Watch Contract Events - Para Docs

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Subscribe to and filter smart contract events to react to on-chain activity in real-time.

Prerequisites

You need Web3 libraries configured with Para authentication.

Watch Contract Events

Ethers.js
Viem
Wagmi

import { ethers } from "ethers";

const ERC20_ABI = [
  "event Transfer(address indexed from, address indexed to, uint256 value)"
];

async function watchTransferEvents(
  provider: ethers.Provider,
  tokenAddress: string
) {
  const contract = new ethers.Contract(
    tokenAddress,
    ERC20_ABI,
    provider
  );

  contract.on("Transfer", (from, to, value, event) => {
    console.log(`Transfer Event: ${from} -> ${to}, Value: ${ethers.formatUnits(value, 18)}`);
    console.log("Event details:", event);
  });

  console.log(`Listening for Transfer events on ${tokenAddress}...`);
}

async function stopWatchingTransferEvents(
  provider: ethers.Provider,
  tokenAddress: string
) {
  const contract = new ethers.Contract(
    tokenAddress,
    ERC20_ABI,
    provider
  );
  contract.off("Transfer");
  console.log(`Stopped listening for Transfer events on ${tokenAddress}.`);
}

import { createPublicClient, http } from "viem";
import { mainnet } from "viem/chains";

const ERC20_ABI = [
  {
    anonymous: false,
    inputs: [
      { indexed: true, name: "from", type: "address" },
      { indexed: true, name: "to", type: "address" },
      { indexed: false, name: "value", type: "uint256" },
    ],
    name: "Transfer",
    type: "event",
  },
] as const;

async function watchTransferEvents(
  publicClient: any,
  tokenAddress: `0x${string}`
) {
  const unwatch = publicClient.watchContractEvent({
    address: tokenAddress,
    abi: ERC20_ABI,
    eventName: "Transfer",
    onLogs: (logs) => {
      for (const log of logs) {
        console.log(`Transfer Event: ${log.args.from} -> ${log.args.to}, Value: ${log.args.value}`);
        console.log("Log details:", log);
      }
    },
  });

  console.log(`Listening for Transfer events on ${tokenAddress}...`);
  return unwatch; // Return the unwatch function to stop listening later
}

import { useWatchContractEvent } from "@getpara/react-sdk/wagmi";

const ERC20_ABI = [
  {
    anonymous: false,
    inputs: [
      { indexed: true, name: "from", type: "address" },
      { indexed: true, name: "to", type: "address" },
      { indexed: false, name: "value", type: "uint256" },
    ],
    name: "Transfer",
    type: "event",
  },
] as const;

function WatchTransferEvents({ tokenAddress }: { tokenAddress: `0x${string}` }) {
  useWatchContractEvent({
    address: tokenAddress,
    abi: ERC20_ABI,
    eventName: "Transfer",
    onLogs: (logs) => {
      for (const log of logs) {
        console.log(`Transfer Event: ${log.args.from} -> ${log.args.to}, Value: ${log.args.value}`);
        console.log("Log details:", log);
      }
    },
  });

  return <div>Listening for Transfer events on {tokenAddress}...</div>;
}

Next Steps