When a ERC1155 works as ERC721 and when as ERC20?

Question

Im trying to track in real time the holders of any ERC1155 contract, and I have this doubt about this interface.

The ERC1155 can work as an NFT or as a Token, but also as both at the same time, right?

If it do, Is there any other way to know if a transaction involves NFTs or tokens as part of checking if the value of the transaction is 1 or not?

Thanks!

Edit 1: Reading the oficial specification of the ERC1155 I found the following:

The following strategies are examples of how you MAY mix fungible and non-fungible tokens together in the same contract. The standard does NOT mandate how an implementation must do this.

Also they propose a posible implementation of how to mix both uses cases:

The top 128 bits of the uint256 _id parameter in any ERC-1155 function MAY represent the base token ID, while the bottom 128 bits MAY represent the index of the non-fungible to make it unique.

uint256 baseTokenNFT = 12345 << 128;
uint128 indexNFT = 50;

uint256 baseTokenFT = 54321 << 128;

balanceOf(msg.sender, baseTokenNFT); // Get balance of the base token for non-fungible set 12345 (this MAY be used to get balance of the user for all of this token set if the implementation wishes as a convenience).
balanceOf(msg.sender, baseTokenNFT + indexNFT); // Get balance of the token at index 50 for non-fungible set 12345 (should be 1 if user owns the individual non-fungible token or 0 if they do not).
balanceOf(msg.sender, baseTokenFT); // Get balance of the fungible base token 54321.

Answer 1

The best way to track holders of ERC1155 tokens is to track those two events defined in ERC1155 specifications :

• event TransferSingle(address indexed _operator, address indexed _from, address indexed _to, uint256 _id, uint256 _value);
• event TransferBatch(address indexed _operator, address indexed _from, address indexed _to, uint256[] _ids, uint256[] _values);

Make sure these events are well implemented in the transfer function of the token's smart contract you want to track.