Intermediate
How to build a Simple Metaverse Contract
This tutorial provides a step-by-step guide on how to create a simple metaverse contract called Polyland on the Harmony blockchain using the HRC-721 token standard (NFT). The contract represents a plot of land that is divided into patches, which can be owned by different accounts. The tutorial uses Chainstack to create a public chain project and join the Harmony devnet, Foundry to create and deploy the contract, and the Harmony explorer to verify the contract and distribute patches to accounts. The tutorial assumes that you have a Chainstack account and have installed Foundry. The tutorial covers creating the project, joining the devnet, accessing node credentials, creating the contract, flattening, compiling, and deploying the contract, verifying the contract on the explorer, and distributing patches to accounts.
A simple metaverse contract with Foundry
This tutorial will guide you through the process of creating a simple metaverse contract on the Harmony blockchain using the HRC-721 token standard, also known as NFT. The contract, called Polyland, represents a plot of land that is divided into patches, which can be owned by different accounts. The tutorial will cover the steps necessary to create the contract, program it to consist of patches of land, deploy it on the Harmony devnet through a node deployed with Chainstack, and distribute patches to different accounts on the Harmony devnet. The goal of this tutorial is to provide you with a basic understanding of how to create and deploy a metaverse contract on the Harmony blockchain using NFTs.
Prerequisites
-
Chainstack account to deploy a Harmony node.
-
Foundry (opens new window)to create and deploy contracts.
Overview
To get from zero to a deployed metaverse contract and patches of land distributed on the Harmony devnet, do the following
With Chainstack, create a public chain project.
With Chainstack, join the Harmony devnet.
With Chainstack, access your Harmony node credentials.
With OpenZeppelin, create an HRC-721 contract.
With Foundry, flatten, compile, and deploy the contract through your Harmony node.
Verify the contract on the Harmony explorer.
Using the Harmony explorer as a web app, distribute the patches of land to accounts.
Step-by-step
Create a public chain project
See Create a project.
Join the Harmony devnet
Get your Harmony node access and credentials
See View node access and credentials.
Install Foundry
See Foundry (opens new window).
Create the contract
Initialize your project with Foundry:
forge init polyland
This will create the project directory polyland
an initialize it.
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 (opens new window).
-
The contract consists of the
Triangle
object with threeedge
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 toTriangle0
.Triangle0
is the default first element that will not represent a patch of land in the Polyland metaverse. -
Through the
maxSupply
variable and thesupplyCap
modifier, the number of patches available to mint is capped at4
. -
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.
-
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/
Flatten the contract
Flatten the contract to make it easier to verify on the Harmony devnet explorer (opens new window).
Run:
forge flatten polyland.sol > polylandFlat.sol
Deploy the contract:
forge create CONTRACT_NAME --contracts CONTRACT_PATH --private-key PRIVATE_KEY --rpc-url HTTPS_ENDPOINT --legacy
where
-
CONTRACT_NAME — the name of the contract as provided in the contract code
contract Polyland is ERC721, Ownable
. -
CONTRACT_PATH — full path to the flattened contract.
-
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 devnet faucet (opens new window). -
HTTPS_ENDPOINT — your Harmony node HTTPS endpoint. See also View node access and credentials and Tools.
-
--legacy
— the Foundry flag to work with the EVM-based networks that are not EIP-1559 (opens new window)activated.
Example:
forge create Polyland --contracts /root/polyland/src/polylandFlat.sol --private-key 671f1be339521731fef5f0aef7957e1f6653ae27fa58b528242f160a8cfd58dc --rpc-url https://nd-123-456-789.p2pify.com/3c6e0b8a9c15224a8228b9a98ca1531d --legacy
Once the contract deploys, note the solc
and the Deployed to
values in the output.
Verify the contract
Open the Harmony devnet explorer (opens new window).
Put in the contract address. Click the Contract tab.
Click Verify and Publish.
In the Contract Name
field, put Polyland
.
Set Chain Type
to devnet
.
In Compiler
, provide the solc
version that the contract compiled with.
In Optimizer
, set Yes
, 200
. Contract bytecode optimization with 200 runs is the default Foundry setting.
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.
Distribute the patches of land
On the contract page in the Harmony devnet explorer (opens new window), click Write Contract
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.
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
.
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.
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 (opens new window).
This tutorial uses devnet, however the exact same instructions and sequence work on the mainnet.