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Item Smart Contract

Let's add another smart contract:

pragma solidity ^0.6.0;

import "./ItemManager.sol";

contract Item {
    uint public priceInWei;
    uint public paidWei;
    uint public index;

    ItemManager parentContract;

    constructor(ItemManager _parentContract, uint _priceInWei, uint _index) public {
        priceInWei = _priceInWei;
        index = _index;
        parentContract = _parentContract;

    receive() external payable {
        require(msg.value == priceInWei, "We don't support partial payments");
        require(paidWei == 0, "Item is already paid!");
        paidWei += msg.value;
        (bool success, ) = address(parentContract).call{value:msg.value}(abi.encodeWithSignature("triggerPayment(uint256)", index));
        require(success, "Delivery did not work");

    fallback () external {



Solidity Changes

Note that call.value(msg.value)(abi.encodeWithSignature("triggerPayment(uint256)",Β index)), due to changes at Solidity version 6.4 it is recommended it should be changed to call{value:msg.value}(abi.encodeWithSignature("triggerPayment(uint256)",Β index)).

And change the ItemManager Smart Contract to use the Item Smart Contract instead of the Struct only:

pragma solidity ^0.6.0;

import "./Item.sol";

contract ItemManager {

    struct S_Item {
        Item _item;
        ItemManager.SupplyChainSteps _step;
        string _identifier;
    mapping(uint => S_Item) public items;
    uint index;

    enum SupplyChainSteps {Created, Paid, Delivered}

    event SupplyChainStep(uint _itemIndex, uint _step, address _address);

    function createItem(string memory _identifier, uint _priceInWei) public {
        Item item = new Item(this, _priceInWei, index);
        items[index]._item = item;
        items[index]._step = SupplyChainSteps.Created;
        items[index]._identifier = _identifier;
        emit SupplyChainStep(index, uint(items[index]._step), address(item));

    function triggerPayment(uint _index) public payable {
        Item item = items[_index]._item;
        require(address(item) == msg.sender, "Only items are allowed to update themselves");
        require(item.priceInWei() == msg.value, "Not fully paid yet");
        require(items[_index]._step == SupplyChainSteps.Created, "Item is further in the supply chain");
        items[_index]._step = SupplyChainSteps.Paid;
        emit SupplyChainStep(_index, uint(items[_index]._step), address(item));

    function triggerDelivery(uint _index) public {
        require(items[_index]._step == SupplyChainSteps.Paid, "Item is further in the supply chain");
        items[_index]._step = SupplyChainSteps.Delivered;
        emit SupplyChainStep(_index, uint(items[_index]._step), address(items[_index]._item));

Now with this we just have to give a customer the address of the Item Smart Contract created during "createItem" and he will be able to pay directly by sending X Wei to the Smart Contract. But the smart contract isn't very secure yet. We need some sort of owner functionality.

Last update: March 28, 2022