SupplyChain.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
...

//2nd step of suppply chain process
function sellItemByFarmer(uint256 _productCode, uint256 _price)
        public
        onlyFarmer // check _msgSender() belongs to farmerRole
        producedByFarmer(_productCode) // check items state has been produced
        verifyCaller(items[_productCode].ownerID) // check _msgSender() is owner
    {
        items[_productCode].itemState = State.ForSaleByFarmer;
        items[_productCode].productPrice = _price;
        emit ForSaleByFarmer(_productCode);
    }
    
// 3rd step of suppply chain process
    function purchaseItemByDistributor(uint256 _productCode)
        public
        payable
        onlyDistributor // check _msgSender() belongs to distributorRole
        forSaleByFarmer(_productCode) // check items state is for ForSaleByFarmer
        paidEnough(items[_productCode].productPrice) // check if distributor sent enough Ether for item
        checkValue(_productCode, payable(_msgSender())) // check if overpayed return remaing funds back to _msgSender()
    {
        address payable ownerAddressPayable = _make_payable(
            items[_productCode].farmerID
        ); // make originFarmID payable
        ownerAddressPayable.transfer(items[_productCode].productPrice); // transfer funds from distributor to farmer
        items[_productCode].ownerID = _msgSender(); // update owner
        items[_productCode].distributorID = _msgSender(); // update distributor
        items[_productCode].itemState = State.PurchasedByDistributor; // update state
        itemsHistory[_productCode].FTD = block.number; // add block number
        emit PurchasedByDistributor(_productCode);
    }
    ...