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Event_Trigger.sol

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

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 wasn't successful, canceling");
    }

    fallback() external {}
}

contract ItemManager {

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

    }

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

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

    function createItem(string memory _identifier, uint _itemPrice) public {
        Item item = new Item(this, _itemPrice, itemIndex);
        items[itemIndex]._item = item;
        items[itemIndex]._identifier = _identifier;
        items[itemIndex]._itemPrice = _itemPrice;
        items[itemIndex]._state = SupplyChainState.Created;
        emit SupplyChainStep(itemIndex, uint(items[itemIndex]._state), address(item));
        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), address(items[_itemIndex]._item));

    }

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

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