在编写同一个 sol 文件，已经导入了 uniswapfactroy 和 uniswapexchange 的接口文件，接下来新建一个 util 文件完成 ethToTokenSwapInput,代码如下

contract UniswapUtils {
    // ropsten testnet
    address public constant UNISWAP_FACTORY_ADDRESS = 0x9c83dCE8CA20E9aAF9D3efc003b2ea62aBC08351;
    // DAI address
    address public constant DAI_ADDRESS = 0xaD6D458402F60fD3Bd25163575031ACDce07538D;

function getUniswapExchange(address _tokenAddress)
        public
        view
        returns (address)
    {
        return
            IUniswapFactory(UNISWAP_FACTORY_ADDRESS).getExchange(_tokenAddress);
    }

/**
     * @dev ETH=>DAI
     * @param _ethAmount amount of ETH sold
     * @return Amount of DAI bought
     */
    function ethToDai(uint256 _ethAmount) 
        public 
        returns (uint256) 
    {
        return ethToDai(_ethAmount, uint256(1));
    }

    /**
     * @dev ETH=>DAI
     * @param _ethAmount amount of ETH sold
     * @param _minTokenAmount minimum DAI bought
     * @return Amount of DAI bought
     */
    function ethToDai(uint256 _ethAmount, uint256 _minTokenAmount)
        public
        returns (uint256)
    {
        IUniswapExchange uniswapExchange=IUniswapExchange(getUniswapExchange(DAI_ADDRESS));
        return uniswapExchange.ethToTokenSwapInput.value(_ethAmount)(_minTokenAmount, uint256(now + 60));
    }

}

在remix上测试ethtoDai(),显示错误,错误原因如下: https://ropsten.etherscan.io/tx/0xe2d3af02a85a1135c88d33d4f94e98dcf1825fa9ffb6b48388d3d970ab380051