ETHUSDOracle.sol

/* ETH price fetcher

This smart contract keeps tracking 
of ETH price in USD

*/

pragma solidity ^0.4.24;
import "github.com/oraclize/ethereum-api/oraclizeAPI_0.4.sol";

// For newer pragma solidity versions, the previous statement
// should be the following:

// pragma solidity ^0.5.0;
// import "github.com/oraclize/ethereum-api/oraclizeAPI.sol";

contract ETHUSD is usingOraclize {
    uint public etherUSD;
    
    constructor () public {
        etherUSD = 0;
    }
    
    // __callback is an overridden function provided by oraclize api
    // to handle API responses
    function __callback(bytes32 myid, string memory result) public {
        // oraclize_cbAddress() ensures that the data provided comes from
        // oraclize provider, and not anyone could update this data
        require(msg.sender == oraclize_cbAddress());
        
        etherUSD = parseInt(result, 2); // Recover 2 decimals
    }
    
    function update() public payable {
        // oraclize_query() is the way to query an API from the smart contract
        // Receives 2 parameters:
        // 1.- Data source
        // 2.- Parser helper with data
        oraclize_query("URL", "json(https://api.kraken.com/0/public/Ticker?pair=ETHUSD).result.XETHZUSD.c.0");
    }
}