Event_Trigger.sol

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.6.0;

contract Ownable {
    address payable _owner;

    constructor() public {
        _owner = msg.sender;
    }

    modifier onlyOwner() {
        require(isOwner(), "You are not the owner");
        _;
    }

    function isOwner() public view returns(bool) {
        return (msg.sender == _owner);
    }
}

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

    ItemManager parentContract;

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

    receive() external payable {
        require(pricePaid == 0, "Item is paid already");
        require(priceInWei == msg.value, "Only full payments allowed");
        pricePaid += msg.value;
        (bool success, ) = address(parentContract).call.value(msg.value)(abi.encodeWithSignature("triggerPayment(uint256)",index));
        require(success, "The transaction was not successful, canceling");
    }

    fallback() external {}
}

contract ItemManager is Ownable {

    enum SupplyChainState{Created, Paid, Delivered}
    
    struct S_Item {
        string _identifier;
        uint _itemPrice;
        ItemManager.SupplyChainState _state;

    }

    mapping(uint => S_Item) public items;
    uint itemIndex;

    event SupplyChainStep(uint _itemIndex, uint _step);

    function createItem(string memory _identifier, uint _itemPrice) public onlyOwner {
        items[itemIndex]._identifier = _identifier;
        items[itemIndex]._itemPrice = _itemPrice;
        items[itemIndex]._state = SupplyChainState.Created;
        emit SupplyChainStep(itemIndex, uint(items[itemIndex]._state));
        itemIndex++;
    }

    function triggerPayment(uint _itemIndex) public payable {
        require(items[_itemIndex]._itemPrice == msg.value, "Only full payments accepted");
        require(items[_itemIndex]._state == SupplyChainState.Created, "Item is further in the chain");
        items[_itemIndex]._state = SupplyChainState.Paid;

        emit SupplyChainStep(_itemIndex, uint(items[_itemIndex]._state));

    }

    function triggerDelivery(uint _itemIndex) public onlyOwner {
        require(items[_itemIndex]._state == SupplyChainState.Paid, "Item is further in the chain");
        items[_itemIndex]._state = SupplyChainState.Delivered;

        emit SupplyChainStep(_itemIndex, uint(items[_itemIndex]._state));
    }
}