@ethereum-waffle/mock-contract

@ethereum-waffle/mock-contract

Library for mocking smart contract dependencies during unit testing.

Warning

@ethereum-waffle/mock-contract is an experimental package. Breaking changes will not result in a new major version.

Installation

In the current version of waffle (v2.x.x) you will install this package as a dependency of the main waffle package - ethereum-waffle.

yarn add --dev ethereum-waffle
npm install --save-dev ethereum-waffle

If you want to use this package directly please install it via:

yarn add --dev @ethereum-waffle/mock-contract
npm install --save-dev @ethereum-waffle/mock-contract

Usage

Create an instance of a mock contract providing the ABI/interface of the smart contract you want to mock:

import {deployMockContract} from '@ethereum-waffle/mock-contract';

...

const mockContract = await deployMockContract(wallet, contractAbi);

Mock contract can now be passed into other contracts by using the address attribute.

Return values for mocked functions can be set using:

await mockContract.mock.<nameOfMethod>.returns(<value>)
await mockContract.mock.<nameOfMethod>.withArgs(<arguments>).returns(<value>)

Methods can also be set up to be reverted using:

await mockContract.mock.<nameOfMethod>.reverts()
await mockContract.mock.<nameOfMethod>.withArgs(<arguments>).reverts()

Example

The example below illustrates how mock-contract can be used to test the very simple AmIRichAlready contract.

pragma solidity ^0.6.3;

import "openzeppelin-solidity/contracts/token/ERC20/IERC20.sol";

contract AmIRichAlready {
    IERC20 private tokenContract;
    uint private constant RICHNESS = 1000000 * 10 ** 18;

    constructor (IERC20 _tokenContract) public {
        tokenContract = _tokenContract;
    }

    function check() public view returns (bool) {
        uint balance = tokenContract.balanceOf(msg.sender);
        return balance > RICHNESS;
    }
}

We are mostly interested in the tokenContract.balanceOf call. Mock contract will be used to mock exactly this call with values that are significant for the return of the check() method.

import chai, {expect} from 'chai';
import chaiAsPromised from 'chai-as-promised';
import ethers from 'ethers';
import {MockProvider} from '@ethereum-waffle/provider';
import {waffleChai} from '@ethereum-waffle/chai';
import {deployMockContract} from '@ethereum-waffle/mock-contract';

import IERC20 from '../../build/IERC20';
import AmIRichAlready from '../../build/AmIRichAlready';

chai.use(chaiAsPromised);
chai.use(waffleChai);

describe('Am I Rich Already?', () => {
  const [user, stranger] = new MockProvider().getWallets();
  let contract; // an instance of the AmIRichAlready contract
  let mockERC20; // an instance of a mock contract for the ERC20 token we want to observe

  beforeEach(async () => {
    mockERC20 = await deployMockContract(user, IERC20.abi);
    const contractFactory = new ContractFactory(AmIRichAlready.abi, AmIRichAlready.bytecode, sender)
    contract = await contractFactory.deploy(mockERC20.address);
  });

  describe('check method', () => {
    it('returns false if the wallet has less then 1000000 DAI', async () => {
      // configure mockERC20 to return 999999 when balanceOf is called
      await mockERC20.mock.balanceOf.returns(ethers.utils.parseEther('999999'));
      expect(await contract.check()).to.be.equal(false);
    });

    it('returns false if the wallet has exactly 1000000 DAI', async () => {
      // subsequent calls override the previous config
      await mockERC20.mock.balanceOf.returns(ethers.utils.parseEther('1000000'));
      expect(await contract.check()).to.equal(false);
    });

    it('returns true if the wallet has more then 1000000 DAI', async () => {
      await mockERC20.mock.balanceOf.returns(ethers.utils.parseEther('1000001'));
      expect(await contract.check()).to.equal(true);
    });

    it('reverts for some reason', async () => {
      await mockERC20.mock.balanceOf.reverts();
      await expect(contract.check()).to.be.revertedWith('Mock revert');
    });

    it('returns 1000001 DAI for my address and 0 otherwise', async () => {
        await mockERC20.mock.balanceOf.withArgs(user.address).returns(ethers.utils.parseEther('1000001'));
        // mock without any arguments will be triggered by default
        await mockERC20.mock.balanceOf.returns('0');

        expect(await contract.check()).to.equal(true);
        expect(await contract.attach(stranger).check()).to.equal(false);
    }); 
  });
});

Special thanks

Special thanks to @spherefoundry for creating the original Doppelganger project.