TLDR:

  • This tutorial showcases how to create an ERC-721 “Polyland” contract on Harmony devnet that treats each land patch as an NFT.
  • Polyland can hold a finite set of triangular patches, and the owner (contract deployer) can mint them to different accounts until the supply is capped.
  • Foundry is used to compile and deploy the contract, then the Harmony explorer is used to verify it and distribute patches.
  • Once deployed, each patch’s owner and properties (triangle edges) can be queried on-chain, illustrating the fundamentals of a metaverse asset contract.

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Main article

The blockchain part of any metaverse that involves decentralization is object ownership.

On Harmony, object ownership can be realized with the HRC-721 token standard, commonly known as NFT.

A very basic example of realizing a metaverse on blockchain is creating a plot of land and distributing the patches of it to different owners—all through an NFT contract.

In this tutorial, you will:

  • Create a simple contract called Polyland. The Polyland contract represents a plot of land.
  • Program Polyland to consist of patches of land.
  • Deploy Polyland on the Harmony devnet through a node deployed with Chainstack.
  • Distribute patches of Polyland to different accounts on the Harmony devnet.

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Prerequisites

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Overview

To get from zero to a deployed metaverse contract and patches of land distributed on the Harmony devnet, do the following:

  1. With Chainstack, create a .
  2. With Chainstack, join the Harmony devnet.
  3. With Chainstack, access your Harmony node credentials.
  4. With OpenZeppelin, create an HRC-721 contract.
  5. With Foundry, flatten, compile, and deploy the contract through your Harmony node.
  6. Verify the contract on the Harmony explorer.
  7. Using the Harmony explorer as a web app, distribute the patches of land to accounts.

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Step-by-step

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Create a public chain project

See Create a project.

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Join the Harmony devnet

See Join a public network.

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Get your Harmony node access and credentials

See View node access and credentials.

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Install Foundry

See Foundry.

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Create the contract

  1. Initialize your project with Foundry:

    forge init polyland

    This will create the project directory polyland and initialize it.

  2. Go to the polyland/src/ directory. In the directory, create your metaverse contract: polyland.sol.

    // SPDX-License-Identifier: MIT
pragma solidity ^0.8;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/utils/Counters.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract Polyland is ERC721, Ownable {
    using Counters for Counters.Counter;

    Counters.Counter private supply;

    uint256 public maxSupply = 4;

    struct Triangle {
        string name;
        int8 edge1;
        int8 edge2;
        int8 edge3;
    }

    Triangle[] public triangles;

    constructor() ERC721("Polyland", "PLLND") {
        triangles.push(Triangle("Triangle0", 0,0,0));
        triangles.push(Triangle("Triangle1", 1,1,1));
        triangles.push(Triangle("Triangle2", 2,2,2));
        triangles.push(Triangle("Triangle3", 3,3,3));
        triangles.push(Triangle("Triangle4", 4,4,4));
    }

    modifier supplyCap {
        require(supply.current() < maxSupply, "All patches minted.");
        _;
    }

    function totalSupply() public view returns (uint256) {
        return supply.current();
    }

    function getTriangles() public view returns (Triangle[] memory) {
        return triangles;
    }

    function mintTriangle(address account)
        public
        onlyOwner
        supplyCap
        returns (uint256)
    {
        supply.increment();

        uint256 newPatchId = supply.current();
        _mint(account, newPatchId);

        return newPatchId;
    }
}

    The contract implementation is the following:

    • The contract uses the OpenZeppelin audited ERC-721 contract templates.
    • The contract consists of a Triangle object with three edge properties. The triangle is a patch of land that has three edges.
    • The constructor function sets the contract up with four triangles. Since this is an array and starts with 0, while the ID of the minted patch starts with 1, the first element is set to Triangle0. Triangle0 is the default first element that will not represent a patch of land in the Polyland metaverse.
    • Through the maxSupply variable and the supplyCap modifier, the number of patches available to mint is capped at 4.
    • Only the address that deploys the contract can mint the patches of land.

    Thus, the contract represents a plot of land called Polyland that consists of four triangular patches of land.

  3. Set up OpenZeppelin with Foundry

    Install OpenZeppelin with Foundry:

    forge install openzeppelin/openzeppelin-contracts

    In the project directory, create a remappings.txt file with the following contents:

    @openzeppelin/=lib/openzeppelin-contracts/

  4. Flatten the contract.

    Flatten the contract to make it easier to verify on the Harmony Testnet explorer.

    Run:

    forge flatten polyland.sol > polylandFlat.sol

  5. Deploy the contract:

    forge create Polyland --contracts /root/polyland/src/polylandFlat.sol --private-key YOUR_PRIVATE_KEY --rpc-url YOUR_CHAINSTACK_ENDPOINT --legacy

    where

    • Polyland — the name of the contract as provided in the contract code contract Polyland is ERC721, Ownable
    • /root/polyland/src/polylandFlat.sol — full path to the flattened contract
    • YOUR_PRIVATE_KEY — the private key to the account that deploys the contract. Must be used without the 0x prefix. Fund the account with devnet ONE using the testnet faucet.
    • YOUR_CHAINSTACK_ENDPOINT — your Harmony node HTTPS endpoint deployed with Chainstack. See also View node access and credentials and Harmony tooling.
    • --legacy — the Foundry flag to work with the EVM-based networks that are not EIP-1559 activated.

    Once the contract deploys, note the solc and the Deployed to values in the output.

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Verify the contract

  1. Open the Harmony Testnet explorer.
  2. Put in the contract address. Click the Contract tab.
  3. Click Verify and Publish.
  4. In the Contract Name field, put Polyland.
  5. Set Chain Type to devnet.
  6. In Compiler, provide the solc version that the contract compiled with.
  7. In Optimizer, set Yes, 200. Contract bytecode optimization with 200 runs is the default Foundry setting.
  8. Paste the entirety of the flattened contract in the contract field and hit Submit.

This will verify the contract. You can now use the explorer as a web app to interact with the contract.

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Distribute the patches of land

  1. On the contract page in the Harmony Testnet explorer, click Write Contract.
  2. In mintTriangle, provide an address to distribute a patch of land to. Distribute the patches to different addresses until you hit the cap with the All patches minted message.
  3. On the Read Contract tab, query the ownerOf field by putting in the tokenId values representing each of the patches of land: 1, 2, 3, 4.
  4. In the triangles field, put in the same tokenId values to get the data on each of the patches: name and the size of each of the three edges.

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Conclusion

This tutorial guided you through the basics of creating and deploying a metaverse contract with object ownership representation.

You created your own plot of land, distributed the finite number of land patches to different owners, and retrieved the data for each of the patches: patch size and patch owner.

You did all of it using Foundry.

This tutorial uses devnet, however, the exact same instructions and sequence work on the mainnet.

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About the author

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Ake

Developer Advocate @ Chainstack

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