author: Witek Radomski <witek@enjin.com>, Andrew Cooke <andrew@enjin.com>, Philippe Castonguay <pc@horizongames.net>, James Therien <james@enjin.com>, Eric Binet <eric@enjin.com>
A standard interface for contracts that manage multiple token types. A single deployed contract may include any combination of fungible tokens, non-fungible tokens, or other configurations (for example, semi-fungible tokens).
This standard outlines a smart contract interface that can represent any number of Fungible and Non-Fungible token types. Existing standards such as ERC-20 require deployment of separate contracts per token type. The ERC-721 standard's Token ID is a single non-fungible index and the group of these non-fungibles is deployed as a single contract with settings for the entire collection. In contrast, the ERC-1155 Multi Token Standard allows for each Token ID to represent a new configurable token type, which may have its own metadata, supply and other attributes.
Tokens standards like ERC-20 and ERC-721 require a separate contract to be deployed for each token type or collection. This places a lot of redundant bytecode on the Ethereum blockchain and limits certain functionality by the nature of separating each token contract into its own permissioned address. With the rise of blockchain games and platforms like Enjin Coin, game developers may be creating thousands of token types, and a new type of token standard is needed to support them. However, ERC-1155 is not specific to games, and many other applications can benefit from this flexibility.
New functionality is possible with this design, such as transferring multiple token types at once, saving on transaction costs. Trading (escrow / atomic swaps) of multiple tokens can be built on top of this standard and it removes the need to "approve" individual token contracts separately. It is also easy to describe and mix multiple fungible or non-fungible token types in a single contract.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
The total value transferred from address 0x0 minus the total value transferred to 0x0 may be used by clients and exchanges to be added to the "circulating supply" for a given token ID.
To broadcast the existence of a token ID with no initial balance, the contract SHOULD emit the TransferSingle event from `0x0` to `0x0`, with the token creator as `_operator`, and a `_value` of 0.
@dev Either TransferSingle or TransferBatch MUST emit when tokens are transferred, including zero value transfers as well as minting or burning.
Operator will always be msg.sender.
Either event from address `0x0` signifies a minting operation.
An event to address `0x0` signifies a burning or melting operation.
The total value transferred from address 0x0 minus the total value transferred to 0x0 may be used by clients and exchanges to be added to the "circulating supply" for a given token ID.
To broadcast the existence of multiple token IDs with no initial balance, this SHOULD emit the TransferBatch event from `0x0` to `0x0`, with the token creator as `_operator`, and a `_value` of 0.
When transfer is complete, this function MUST check if `_to` is a smart contract (code size > 0). If so, it MUST call `onERC1155Received` on `_to` and revert if the return value is not `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"))`.
When transfer is complete, this function MUST check if `_to` is a smart contract (code size > 0). If so, it MUST call `onERC1155BatchReceived` on `_to` and revert if the return value is not `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))`.
@notice Handle the receipt of a single ERC1155 token type.
@dev An ERC1155-compliant smart contract MUST call this function on the token recipient contract, at the end of a `safeTransferFrom` after the balance has been updated.
@notice Handle the receipt of multiple ERC1155 token types.
@dev An ERC1155-compliant smart contract MUST call this function on the token recipient contract, at the end of a `safeBatchTransferFrom` after the balances have been updated.
The URI value allows for ID substitution by clients. If the string `{id}` exists in any URI, clients MUST replace this with the actual token ID in hexadecimal form. This allows for large number of tokens to use the same on-chain string by defining a URI once, for a large collection of tokens. Example of such a URI: `https://token-cdn-domain/{id}.json` would be replaced with `https://token-cdn-domain/780000000000001e000000000000000000000000000000000000000000000000.json` if the client is referring to token ID `780000000000001e000000000000000000000000000000000000000000000000`.
The following optional extensions can be identified with the (ERC-165 Standard Interface Detection)[https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md].
Changes to the URI MUST emit the `URI` event if the change can be expressed with an event. If the optional ERC1155Metadata_URI extension is included, the value returned by this function SHOULD be used to retrieve values for which no event was emitted. The function MUST return the same value as the event if it was emitted.
This JSON schema is loosely based on the "ERC721 Metadata JSON Schema", but includes optional formatting to allow for ID substitution by clients. If the string `{id}` exists in any JSON value, it MUST be replaced with the actual token ID, by all client software that follows this standard.
"description": "The number of decimal places that the token amount should display - e.g. 18, means to divide the token amount by 1000000000000000000 to get its user representation.",
"description": "Describes the asset to which this token represents",
},
"image": {
"type": "string",
"description": "A URI pointing to a resource with mime type image/* representing the asset to which this token represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive.",
},
"properties": {
"type": "object",
"description": "Arbitrary properties. Values may be strings, numbers, object or arrays.",
},
}
}
```
An example of an ERC-1155 Metadata JSON file follows. The properties array proposes some SUGGESTED formatting for token-specific display properties and metadata.
Metadata localization should be standardized to increase presentation uniformity accross all languages. As such, a simple overlay method is proposed to enable localization. If the metadata JSON file contains a `localization` attribute, its content may be used to provide localized values for fields that need it. The `localization` attribute should be a sub-object with three attributes: `uri`, `default` and `locales`. If the string `{locale}` exists in any URI, it MUST be replaced with the chosen locale by all client software.
#### JSON Schema
```json
{
"title": "Token Metadata",
"type": "object",
"properties": {
"name": {
"type": "string",
"description": "Identifies the asset to which this token represents",
},
"decimals": {
"type": "integer",
"description": "The number of decimal places that the token amount should display - e.g. 18, means to divide the token amount by 1000000000000000000 to get its user representation.",
},
"description": {
"type": "string",
"description": "Describes the asset to which this token represents",
},
"image": {
"type": "string",
"description": "A URI pointing to a resource with mime type image/* representing the asset to which this token represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive.",
},
"properties": {
"type": "object",
"description": "Arbitrary properties. Values may be strings, numbers, object or arrays.",
},
"localization": {
"type": "object",
"required": ["uri", "default", "locales"],
"properties": {
"uri": {
"type": "string",
"description": "The URI pattern to fetch localized data from. This URI should contain the substring `{locale}` which will be replaced with the appropriate locale value before sending the request."
},
"default": {
"type": "string",
"description": "The locale of the default data within the base JSON"
},
"locales": {
"type": "array",
"description": "The list of locales for which data is available. These locales should conform to those defined in the Unicode Common Locale Data Repository (http://cldr.unicode.org/)."
}
}
},
}
}
```
#### Localized Sample
Base URI:
```json
{
"name": "Advertising Space",
"description": "Each token represents a unique Ad space in the city.",
"description": "Cada token representa un espacio publicitario único en la ciudad."
}
```
fr.json:
```json
{
"name": "Espace Publicitaire",
"description": "Chaque jeton représente un espace publicitaire unique dans la ville."
}
```
</details>
## Approval
The function `setApprovalForAll` allows an operator to manage one's entire set of tokens on behalf of the approver. To permit approval on a subset of tokens, standardized scoped approval is available as an extension. More complex approval schemes will require the use of an external contract enforcing custom rules.
### Scoped Approval Extension
This extension enables restrictions on approval's reach using a standardized method. When considering a smart contract
managing tokens from multiple different domains, it makes sense to limit approvals to those domains. Scoped approval is a
generalization of this idea. ERC-1155 implementors can define scopes as needed.
<details>
<summary>Interface</summary>
```solidity
pragma solidity ^0.5.2;
/**
Note: The ERC-165 identifier for this interface is 0x30168307.
@notice Returns the number of scopes that contain _id.
@param _id The token ID
@return The number of scopes containing the ID
*/
function scopeCountForId(uint256 _id) public view returns (uint32);
/**
@notice Returns a scope that contains _id.
@param _id The token ID
@param _scopeIndex The scope index to query (valid values are 0 to scopeCountForId(_id)-1)
@return The Nth scope containing the ID
*/
function scopeForId(uint256 _id, uint32 _scopeIndex) public view returns (bytes32);
/**
@notice Returns a URI that can be queried to get scope metadata. This URI should return a JSON document containing, at least the scope name and description. Although supplying a URI for every scope is recommended, returning an empty string "" is accepted for scopes without a URI.
@param _scope The queried scope
@return The URI describing this scope.
*/
function scopeUri(bytes32 _scope) public view returns (string memory);
/**
@notice Enable or disable approval for a third party ("operator") to manage the caller's tokens in the specified scope.
@dev MUST emit the ApprovalForScope event on success.
@param _operator Address to add to the set of authorized operators
@param _scope Approval scope (can be identified by calling scopeForId)
@param _approved True if the operator is approved, false to revoke approval
*/
function setApprovalForScope(address _operator, bytes32 _scope, bool _approved) external;
/**
@notice Queries the approval status of an operator for a given owner, within the specified scope.
@param _owner The owner of the Tokens
@param _operator Address of authorized operator
@param _scope Scope to test for approval (can be identified by calling scopeForId)
@return True if the operator is approved, false otherwise
*/
function isApprovedForScope(address _owner, address _operator, bytes32 _scope) public view returns (bool);
}
```
</details>
<details>
<summary>Scope Metadata JSON Schema</summary>
This shema is similar to the token metadata schema and also allows localization. `{id}` and `{locale}` should be replaced with the proper values.
```json
{
"title": "Scope Metadata",
"type": "object",
"required": ["name"],
"properties": {
"name": {
"type": "string",
"description": "Identifies the scope in a human-readable way.",
},
"description": {
"type": "string",
"description": "Describes the scope to allow users to make informed approval decisions.",
},
"localization": {
"type": "object",
"required": ["uri", "default", "locales"],
"properties": {
"uri": {
"type": "string",
"description": "The URI pattern to fetch localized data from. This URI should contain the substring `{locale}` which will be replaced with the appropriate locale value before sending the request."
},
"default": {
"type": "string",
"description": "The locale of the default data within the base JSON"
},
"locales": {
"type": "array",
"description": "The list of locales for which data is available. These locales should conform to those defined in the Unicode Common Locale Data Repository (http://cldr.unicode.org/)."
The `symbol` function (found in the ERC-20 and ERC-721 standards) was not included as we do not believe this is a globally useful piece of data to identify a generic virtual item / asset and are also prone to collisions. Short-hand symbols are used in tickers and currency trading, but they aren't as useful outside of that space.
The `name` function (for human-readable asset names, on-chain) was removed from the standard to allow the Metadata JSON to be the definitive asset name and reduce duplication of data. This also allows localization for names, which would otherwise be prohibitively expensive if each language string was stored on-chain, not to mention bloating the standard interface. While this decision may add a small burden on implementers to host a JSON file containing metadata, we believe any serious implementation of ERC-1155 will already utilize JSON Metadata.
The requirement to emit `TransferSingle` or `TransferBatch` on balance change implies that a valid implementation of ERC-1155 redeploying to a new contract address MUST emit events from the new contract address to replicate the deprecated contract final state. It is valid to only emit a minimal number of events to reflect only the final balance and omit all the transactions that led to that state. The event emit requirement is to ensure that the current state of the contract can always be traced only through events. To alleviate the need to emit events when changing contract address, consider using the proxy pattern, such as described in ERC-1538. This will also have the added benefit of providing a stable contract address for users.
The standard supports `safeTransferFrom` and `onERC1155Received` functions because they are significantly cheaper for single token-type transfers, which is arguably a common use case.
The standard only supports safe-style transfers, making it possible for receiver contracts to depend on `onERC1155Received` or `onERC1155BatchReceived` function to be always called at the end of a transfer.
As the Ethereum ecosystem continues to grow, many dapps are relying on traditional databases and explorer API services to retrieve and categorize data. The ERC-1155 standard guarantees that event logs emitted by the smart contract will provide enough data to create an accurate record of all current token balances. A database or explorer may listen to events and be able to provide indexed and categorized searches of every ERC-1155 token in the contract.
This standard can be used to represent multiple token types for an entire domain. Both Fungible and Non-Fungible tokens can be stored in the same smart-contract.
The `safeBatchTransferFrom` function allows for batch transfers of multiple token ids and values. The design of ERC-1155 makes batch transfers possible without the need for a wrapper contract, as with existing token standards. This reduces gas costs when more than one token type is included in a batch transfer, as compared to single transfers with multiple transactions.
Another advantage of standardized batch transfers is the ability for a smart contract to respond to the batch transfer in a single operation using `onERC1155BatchReceived`.
### Batch Balance
The `balanceOfBatch` function allows clients to retrieve balances of multiple owners and token ids with a single call.
In order to keep storage requirements light for contracts implementing ERC-1155, enumeration (discovering the IDs and values of tokens) must be done using event logs. It is RECOMMENDED that clients such as exchanges and blockchain explorers maintain a local database containing the Token ID, Supply, and URI at the minimum. This can be built from each TransferSingle, TransferBatch, and URI event, starting from the block the smart contract was deployed until the latest block.
ERC-1155 contracts must therefore carefully emit TransferSingle or TransferBatch events in any instance where tokens are created, minted, or destroyed.
</details>
<details>
<summary>
Non-Fungible Tokens</summary>
### Non-Fungible Tokens
The following strategy is an example of how to mix fungible and non-fungible tokens together in the same contract. The top 128 bits of the uint256 `_id` parameter in any ERC-1155 function could represent the base token ID, while the bottom 128 bits might be used for any extra data passed to the contract.
Non-Fungible tokens can be interacted with using an index based accessor into the contract/token data set. Therefore to access a particular token set within a mixed data contract and particular NFT within that set, `_id` could be passed as `<uint128: base token id><uint128: index of NFT>`.
Inside the contract code the two pieces of data needed to access the individual NFT can be extracted with uint128(~0) and the same mask shifted by 128.
- [Here Be Dragons - Going Beyond ERC-20 and ERC-721 To Reduce Gas Cost by ~80%](https://medium.com/horizongames/going-beyond-erc20-and-erc721-9acebd4ff6ef)
