Introduction

An architecture for composing ERC721 tokens with ERC1155 tokens by adding extensions to these already widely used standards.

TOC

• Architecture
• Composable NFT Extensions
  • ERC721CUniqueComponents
  • ERC721CCompatibleContracts
  • ERC721CMutexComponents
• Application examples
• Examples

Architecture

To implement composability two extensions have been developed that add public methods to ERC721 and ERC1155 standards, so that ERC1155 Component (ERC1155C) tokens can be attached to ERC721 Composable (ERC721C) tokens.
The new methods are:

ERC721C:

• getComponentsContracts -> view
• getComponents -> view
• getComponentAmount -> view
• attachComponent -> write
• detachComponent -> write

ERC1155C:

• getAttachedTokensQuantity -> view
• attachedSafeBatchTransferFrom -> write
• attach -> write
• detach -> write

To manage composability devs need to use only the write methods on the ERC721C: attachComponent and detachComponent. All the three write methods in the ERC1155C extension are callable only by an instance of an ERC721C contract instance:

• attach is called by attachComponent
• detach is called by detachComponent
• attachedSafeBatchTransferFrom is called when a composable NFT is transferred

When an ERC721C token is transferred, all the attached ERC1155C tokens will be transferred too, so the ERC721C contract needs to be approved otherwise the transfer transaction gets reverted. ERC1155C tokens cannot be transferred alone if attached.

ERC721C constructor requires uint256 maxComponentsQnt as additional argument to define the quantity of components that a composable NFT can support, it can be set to 0 for no limit, but it is not recommended.

Composable NFT Extensions

The base implementation does not define limits on which components can be attached, some extensions have been developed to cover general cases:

• ERC721CUniqueComponents
• ERC721CCompatibleContracts
• ERC721CMutexComponents

ERC721CUniqueComponents

This extension ensures that all attached components are unique.

ERC721CCompatibleContracts

This extension limits attachable components to specific ERC1155C contracts. Compatible contracts can be managed with the two methods:

• _addCompatibleContracts(address[] memory contracts)
• _removeCompatibleContracts(address[] memory contracts)

ERC721CMutexComponents

This extension ensures mutual exclusivity between components. Configuration can be set using the method _setMutexComponents(Component[][] memory components).

Only components set with _setMutexComponents can be attached to this type of composable NFT and they must be unique.

Component is defined as:

struct Component {
    address addr;
    uint256 id;
}

where addr is the ERC1155C contract address and id is the token id.

The argument components is an array of arrays of Component where the internal ones are lists of mutually exclusive components.

Gas consumption of _setMutexComponents grows proportionally with the length of the argument components.

Application examples

Avatars:

• ERC721C -> Base avatar
• ERC1155C -> Clothes, accessories, weapons, etc...

Cars:

• ERC721C -> Chassis
• ERC1155C -> Engines, wheels, transmissions, etc...

Boxes:

• ERC721C -> Box
• ERC1155C -> Any token

Houses:

• ERC721C -> Building
• ERC1155C -> Furnitures

Examples

Examples on how to use these contracts can be found here and here.