However, this is would not work. How do we define/pre-allocate an address automatically to each item on creation? We do not control address creation and allocation on the EVM.
Solution
To get a unique address per item -> each item has its own smart contract. Therefore, the create() should result in deployment on a new item AND its smart contract.
We will split the code up, keeping the item smart contract as a separate child contract.
//SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;
import "./ItemManager.sol";
contract Item {
uint public priceInWei;
uint public paidWei;
uint public index;
//variable of type ItemManager -> contract object.
ItemManager parentContract;
// creation requires inputs: price & index.
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 {
}
}
//SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;
contract ItemManager {
enum itemState{
Created, Paid, Delivered //created - 0, paid -1,..
}
struct Item {
Item _item; //_item variable is contract object? by above logic
string id;
uint price;
itemState state;
}
// to create a dataframe-like structure
mapping(uint => Item) public item_list;
uint item_index;
event SupplyChainStep(uint _index, uint _state);
function createItem(string memory _id, uint _price) public {
Item item = new Item(this, _price, item_index); //create new Item contract. this: the current contract, explicitly convertible to address
item_list[item_index]._item = item;
item_list[item_index].id = _id;
item_list[item_index].price = _price;
item_list[item_index].state = itemState.Created;
emit SupplyChainStep(item_index, uint(item_list[item_index].state));
item_index++;
}
function triggerPayment(uint _index) public payable {
require(item_list[_index].price == msg.value, "please pay exact full amount");
require(item_list[_index].state == itemState.Created,"Item is not available");
item_list[_index].state = itemState.Paid; //update state to paid
// emit payment event
emit SupplyChainStep(item_index, uint(item_list[item_index].state));
}
function triggerDeliver(uint _index) public {
require(item_list[_index].state == itemState.Paid,"Item is not for delivery");
item_list[_index].state == itemState.Delivered;
// emit delivery
emit SupplyChainStep(item_index, uint(item_list[item_index].state));
}
}
How do these 2 contracts interact?
createItem
When createItem() is called Item item = new Item(this, _price, item_index), creates a new instance of the Item contract based on the price provided and index counter.
this : the current contract, explicitly convertible to address.
this is passed into the child contract's constructor (Item{}), for the field ItemManager _parentContract, where it eventually assigned to variable parentContract
parentContract = _parentContract;
// evaluates to:
parentContract = this;
This is to store the parent contract association.
struc Item {}
struc Item now has Item _item; which is a contract object. It is assigned value under the createItem(), after the new item contract object has been created.
item_list[item_index]._item = item;
Thoughts
This approach allows us to create an item using the parent contract, assign values to both parent and child