Implement Game Contract

Spending conditions are scripts providing the ability to apply specific rules and conditions to the transfer of funds. You can get more details in previous sections of this documentation

Create new Solidity file inside server/contracts folder and call it WordGame.sol.
Open this file in your code editor. Setup your pragma and import IERC20 interface from openzeppelin library:

pragma solidity ^0.5.0;
import "openzeppelin-solidity/contracts/token/ERC20/IERC20.sol";

Then define contract and some constants we will use later

contract WordGame {
    // Here we will define several constants that we replace at later stage
    // Think about those as simple masks. You can use any combination of hexadecimal numbers
    bytes32 constant ROUND_ID = 0x1337133713371337133713371337133713371337133713371337133713371337;

    // We will store our answer as bytes32 representation of selected word
    bytes32 constant ANSWER = 0x1234123412341234123412341234123412341234123412341234123412341234;

    // You can process the same contract using different tokens, by replacing this address
    address constant TOKEN_ADDR = 0x1222222222222222222222222222222222222222;

    // Player and house addresses to receive funds, based on outcome of the round
    address constant PLAYER = 0x1333333333333333333333333333333333333333;
    address constant HOUSE = 0x1444444444444444444444444444444444444444;

    // We will implement contract methods here

For the sake of simplicity our contract will have two methods: - roundResult which will process player answer and transfer funds according to it - cancelRound will simply return balance of the contract back to house wallet

function roundResult(bytes32 _playerAnswer, bytes32 _roundId) public {
    // Check that player is trying to unlock funds for correct answer 
    require(_roundId == ROUND_ID, "wrong round ID");

    // We will need IERC20 interface to get balance of our contract
    IERC20 token = IERC20(TOKEN_ADDR);
    uint balance = token.balanceOf(address(this));

    // And then, if answer is correct, contract will transfer funds either to player or house 
    if (_playerAnswer == ANSWER) {
        token.transfer(PLAYER, balance);
    } else {
        token.transfer(HOUSE, balance);

function cancelRound() public {
    IERC20 token = IERC20(TOKEN_ADDR);
    uint balance = token.balanceOf(address(this));
    token.transfer(HOUSE, balance);

Full listing WordGame.sol

Compile Contract

Open terminal inside of your server folder and call:

truffle compile

This will create a new folder "build/contract" with three files in it - IERC20.json, Migrations.json and WordGame.json. In next section we will use IERC20.json to get toke balance and later WordGame.json to deploy spending condition.