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Local testing using a Mock contract

This guide explains how to test Chainlink VRF v2 on a Remix IDE sandbox blockchain environment. Note: You can reuse the same logic on another development environment, such as Hardhat or Truffle. For example, read the Hardhat Starter Kit RandomNumberConsumer unit tests.

Benefits of local testing

Testing locally using mock contracts saves you time and resources during development. Some of the key benefits include:

  • Faster feedback loop: Immediate feedback on the functionality and correctness of your smart contracts. This helps you quickly identify and fix issues without waiting for transactions to be mined/validated on a testnet.
  • Saving your native testnet gas: Deploying and interacting with contracts requires paying gas fees. Although native testnet gas does not have any associated value, supply is limited by public faucets. Using mock contracts locally allows you to test your contracts freely without incurring any expenses.
  • Controlled environment: Local testing allows you to create a controlled environment where you can manipulate various parameters, such as block time and gas prices, to test your smart contracts' function as expected under different conditions.
  • Isolated testing: You can focus on testing individual parts of your contract, ensuring they work as intended before integrating them with other components.
  • Easier debugging: Because local tests run on your machine, you have better control over the debugging process. You can set breakpoints, inspect variables, and step through your code to identify and fix issues.
  • Comprehensive test coverage: You can create test cases to cover all possible scenarios and edge cases.

Testing logic

Complete the following tasks to test your VRF v2 consumer locally:

  1. Deploy the VRFCoordinatorV2Mock. This contract is a mock of the VRFCoordinatorV2 contract.
  2. Call the VRFCoordinatorV2Mock createSubscription function to create a new subscription.
  3. Call the VRFCoordinatorV2Mock fundSubscription function to fund your newly created subscription. Note: You can fund with an arbitrary amount.
  4. Deploy your VRF consumer contract.
  5. Call the the VRFCoordinatorV2Mock addConsumer function to add your consumer contract to your subscription.
  6. Request random words from your consumer contract.
  7. Call the VRFCoordinatorV2Mock fulfillRandomWords function to fulfill your consumer contract request.

Testing

Open the contracts on RemixIDE

Open VRFCoordinatorV2Mock and compile in Remix:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;
import "@chainlink/contracts/src/v0.8/mocks/VRFCoordinatorV2Mock.sol";
Open in Remix What is Remix?

Open VRFv2Consumer and compile in Remix:

// SPDX-License-Identifier: MIT
// An example of a consumer contract that relies on a subscription for funding.
pragma solidity ^0.8.7;

import "@chainlink/contracts/src/v0.8/interfaces/VRFCoordinatorV2Interface.sol";
import "@chainlink/contracts/src/v0.8/vrf/VRFConsumerBaseV2.sol";

/**
 * THIS IS AN EXAMPLE CONTRACT THAT USES HARDCODED VALUES FOR CLARITY.
 * THIS IS AN EXAMPLE CONTRACT THAT USES UN-AUDITED CODE.
 * DO NOT USE THIS CODE IN PRODUCTION.
 */

/**
 * @title The RandomNumberConsumerV2 contract
 * @notice A contract that gets random values from Chainlink VRF V2
 */
contract RandomNumberConsumerV2 is VRFConsumerBaseV2 {
    VRFCoordinatorV2Interface immutable COORDINATOR;

    // Your subscription ID.
    uint64 immutable s_subscriptionId;

    // The gas lane to use, which specifies the maximum gas price to bump to.
    // For a list of available gas lanes on each network,
    // see https://docs.chain.link/docs/vrf-contracts/#configurations
    bytes32 immutable s_keyHash;

    // Depends on the number of requested values that you want sent to the
    // fulfillRandomWords() function. Storing each word costs about 20,000 gas,
    // so 100,000 is a safe default for this example contract. Test and adjust
    // this limit based on the network that you select, the size of the request,
    // and the processing of the callback request in the fulfillRandomWords()
    // function.
    uint32 constant CALLBACK_GAS_LIMIT = 100000;

    // The default is 3, but you can set this higher.
    uint16 constant REQUEST_CONFIRMATIONS = 3;

    // For this example, retrieve 2 random values in one request.
    // Cannot exceed VRFCoordinatorV2.MAX_NUM_WORDS.
    uint32 constant NUM_WORDS = 2;

    uint256[] public s_randomWords;
    uint256 public s_requestId;
    address s_owner;

    event ReturnedRandomness(uint256[] randomWords);

    /**
     * @notice Constructor inherits VRFConsumerBaseV2
     *
     * @param subscriptionId - the subscription ID that this contract uses for funding requests
     * @param vrfCoordinator - coordinator, check https://docs.chain.link/docs/vrf-contracts/#configurations
     * @param keyHash - the gas lane to use, which specifies the maximum gas price to bump to
     */
    constructor(
        uint64 subscriptionId,
        address vrfCoordinator,
        bytes32 keyHash
    ) VRFConsumerBaseV2(vrfCoordinator) {
        COORDINATOR = VRFCoordinatorV2Interface(vrfCoordinator);
        s_keyHash = keyHash;
        s_owner = msg.sender;
        s_subscriptionId = subscriptionId;
    }

    /**
     * @notice Requests randomness
     * Assumes the subscription is funded sufficiently; "Words" refers to unit of data in Computer Science
     */
    function requestRandomWords() external onlyOwner {
        // Will revert if subscription is not set and funded.
        s_requestId = COORDINATOR.requestRandomWords(
            s_keyHash,
            s_subscriptionId,
            REQUEST_CONFIRMATIONS,
            CALLBACK_GAS_LIMIT,
            NUM_WORDS
        );
    }

    /**
     * @notice Callback function used by VRF Coordinator
     *
     * @param  - id of the request
     * @param randomWords - array of random results from VRF Coordinator
     */
    function fulfillRandomWords(
        uint256 /* requestId */,
        uint256[] memory randomWords
    ) internal override {
        s_randomWords = randomWords;
        emit ReturnedRandomness(randomWords);
    }

    modifier onlyOwner() {
        require(msg.sender == s_owner);
        _;
    }
}
Open in Remix What is Remix?

Your RemixIDE file explorer should display VRFCoordinatorV2Mock.sol and VRFv2Consumer.sol:

Deploy VRFCoordinatorV2Mock

  1. Open VRFCoordinatorV2Mock.sol.

  2. Under DEPLOY & RUN TRANSACTIONS, select VRFCoordinatorV2Mock.

  3. Under DEPLOY, fill in the _BASEFEE and _GASPRICELINK. These variables are used in the VRFCoordinatorV2Mock contract to represent the base fee and the gas price (in LINK tokens) for the VRF requests. You can set: _BASEFEE=100000000000000000 and _GASPRICELINK=1000000000.

  4. Click on transact to deploy the VRFCoordinatorV2Mock contract.

  5. Once deployed, you should see the VRFCoordinatorV2Mock contract under Deployed Contracts.

  6. Note the address of the deployed contract.

Create and fund a subscription

  1. Click on createSubscription to create a new subscription.

  2. In the RemixIDE console, read your transaction decoded output to find the subscription ID. In this example, the subscription ID is 1.

  3. Click on fundSubscription to fund your subscription. In this example, you can set the _subid to 1 (which is your newly created subscription ID) and the _amount to 1000000000000000000.

Deploy the VRF consumer contract

  1. In the file explorer, open VRFv2Consumer.sol.

  2. Under DEPLOY & RUN TRANSACTIONS, select RandomNumberConsumerV2.

  3. Under DEPLOY, fill in SUBSCRIPTIONID with your subscription ID, vrfCoordinator with the deployed VRFCoordinatorV2Mock address and, KEYHASH with an arbitrary bytes32 (In this example, you can set the KEYHASH to 0xd89b2bf150e3b9e13446986e571fb9cab24b13cea0a43ea20a6049a85cc807cc).

  4. Click on transact to deploy the RandomNumberConsumerV2 contract.

  5. After the consumer contract is deployed, you should see the RandomNumberConsumerV2 contract under Deployed Contracts.

  6. Note the address of the deployed contract.

Add the consumer contract to your subscription

  1. Under Deployed Contracts, open the functions list of your deployed VRFCoordinatorV2Mock contract.

  2. Click on addConsumer and fill in the _subid with your subscription ID and _consumer with your deployed consumer contract address.

  3. Click on transact.

Request random words

  1. Under Deployed Contracts, open the functions list of your deployed RandomNumberConsumerV2 contract.

  2. Click on requestRandomWords.

  3. In the RemixIDE console, read your transaction logs to find the VRF request ID. In this example, the request ID is 1.

  4. Note your request ID.

Fulfill the VRF request

Because you are testing on a local blockchain environment, you must fulfill the VRF request yourself.

  1. Under Deployed Contracts, open the functions list of your deployed VRFCoordinatorV2Mock contract.

  2. Click fulfillRandomWords and fill in _requestId with your VRF request ID and _consumer with your consumer contract address.

  3. Click on transact.

Check the results

  1. Under Deployed Contracts, open the functions list of your deployed RandomNumberConsumerV2 contract.

  2. Click on s_requestId to display the last request ID. In this example, the output is 1.

  3. Each time you make a VRF request, your consumer contract requests two random words. After the request is fulfilled, the two random words are stored in the s_randomWords array. You can check the stored random words by reading the two first indexes of the s_randomWords array. To do so, click on the s_randomWords function and:

    1. Fill in the index with 0 then click on call to read the first random word.

    2. Fill in the index with 1 then click on call to read the second random word.

Next steps

This guide demonstrated how to test a VRF v2 consumer contract on your local blockchain. We made the guide on RemixIDE for learning purposes, but you can reuse the same testing logic on another development environment, such as Truffle or Hardhat. For example, read the Hardhat Starter Kit RandomNumberConsumer unit tests.

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