Git

Set Up Your Environment

Clone the Hardhat contract template:

git clone https://github.com/zeta-chain/template

Install dependencies:

cd template
yarn

Create the Contract

To create a new omnichain contract you will use the omnichain Hardhat task available by default in the template.

npx hardhat omnichain MyContract

The omnichain task can also accept a list of arguments (optionally with types) to create a contract that accepts specific data from a connected chain. You can learn more about passing arguments in the following tutorials. In this tutorial you will create a contract that does not accept any arguments.

This omnichain task has created:

• contracts/MyContract.sol: a Solidity omnichain smart contract
• tasks/deploy.ts: a Hardhat task to deploy the contract
• tasks/interact.ts: a Hardhat task to interact with the contract

Omnichain Contract

Let's review the contents of the MyContract contract:

contracts/MyContract.sol

// SPDX-License-Identifier: MIT
pragma solidity 0.8.7;

import "@zetachain/protocol-contracts/contracts/zevm/SystemContract.sol";
import "@zetachain/protocol-contracts/contracts/zevm/interfaces/zContract.sol";

contract MyContract is zContract {
    error SenderNotSystemContract();

    SystemContract public immutable systemContract;

    constructor(address systemContractAddress) {
        systemContract = SystemContract(systemContractAddress);
    }

    function onCrossChainCall(
        zContext calldata context,
        address zrc20,
        uint256 amount,
        bytes calldata message
    ) external virtual override {
        if (msg.sender != address(systemContract)) {
            revert SenderNotSystemContract();
        }
        // TODO: implement the logic
    }
}

MyContract is a simple contract that inherits from the zContract interface.

The contract declares a state variable of type SystemContract that stores a reference to the system contract.

The constructor function accepts the address of the system contract and stores it in the systemContract state variable.

onCrossChainCall is a function that is called when the contract gets called by a token transfer transaction sent to the TSS address on a connected chain. The function receives the following inputs:

• context: is a struct of type zContext that contains the following values:
  • origin: EOA address that sent the token transfer transaction to the TSS address (triggering the omnichain contract)
  • chainID: interger ID of the connected chain from which the omnichain contract was triggered.
  • sender (reserved for future use, currently empty)
• zrc20: the address of the ZRC-20 token contract that represents an asset from a connected chain on ZetaChain.
• amount: the amount of tokens that were transferred to the TSS address.
• message: the contents of the data field of the token transfer transaction.

The onCrossChainCall function should only be called by the system contract (in other words, by the ZetaChain protocol) to prevent a caller from supplying arbitrary values in context. The following check ensures that the function is called only as a response to a token transfer transaction sent to the TSS address:

if (msg.sender != address(systemContract)) {
    revert SenderNotSystemContract();
}

By default, the onCrossChainCall function doesn't do anything else. You will implement the logic yourself based on your use case.

Deploy Task

The omnichain task has created a Hardhat task to deploy the contract:

tasks/deploy.ts

import { getAddress } from "@zetachain/protocol-contracts";
import { task } from "hardhat/config";
import { HardhatRuntimeEnvironment } from "hardhat/types";

const main = async (args: any, hre: HardhatRuntimeEnvironment) => {
  if (hre.network.name !== "zeta_testnet") {
    throw new Error(
      '🚨 Please use the "zeta_testnet" network to deploy to ZetaChain.'
    );
  }

  const [signer] = await hre.ethers.getSigners();
  console.log(`🔑 Using account: ${signer.address}\n`);

  const systemContract = getAddress("systemContract", "zeta_testnet");

  const factory = await hre.ethers.getContractFactory("MyContract");
  const contract = await factory.deploy(systemContract);
  await contract.deployed();

  console.log(`🚀 Successfully deployed contract on ZetaChain.
📜 Contract address: ${contract.address}
🌍 Explorer: https://athens3.explorer.zetachain.com/address/${contract.address}
`);
};

task("deploy", "Deploy the contract", main);

Omnichain contracts are supposed to be deployed to ZetaChain, so the task checks that the --network flag value is always zeta_testnet.

The task uses the getAddress function from @zetachain/protocol-contracts to get the address of the system contract on ZetaChain.

The task then uses Ethers.js to deploy the contract to ZetaChain.

Interact Task

The omnichain task has also created a Hardhat task to interact with the contract:

tasks/interact.ts

import { task } from "hardhat/config";
import { HardhatRuntimeEnvironment } from "hardhat/types";
import { parseEther } from "@ethersproject/units";
import { getAddress } from "@zetachain/protocol-contracts";
import { prepareData, trackCCTX } from "@zetachain/toolkit/helpers";

const main = async (args: any, hre: HardhatRuntimeEnvironment) => {
  const [signer] = await hre.ethers.getSigners();
  console.log(`🔑 Using account: ${signer.address}\n`);

  const data = prepareData(args.contract, [], []);
  const to = getAddress("tss", hre.network.name);
  const value = parseEther(args.amount);

  const tx = await signer.sendTransaction({ data, to, value });

  console.log(`
🚀 Successfully broadcasted a token transfer transaction on ${hre.network.name} network.
📝 Transaction hash: ${tx.hash}
`);
  await trackCCTX(tx.hash);
};

task("interact", "Interact with the contract", main)
  .addParam("contract", "The address of the withdraw contract on ZetaChain")
  .addParam("amount", "Amount of tokens to send");

The task uses the prepareData function from @zetachain/toolkit/helpers to prepare the data field of the token transfer transaction. prepareData accepts an omnichain contract address on ZetaChain, a list of argument types, and a list of argument names. The data field contains the following information:

• the address of the contract on ZetaChain
• the arguments to pass to the onCrossChainCall function in the message parameter

In the code generated above there are no arguments, so the data field is simply the address of the contract on ZetaChain.

getAddress retrieves the address of the TSS on the current network.

The task then uses Ethers.js to send a token transfer transaction to the TSS address. The transaction contains the following information:

• data: the data field prepared by prepareData
• to: the address of the TSS
• value: the amount of tokens to transfer

Finally, the task uses the trackCCTX function from @zetachain/toolkit/helpers to track the token transfer transaction. The function waits for the transaction to appear on ZetaChain and tracks the status of the transaction. Transaction tracking is optional, but helpful to know when the transaction has been processed by ZetaChain.

Create an Account

To deploy and interact with the contract you will need a wallet with tokens.

Create a new wallet account:

npx hardhat account --save

This command generates a random wallet, prints information about the wallet to the terminal, and saves the private key to a .env file to make it accessible to Hardhat.

Use the Faucet to Request Tokens

Request testnet ZETA tokens from the ZetaChain faucet:

npx hardhat faucet

This command requests tokens from the faucet for the account address derived from the private key specified in the .env. Tokens sent to the address on ZetaChain.

Using the faucet task you can get ZETA tokens on ZetaChain as well as ZETA tokens on connected chains.

You will, however, need to request native tokens on connected chains from one of the publicly available faucets.

Check Token Balances

Check token balances to ensure you have tokens on ZetaChain and at least one of the connected chains (Goerli in this example):

npx hardhat balances

Deploy the Contract

Clear the cache and artifacts, then compile the contract:

npx hardhat compile --force

Deploy the contract to ZetaChain:

npx hardhat deploy --network zeta_testnet

🔑 Using account: 0x1bE17D79b60182D7F3573576B7807F6C20Ae7C99

🚀 Successfully deployed contract on ZetaChain.
📜 Contract address: 0xE26F2e102E2f3267777F288389435d3037D14bb3
🌍 Explorer: https://athens3.explorer.zetachain.com/address/0xE26F2e102E2f3267777F288389435d3037D14bb3

Interact with the Contract

Call the interact task to interact with the contract:

npx hardhat interact --contract 0xE26F2e102E2f3267777F288389435d3037D14bb3 --amount 0.1 --network goerli_testnet

🔑 Using account: 0x2cD3D070aE1BD365909dD859d29F387AA96911e1

🚀 Successfully broadcasted a token transfer transaction on goerli_testnet network.
📝 Transaction hash: 0x93b441dc2ddb751a60a2f4c0fc52dbbd447ed70eb962b1a01072328aa6872b73

✔ CCTX hash found: 0x31310706ac4b33aa468e62a77d5db358e52a60dad3854210db8fc06c870186b6

ℹ Status updated to "OutboundMined": Remote omnichain contract call completed

✔ CCTX has been finalized on ZetaChain

Once the transaction is finalized on ZetaChain, you should be able to review the transaction on the ZetaChain explorer:

https://athens3.explorer.zetachain.com/cc/tx/0x31310706ac4b33aa468e62a77d5db358e52a60dad3854210db8fc06c870186b6

The interact task has sent a token transfer transaction to the TSS address on Goerli. The transaction contains the address of the contract on ZetaChain in the data field. ZetaChain detects the transaction and triggers the onCrossChainCall function of the contract. The onCrossChainCall function does nothing in this example, but you can modify it to implement your own logic.

Congratulations! 🎉 In this tutorial you have:

• cloned the Hardhat contract template
• used npx hardhat omnichain to create a new omnichain contract
• reviewed the contents of the generated contract and the tasks to deploy and interact with the contract
• successfully deployed the contract to ZetaChain
• interacted with the contract by sending a token transfer transaction to the TSS address on a connected chain and triggering the onCrossChainCall function of the omnichain contract on ZetaChain