UniswapTrade.sol

pragma solidity ^0.5.12;

// A simple contract that interacts with Uniswap V1 to swap the ether you send with your desired ERC20 token
// I'm swapping for DAI in this particular example
contract UniswapTrade {

    // Initialisation parameters for uniswap on kovan testnet
    address kovanFactoryAddress = 0xD3E51Ef092B2845f10401a0159B2B96e8B6c3D30;
    address kovanDaiExchange = 0x8779C708e2C3b1067de9Cd63698E4334866c691C;
    address kovanDaiAddress = 0x4C38cDC08f1260F5c4b21685654393BB1e66a858;
    UniswapFactoryInterface uniswapFactory = UniswapFactoryInterface(kovanFactoryAddress);

    // Buys the ERC20 token from one of the liquidity pools
    // Payable: the ether you want to trade for the ERC20 token
    function buy() external payable {
    
       // pointer to the uniswap exchange
       UniswapExchangeInterface uniswapExchange = UniswapExchangeInterface(
           uniswapFactory.getExchange(kovanDaiAddress));
       
       // calculates how many of this ERC20 token you can get with the amount of ether you're sending with this buy() call
       uint tokenAmount = uniswapExchange.getEthToTokenInputPrice(msg.value); 
       
       // transfers the equivalent amount of the ERC20 token to the caller of this contract
       uniswapExchange.ethToTokenTransferInput.value(msg.value)(
           tokenAmount, // minimum amount of tokens to receive i.e. equivalent of eth you sent
           now + 300, // valid period of this trade (5 mins)
           msg.sender // address of the recipient i.e. you
       );
   }
}

// Factory interface imported from https://docs.uniswap.io/smart-contract-integration/interface
contract UniswapFactoryInterface {
    // Public Variables
    address public exchangeTemplate;
    uint256 public tokenCount;
    // Create Exchange
    function createExchange(address token) external returns (address exchange);
    // Get Exchange and Token Info
    function getExchange(address token) external view returns (address exchange);
    function getToken(address exchange) external view returns (address token);
    function getTokenWithId(uint256 tokenId) external view returns (address token);
    // Never use
    function initializeFactory(address template) external;
}

// Exchange interface imported from https://docs.uniswap.io/smart-contract-integration/interface
contract UniswapExchangeInterface {
    // Address of ERC20 token sold on this exchange
    function tokenAddress() external view returns (address token);
    // Address of Uniswap Factory
    function factoryAddress() external view returns (address factory);
    // Provide Liquidity
    function addLiquidity(uint256 min_liquidity, uint256 max_tokens, uint256 deadline) external payable returns (uint256);
    function removeLiquidity(uint256 amount, uint256 min_eth, uint256 min_tokens, uint256 deadline) external returns (uint256, uint256);
    // Get Prices
    function getEthToTokenInputPrice(uint256 eth_sold) external view returns (uint256 tokens_bought);
    function getEthToTokenOutputPrice(uint256 tokens_bought) external view returns (uint256 eth_sold);
    function getTokenToEthInputPrice(uint256 tokens_sold) external view returns (uint256 eth_bought);
    function getTokenToEthOutputPrice(uint256 eth_bought) external view returns (uint256 tokens_sold);
    // Trade ETH to ERC20
    function ethToTokenSwapInput(uint256 min_tokens, uint256 deadline) external payable returns (uint256  tokens_bought);
    function ethToTokenTransferInput(uint256 min_tokens, uint256 deadline, address recipient) external payable returns (uint256  tokens_bought);
    function ethToTokenSwapOutput(uint256 tokens_bought, uint256 deadline) external payable returns (uint256  eth_sold);
    function ethToTokenTransferOutput(uint256 tokens_bought, uint256 deadline, address recipient) external payable returns (uint256  eth_sold);
    // Trade ERC20 to ETH
    function tokenToEthSwapInput(uint256 tokens_sold, uint256 min_eth, uint256 deadline) external returns (uint256  eth_bought);
    function tokenToEthTransferInput(uint256 tokens_sold, uint256 min_eth, uint256 deadline, address recipient) external returns (uint256  eth_bought);
    function tokenToEthSwapOutput(uint256 eth_bought, uint256 max_tokens, uint256 deadline) external returns (uint256  tokens_sold);
    function tokenToEthTransferOutput(uint256 eth_bought, uint256 max_tokens, uint256 deadline, address recipient) external returns (uint256  tokens_sold);
    // Trade ERC20 to ERC20
    function tokenToTokenSwapInput(uint256 tokens_sold, uint256 min_tokens_bought, uint256 min_eth_bought, uint256 deadline, address token_addr) external returns (uint256  tokens_bought);
    function tokenToTokenTransferInput(uint256 tokens_sold, uint256 min_tokens_bought, uint256 min_eth_bought, uint256 deadline, address recipient, address token_addr) external returns (uint256  tokens_bought);
    function tokenToTokenSwapOutput(uint256 tokens_bought, uint256 max_tokens_sold, uint256 max_eth_sold, uint256 deadline, address token_addr) external returns (uint256  tokens_sold);
    function tokenToTokenTransferOutput(uint256 tokens_bought, uint256 max_tokens_sold, uint256 max_eth_sold, uint256 deadline, address recipient, address token_addr) external returns (uint256  tokens_sold);
    // Trade ERC20 to Custom Pool
    function tokenToExchangeSwapInput(uint256 tokens_sold, uint256 min_tokens_bought, uint256 min_eth_bought, uint256 deadline, address exchange_addr) external returns (uint256  tokens_bought);
    function tokenToExchangeTransferInput(uint256 tokens_sold, uint256 min_tokens_bought, uint256 min_eth_bought, uint256 deadline, address recipient, address exchange_addr) external returns (uint256  tokens_bought);
    function tokenToExchangeSwapOutput(uint256 tokens_bought, uint256 max_tokens_sold, uint256 max_eth_sold, uint256 deadline, address exchange_addr) external returns (uint256  tokens_sold);
    function tokenToExchangeTransferOutput(uint256 tokens_bought, uint256 max_tokens_sold, uint256 max_eth_sold, uint256 deadline, address recipient, address exchange_addr) external returns (uint256  tokens_sold);
    // ERC20 comaptibility for liquidity tokens
    bytes32 public name;
    bytes32 public symbol;
    uint256 public decimals;
    function transfer(address _to, uint256 _value) external returns (bool);
    function transferFrom(address _from, address _to, uint256 value) external returns (bool);
    function approve(address _spender, uint256 _value) external returns (bool);
    function allowance(address _owner, address _spender) external view returns (uint256);
    function balanceOf(address _owner) external view returns (uint256);
    function totalSupply() external view returns (uint256);
    // Never use
    function setup(address token_addr) external;
}