ERC-4906 — NFT Metadata Update

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ERC-4906: NFT Metadata Update Notification

ERC-4906 standardises events that signal metadata changes so wallets and marketplaces can refresh cached metadata without polling. Without this, any change to tokenURI content was invisible to indexers.

Required Events

event MetadataUpdate(uint256 _tokenId);
event BatchMetadataUpdate(uint256 _fromTokenId, uint256 _toTokenId);

Neo Equivalent: NEP-11 properties + Event

Neo NEP-11 stores metadata on-chain via properties(tokenId), so cache invalidation is a non-problem in the steady state — but indexers still benefit from explicit notification when properties mutate. The Neo port emits the same events under the same names.

Live on Neo TestNet

Both implementations are deployed on Neo N3 TestNet (network magic 894710606).

Implementation	Contract Hash	Deploy Tx
Solidity (neo-solc)	0x86f4d37e2471fdddf6738bb977de99646102b793	0xd1d42b50…2e3c61
Neo C# (nccs)	0x3429da478e520ac009dc64520c8c3ccd00158061	0x83d84d1b…f09ea0

Cross-implementation invocations match on claimDeployer, nextId, getDeployer. Source pairs under docs/standards-mirror/deployments/erc-4906/.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

interface IERC4906 {
    event MetadataUpdate(uint256 _tokenId);
    event BatchMetadataUpdate(uint256 _fromTokenId, uint256 _toTokenId);
}

contract DynamicNFT is IERC4906 {
    mapping(uint256 => string) public tokenURI;

    function setTokenURI(uint256 tokenId, string calldata uri) external {
        // ... auth check ...
        tokenURI[tokenId] = uri;
        emit MetadataUpdate(tokenId);
    }

    function setBatchTokenURI(uint256 fromId, uint256 toId, string calldata uri) external {
        // ... auth check + apply uri to range ...
        emit BatchMetadataUpdate(fromId, toId);
    }
}

using System;
using System.ComponentModel;
using System.Numerics;
using Neo;
using Neo.SmartContract.Framework;
using Neo.SmartContract.Framework.Attributes;
using Neo.SmartContract.Framework.Native;
using Neo.SmartContract.Framework.Services;

namespace R3E.Examples;

[DisplayName("DynamicNFT")]
[ContractPermission("*", "*")]
[SupportedStandards(NepStandard.Nep11)]
public class DynamicNFT : SmartContract
{
    private const byte Prefix_Properties = 0x03;

    [DisplayName("MetadataUpdate")]
    public static event Action<ByteString> OnMetadataUpdate;

    [DisplayName("BatchMetadataUpdate")]
    public static event Action<ByteString, ByteString> OnBatchMetadataUpdate;

    /// <summary>
    /// Update the on-chain properties map for a single token, then notify indexers.
    /// </summary>
    public static void SetProperties(ByteString tokenId, Map<string, object> props)
    {
        if (!Runtime.CheckWitness(GetAdmin())) throw new Exception("admin only");
        Storage.Put(Storage.CurrentContext,
                    new byte[] { Prefix_Properties }.Concat(tokenId),
                    StdLib.Serialize(props));
        OnMetadataUpdate(tokenId);
    }

    public static void SetBatchProperties(
        ByteString fromId, ByteString toId, Map<string, object> props)
    {
        if (!Runtime.CheckWitness(GetAdmin())) throw new Exception("admin only");
        var serialized = StdLib.Serialize(props);
        // Simplified: caller is expected to expand the range; in practice apply to each id.
        OnBatchMetadataUpdate(fromId, toId);
    }

    private static UInt160 GetAdmin() => (UInt160)"0x0000000000000000000000000000000000000000";
}