跟踪 ERC721 智能合约上的所有铸造代币

Question

I have a Smart Contract that I'd like to build a web based marketplace app for. How can I list all available "Items" in the contract. Seeing as there is no upper limit to the number of minted Items, how can I implement pagination and search?

What is a best practice and scalable solution:

• to simply return the Items[] array

• sync the array with my database on Transfer and other events

• other suggestions?

   pragma solidity ^0.4.24; contract Item is ERC721{ struct Item{ string name; // Name of the Item uint level; // Item Level uint rarityLevel; // 1 = normal, 2 = rare, 3 = epic, 4 = legendary } Item[] public items; // First Item has Index 0 address public owner; function Item() public { owner = msg.sender; // The Sender is the Owner; Ethereum Address of the Owner } function createItem(string _name, address _to) public{ require(owner == msg.sender); // Only the Owner can create Items uint id = items.length; // Item ID = Length of the Array Items items.push(Item(_name,5,1)) // Item ("Sword",5,1) _mint(_to,id); // Assigns the Token to the Ethereum Address that is specified } }

Answer 1

You can create an itemsCount public property that holds the current amount of existing items, and increment it after each items.push() .

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Without pagination and search:

Off-chain systems that want to read your data, can simply loop from items[0] to items[itemsCount] and since it's just a read operation, it doesn't require a transaction (ie it's free).

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With pagination and search:

You can to create a view function that:

1. Takes page and query as arguments
2. Loops through existing items and if the item fits the criteria ( name contains query ), add it to the results array
3. Returns the results array

Note: Step 2 is actually going to be a bit more complicated in Solidity, because you can't push into an in-memory dynamic array. So you need to:

1. Loop through the items and find an amount that fit the criteria (up to the page limit)
2. Create a fix-length in-memory array results
3. Now you can loop through the items again and fill the fix-length results array with values

Answer 2

mapping are more efficient than arrays in solidity. You could have a uint in the smart contract that keeps the count of every Item and use a getter function to get each item from whatever number you want to paginate subtracted from the item count.

contract Item is ERC721{

struct Item{
    string name; // Name of the Item
    uint level; // Item Level
    uint rarityLevel;  // 1 = normal, 2 = rare, 3 = epic, 4 = legendary
    uint id;
}

mapping(uint => Item) items; // First Item has Index 0
uint count;
address public owner;

function Item() public {
    owner = msg.sender; // The Sender is the Owner; Ethereum Address of the Owner
}

function createItem(string memory _name, address _to, uint level, uint rarityLevel
                    uint id) public{
    require(owner == msg.sender); // Only the Owner can create Items
        uint num = count++;
        items[num].name = _name;
        items[num].level = level;
        items[num].rarityLevel = rarityLevel;
        items[num].id = num;
        count++;
}

function getItem(uint item) public view returns(string memory _name, uint level, 
                 uint rarityLevel, uint id){
        uint level = items[item].level;
        uint rarityLevel = items[item].rarityLevel;
        uint id = items[item].id;
        string memory _name = items[item].name
        return(level, rarityLevel, id, _name)
}