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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

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.

The 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

It also modified hardhat.config.ts to import both deploy and interact tasks.

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";
import "@zetachain/toolkit/contracts/OnlySystem.sol";
 
contract MyContract is zContract, OnlySystem {
    SystemContract public systemContract;
 
    constructor(address systemContractAddress) {
        systemContract = SystemContract(systemContractAddress);
    }
 
    function onCrossChainCall(
        zContext calldata context,
        address zrc20,
        uint256 amount,
        bytes calldata message
    ) external virtual override onlySystem(systemContract) {
        // TODO: implement the logic
    }
}

MyContract is a simple contract that inherits from the zContract interface (opens in a new tab).

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 (when a gas token is deposited) or a deposit method call on the ERC-20 custody contract (when an ERC-20 token is deposited). The function receives the following inputs:

  • context: is a struct of type zContext (opens in a new tab) that contains the following values:
    • origin: EOA address that sent the token transfer transaction to the TSS address (triggering the omnichain contract) or the value passed to the deposit method call on the ERC-20 custody 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 or an amount of tokens that were deposited to the ERC-20 custody contract.
  • 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 onlySystem modifier ensures that the function is called only as a response to a token transfer transaction sent to the TSS address or an ERC-20 custody contract.

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, ParamChainName } from "@zetachain/protocol-contracts";
import { task } from "hardhat/config";
import { HardhatRuntimeEnvironment } from "hardhat/types";
 
const main = async (args: any, hre: HardhatRuntimeEnvironment) => {
  const network = hre.network.name as ParamChainName;
 
  if (!/zeta_(testnet|mainnet)/.test(network)) {
    throw new Error(
      '🚨 Please use either "zeta_testnet" or "zeta_mainnet" network to deploy to ZetaChain.'
    );
    }
 
  const [signer] = await hre.ethers.getSigners();
  if (signer === undefined) {
    throw new Error(
      `Wallet not found. Please, run "npx hardhat account --save" or set PRIVATE_KEY env variable (for example, in a .env file)`
    );
  }
 
  const systemContract = getAddress("systemContract", network);
 
  const factory = await hre.ethers.getContractFactory(args.name);
  const contract = await factory.deploy(systemContract);
  await contract.deployed();
 
  const isTestnet = network === "zeta_testnet";
  const zetascan = isTestnet ? "athens.explorer" : "explorer";
  const blockscout = isTestnet ? "zetachain-athens-3" : "zetachain";
 
  if (args.json) {
    console.log(JSON.stringify(contract));
  } else {
    console.log(`🔑 Using account: ${signer.address}
 
🚀 Successfully deployed contract on ${network}.
📜 Contract address: ${contract.address}
🌍 ZetaScan: https://${zetascan}.zetachain.com/address/${contract.address}
🌍 Blockcsout: https://${blockscout}.blockscout.com/address/${contract.address}
`);
  }
};
 
task("deploy", "Deploy the contract", main)
  .addFlag("json", "Output in JSON")
  .addOptionalParam("name", "Contract to deploy", "MyContract");

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 { parseUnits } from "@ethersproject/units";
import { getAddress } from "@zetachain/protocol-contracts";
import ERC20Custody from "@zetachain/protocol-contracts/abi/evm/ERC20Custody.sol/ERC20Custody.json";
import { prepareData } from "@zetachain/toolkit/client";
import { utils, ethers } from "ethers";
import ERC20 from "@openzeppelin/contracts/build/contracts/ERC20.json";
 
const main = async (args: any, hre: HardhatRuntimeEnvironment) => {
  const [signer] = await hre.ethers.getSigners();
 
  const data = prepareData(
    args.contract,
    [],
    []
  );
 
  let tx;
 
  if (args.token) {
    const custodyAddress = getAddress("erc20Custody", hre.network.name as any);
    if (!custodyAddress) {
      throw new Error(
        `No ERC20 Custody contract found for ${hre.network.name} network`
      );
    }
 
    const custodyContract = new ethers.Contract(
      custodyAddress,
      ERC20Custody.abi,
      signer
    );
    const tokenContract = new ethers.Contract(args.token, ERC20.abi, signer);
    const decimals = await tokenContract.decimals();
    const value = parseUnits(args.amount, decimals);
    const approve = await tokenContract.approve(custodyAddress, value);
    await approve.wait();
 
    tx = await custodyContract.deposit(signer.address, args.token, value, data);
    tx.wait();
  } else {
    const value = parseUnits(args.amount, 18);
    const to = getAddress("tss", hre.network.name as any);
    tx = await signer.sendTransaction({ data, to, value });
  }
 
  if (args.json) {
    console.log(JSON.stringify(tx, null, 2));
  } else {
    console.log(`🔑 Using account: ${signer.address}\n`);
 
    console.log(`🚀 Successfully broadcasted a token transfer transaction on ${hre.network.name} network.
📝 Transaction hash: ${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")
  .addOptionalParam("token", "The address of the token to send")
  .addFlag("json", "Output in JSON")

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.

Calling omnichain contracts is differs depending on whether a gas token is being deposited or an ERC-20 token.

If an ERC-20 token address is passed to the --token optional parameter, the interact task assumes you want to deposit an ERC-20 token in an omnichain contract.

To deposit an ERC-20 token into an omnichain contract you need to call the deposit method of the ERC-20 custody contract. The task first gets the address of the custody contract on the current network, creates an instance of a token contract, gets the number of decimals of the token, and approves the custody contract to spend the specified amount of ERC-20 tokens. The task then calls the deposit method of the custody contract, passing the following information:

  • signer.address: the sender address that will be available in the origin field of the context parameter of the onCrossChainCall function
  • args.token: the address of the ERC-20 token being deposited
  • value: the amount of tokens being deposited
  • data: the contents of the message

If the --token optional parameter is not used, the interact task assumes you want to deposit a gas token. To deposit a gas token you need to send a token transfer transaction to the TSS address on a connected chain.

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

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:

npx hardhat balances

Learn more about these and other ZetaChain toolkit commands avaialble in the template.

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 sepolia_testnet
🔑 Using account: 0x2cD3D070aE1BD365909dD859d29F387AA96911e1

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

Track the transaction:

npx hardhat cctx 0x93b441dc2ddb751a60a2f4c0fc52dbbd447ed70eb962b1a01072328aa6872b73
✓ CCTXs on ZetaChain found.

✓ 0x31310706ac4b33aa468e62a77d5db358e52a60dad3854210db8fc06c870186b6: 5 → 7001:
OutboundMined (Remote omnichain contract call completed)

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

https://explorer.zetachain.com/cc/tx/0x31310706ac4b33aa468e62a77d5db358e52a60dad3854210db8fc06c870186b6 (opens in a new tab)

The interact task has sent a token transfer transaction to the TSS address on Sepolia. 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
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