Contract event listener is not firing when running hardhat tests

[adamdry]

Here is a very small repo to show the issue: https://github.com/adamdry/ethers-event-issue

But I'll explain it here too. This is my contract:

//SPDX-License-Identifier: UNLICENSED;
pragma solidity 0.8.4;

contract ContractA {

    event TokensMinted(uint amount);

    function mint(uint amount) public {
        emit TokensMinted(amount);
    }

}

And this is my test code:

import * as chai from 'chai'
import { BigNumber, ContractTransaction } from 'ethers'
import { ethers } from 'hardhat'
import { ContractA, ContractAFactory } from '../typechain'

const expect = chai.expect

describe("Example test", function () {
    it("should fire the event", async function () {
        const [owner] = await ethers.getSigners();

        const contractAFactory = (await ethers.getContractFactory(
            'ContractA',
            owner,
        )) as ContractAFactory

        const contractA: ContractA = await contractAFactory.deploy()

        contractA.on('TokensMinted', (amount: BigNumber) => {
            // THIS LINE NEVER GETS HIT
            console.log('###########')
        })

        const contractTx: ContractTransaction = await contractA.mint(123)
        const contractReceipt: ContractReceipt = await contractTx.wait()

        for (const event of contractReceipt.events!) {
            console.log(JSON.stringify(event))
        }
    });
});

I was expecting the ########### to get printed to the console however it doesn't so the listener function isn't being executed for some reason.

If I dig into the ContractReceipt the correct event data is there:

{
  "transactionIndex": 0,
  "blockNumber": 2,
  "transactionHash": "0x55d118548c8200e5e6c19759d9aab56cb2e6a274186a92643de776d617d51e1a",
  "address": "0x5FbDB2315678afecb367f032d93F642f64180aa3",
  "topics": [
    "0x772f66a00a405709c30e7f18feadcc8f123b20c09c7260165d3eec36c9f21372"
  ],
  "data": "0x000000000000000000000000000000000000000000000000000000000000007b",
  "logIndex": 0,
  "blockHash": "0x808e6949118509b5a9e482e84cf47921a2fcffbcd943ebbd8ce4f6671469ee01",
  "args": [
    {
      "type": "BigNumber",
      "hex": "0x7b"
    }
  ],
  "event": "TokensMinted",
  "eventSignature": "TokensMinted(uint256)"
}

[fvictorio]

This might be a duplicate of #588

[fvictorio]

Hey @adamdry, the reason this doesn't work is that ethers.js, by default, uses polling to get events, and the polling interval is 4 seconds. If you add this at the end of your test:

await new Promise(res => setTimeout(() => res(null), 5000));

you'll see the log being printed. (The null is just for the test to compile.) As far as I know, the only way for ethers to not do this is to use a WebSocketProvider, but we can't do that for the in-process Hardhat Network.

Also, I don't think the polling interval can be configured 🙁 But maybe you can check ethers.js discussions board to see if someone asked about that.

Closing this for now, but please re-open if you think there's something we can do on our side.

[feuGeneA]

Also, I don't think the polling interval can be configured slightly_frowning_face But maybe you can check ethers.js discussions board to see if someone asked about that.

Actually the polling interval can be configured. See how I did it in this test: https://github.com/nomiclabs/hardhat/blob/master/packages/hardhat-ethers/test/index.ts#L642

Also, I commented in that test where I got this technique, but just convenience, here too is the source: ethers-io/ethers.js#615 (comment)

[fvictorio]

@feuGeneA oh, nice. I tried modifying ethers.provider that way and couldn't do it, but I think that happens because hre.ethers is a proxied object.

[adamdry]

@fvictorio @feuGeneA Thank you for your input! These are both very helpful to know. This solved my problem. Cheers!