contracts_example

Game {
    Gold gold;
    Details details;

    buyDetailForEther(type) payable {
        uint256 price = details.getPrice(type);
        gold.buyGoldRemote(msg.value, price);
        address(gold).transfer(msg.value); // transfer ether to Gold contract
        details.buyDetail(msg.sender, type);
        gold.transfer(details, price); // transfer gold to Details contract
    }
}

Gold { // erc20
    mapping(address => uint256) ownedTokens;
    uint256 price;

    buyGoldRemote(sender, etherValue, amount) onlyGame { // onlyGame - modificator, that allows call this method only by Game
        require(amount * price <= etherValue);
        ownedTokens[msg.sender] += amount; // msg.sender == Game
    }
}

Details { // erc721
    // there some logic from erc721 standard

    uint256[] prices;

    struct Detail {
        uint8 type;
    }

    buyDetail(sender, type) onlyGame {
        Detail detail = Detail(type);
        ownedTokens[sender].push(detail);
    }

    getDetailPrice(type) {
        return prices[type];
    }
}