Smart Contract Quick start

Developing smart contracts for Ethereum involves a bevy of off-chain tools used for producing and testing bytecode that runs on the Ethereum Virtual Machine (EVM). Some tools also include workflows for deploying this bytecode to the Ethereum network and testnets. There are many options for these tools. This guide walks you through writing and testing a simple smart contract that interacts with the Uniswap Protocol using one specific set of tools (truffle + npm + mocha).

Requirements

To follow this guide, you must have the following installed:

Bootstrapping a project

You can start from scratch, but it’s easier to use a tool like truffle to bootstrap an empty project. Create an empty directory and run npx truffle init inside that directory to unbox the default Truffle box.

mkdir demo
cd demo
npx truffle init

Setting up npm

In order to reference the Uniswap V2 contracts, you should use the npm artifacts we deploy containing the core and periphery smart contracts and interfaces. To add npm dependencies, we first initialize the npm package. We can run npm init in the same directory to create a package.json file. You can accept all the defaults and change it later.

npm init

Adding dependencies

Now that we have an npm package, we can add our dependencies. Let’s add both the @uniswap/v2-core and @uniswap/v2-periphery packages.

npm i --save @uniswap/v2-core
npm i --save @uniswap/v2-periphery

If you check the node_modules/@uniswap directory, you can now find the Uniswap V2 contracts.

moody@MacBook-Pro ~/I/u/demo> ls node_modules/@uniswap/v2-core/contracts
UniswapV2ERC20.sol UniswapV2Pair.sol libraries/
UniswapV2Factory.sol interfaces/ test/
moody@MacBook-Pro ~/I/u/demo> ls node_modules/@uniswap/v2-periphery/contracts/
UniswapV2Migrator.sol examples/ test/
UniswapV2Router01.sol interfaces/
UniswapV2Router02.sol libraries/

These packages include both the smart contract source code and the build artifacts.

Writing our contract

We can now get started writing our example contract. For writing Solidity, we recommend IntelliJ or VSCode with a solidity plugin, but you can use any text editor. Let’s write a contract that returns the value of some amount of liquidity shares for a given token pair. First create a couple of files:

mkdir contracts/interfaces
touch contracts/interfaces/ILiquidityValueCalculator.sol
touch contracts/LiquidityValueCalculator.sol

This will be the interface of the contract we implement. Put it in contracts/interfaces/ILiquidityValueCalculator.sol.

pragma solidity =0.6.6;

interface ILiquidityValueCalculator {
function computeLiquidityShareValue(uint liquidity, address tokenA, address tokenB) external returns (uint tokenAAmount, uint tokenBAmount);
}

Now let’s start with the constructor. You need to know where the UniswapV2Factory is deployed in order to compute the address of the pair and look up the total supply of liquidity shares, plus the amounts for the reserves. We can store this as an address passed to the constructor.

The factory address is constant on mainnet and all testnets, so it may be tempting to make this value a constant in your contract, but since we need to unit test the contract it should be an argument. You can use solidity immutables to save on gas when accessing this variable.

pragma solidity =0.6.6;

import './interfaces/ILiquidityValueCalculator.sol';

contract LiquidityValueCalculator is ILiquidityValueCalculator {
address public factory;
constructor(address factory_) public {
factory = factory_;
}
}

Now we need to be able to look up the total supply of liquidity for a pair, and its token balances. Let’s put this in a separate function. To implement it, we must:

  1. Look up the pair address
  2. Get the reserves of the pair
  3. Get the total supply of the pair liquidity
  4. Sort the reserves in the order of tokenA, tokenB

The UniswapV2Library has some helpful methods for this.

pragma solidity =0.6.6;

import '@uniswap/v2-periphery/contracts/libraries/UniswapV2Library.sol';
import '@uniswap/v2-core/contracts/interfaces/IUniswapV2Pair.sol';

contract LiquidityValueCalculator is ILiquidityValueCalculator {
function pairInfo(address tokenA, address tokenB) internal view returns (uint reserveA, uint reserveB, uint totalSupply) {
IUniswapV2Pair pair = IUniswapV2Pair(UniswapV2Library.pairFor(factory, tokenA, tokenB));
totalSupply = pair.totalSupply();
(uint reserves0, uint reserves1,) = pair.getReserves();
(reserveA, reserveB) = tokenA == pair.token0() ? (reserves0, reserves1) : (reserves1, reserves0);
}
}

Finally we just need to compute the share value. We will leave that as an exercise to the reader.

pragma solidity =0.6.6;

import './interfaces/ILiquidityValueCalculator.sol';
import '@uniswap/v2-periphery/contracts/libraries/UniswapV2Library.sol';
import '@uniswap/v2-core/contracts/interfaces/IUniswapV2Pair.sol';

contract LiquidityValueCalculator is ILiquidityValueCalculator {
address public factory;
constructor(address factory_) public {
factory = factory_;
}

function pairInfo(address tokenA, address tokenB) internal view returns (uint reserveA, uint reserveB, uint totalSupply) {
IUniswapV2Pair pair = IUniswapV2Pair(UniswapV2Library.pairFor(factory, tokenA, tokenB));
totalSupply = pair.totalSupply();
(uint reserves0, uint reserves1,) = pair.getReserves();
(reserveA, reserveB) = tokenA == pair.token0() ? (reserves0, reserves1) : (reserves1, reserves0);
}
function computeLiquidityShareValue(uint liquidity, address tokenA, address tokenB) external override returns (uint tokenAAmount, uint tokenBAmount) {
revert('TODO');
}
}

Writing tests

In order to test your contract, you need to:

  1. Bring up a testnet
  2. Deploy the UniswapV2Factory
  3. Deploy at least 2 ERC20 tokens for a pair
  4. Create a pair for the factory
  5. Deploy your LiquidityValueCalculator contract
  6. Call LiquidityValueCalculator#computeLiquidityShareValue
  7. Verify the result with an assertion

#1 is handled for you automatically by the truffle test command.

Note you should only deploy the precompiled Uniswap contracts in the build directories for unit tests. This is because solidity appends a metadata hash to compiled contract artifacts which includes the hash of the contract source code path, and compilations on other machines will not result in the exact same bytecode. This is problematic because in Uniswap V2 we use the hash of the bytecode in the v2-periphery UniswapV2Library, to compute the pair address.

To get the bytecode for deploying UniswapV2Factory, you can import the file via:

const UniswapV2FactoryBytecode = require('@uniswap/v2-core/build/UniswapV2Factory.json').bytecode

We recommend using a standard ERC20 from @openzeppelin/contracts for deploying an ERC20.

You can read more about deploying contracts and writing tests using Truffle here.

Compiling and deploying the contract

Learn more about compiling and deploying contracts using Truffle here and here respectively.

WIP

This guide is a WIP. Please contribute to this guide with the edit button below!