VRFCoordinatorV2Mock's fulfillRandomWords not being called in hardhat local deployment

Question

I'm building a smart contract that generates 3 random numbers using Chainlink VRF. My contract (SlotMachine.sol) implements the VRFConsumerBaseV2 and the constructor has 2 parameters: _subscriptionId and _vrfCoordinator:

constructor(
        uint64 _subscriptionId,
        address _vrfCoordinator
    ) payable VRFConsumerBaseV2(_vrfCoordinator) 

I have a play() function which calls the requestRandonWords through the vrfCoordinator and I overrided the fulfillRandomWord function to use the generated random words.

I want to do 2 things: 1) create the unit tests in slotmachine.js and 2) test my contract in the hardhat network (chainId = 31337) through deploy.js

For this I'm using the VRFCoordinatorV2Mock which helps me mock an oracle's behavior (as I understand).

My unit tests in slotmachine.js are working well. The fulfillRandomWords is called and then my tests are passing. However when I add a similar logic to the deploy.js file (deploy the mock contract and then the slot machine contract) the fulfillRandomWords is not being called when I call the play() function (which has the requestRandomWords inside it) from my deployed contract.

slotmachine.js

const { ethers } = require("hardhat");
const { expect } = require("chai");
const { BigNumber } = require("ethers");

const provider = ethers.getDefaultProvider();

describe("Decentralized Slot Machine", async function () {
  let myContract;
  let hardhatVrfCoordinatorV2Mock;

  describe("Testing Decentralized Slot Machine", function () {
    //1. Contract deployment
    it("Should deploy Slot Machine Contract", async function () {
      const SlotMachine = await ethers.getContractFactory("SlotMachine");
      let vrfCoordinatorV2Mock = await ethers.getContractFactory(
        "VRFCoordinatorV2Mock"
      );

      hardhatVrfCoordinatorV2Mock = await vrfCoordinatorV2Mock.deploy(0, 0);

      await hardhatVrfCoordinatorV2Mock.createSubscription();

      await hardhatVrfCoordinatorV2Mock.fundSubscription(
        1,
        ethers.utils.parseEther("7")
      );

      myContract = await SlotMachine.deploy(
        1,
        hardhatVrfCoordinatorV2Mock.address,
        {
          value: ethers.utils.parseEther("100"),
        }
      );

      await hardhatVrfCoordinatorV2Mock.addConsumer(1, myContract.address);
    });

    //2. Play
    describe("First Play - First Player", function () {
      it("Contract should receive random numbers", async () => {
        const [account1, account2] = await ethers.getSigners();

        let tx = await myContract.play(ethers.constants.AddressZero, {
          value: ethers.utils.parseEther("1"),
        });
        let { events } = await tx.wait();

        let [reqId] = events.filter((x) => x.event === "RequestedRandomness")[0]
          .args;

        await expect(
          hardhatVrfCoordinatorV2Mock.fulfillRandomWords(
            reqId,
            myContract.address
          )
        ).to.emit(myContract, "ReceivedRandomness");

        let round = await myContract.rounds(reqId);

        expect(round.userAddress).to.be.equal(account1.address);
        expect(round.number1).to.be.equal(1);
        expect(round.number2).to.be.equal(9);
        expect(round.number3).to.be.equal(6);
      });       

    });
  });
});

deploy.js

const { ethers } = require("hardhat");

const localChainId = "31337";

module.exports = async ({ getNamedAccounts, deployments, getChainId }) => {
  const { deploy } = deployments;
  const { deployer } = await getNamedAccounts();
  const chainId = await getChainId();

  await deploy("VRFCoordinatorV2Mock", {
    from: deployer,
    args: [0, 0],
    log: true,
    waitConfirmations: 5,
  });

  const hardhatVrfCoordinatorV2Mock = await ethers.getContract(
    "VRFCoordinatorV2Mock",
    deployer
  );

  await hardhatVrfCoordinatorV2Mock.createSubscription();

  await hardhatVrfCoordinatorV2Mock.fundSubscription(
    1,
    ethers.utils.parseEther("7")
  );

  const myContract = await deploy("SlotMachine", {
    from: deployer,
    args: [1, hardhatVrfCoordinatorV2Mock.address],
    log: true,
    waitConfirmations: 5,
  });

  await hardhatVrfCoordinatorV2Mock.addConsumer(1, myContract.address);

  console.log("Contract address: ", myContract.address);
};
module.exports.tags = ["SlotMachine"];

Question

Can I deploy my contract in the hardhat network with the expected behavior of the VRFCoordinatorV2Mock which is to call automatically fulfillRandomWords? If so, how can I do this? What do I have to change in my code? If the answer is no, is there another alternative than deploying to a testnet?

Answer 1

You need to create a script which calls the ‘play()’ function on deployed contract locally in hardhat you can do it with ‘yarn hardhat run scripts/callPlay.js’ (considering you have callPlay.js inside your ‘scripts’ directory)

Also feel free to checkout https://github.com/technophile-04/Lottery-SmartContract-PureRandom it has mocks and and scripts to mock it locally(here I have used Chainlink keepers too so it might be a bit messy but you will get the idea) also checkout https://youtu.be/gyMwXuJrbJQ at timestamp 13:41:07 you will get the full idea how to do it.

Answer 2

I solved my problem.

Nope, the fulfillRandomWords cannot be called automatically because it is called by Chainlink's VRFCoordinator and it only works on a mainnet or a testnet. So I have to call it manually from my VRFCoordinatorV2Mock, after I call my play function which returns the requestId I have to use as a parameter.