git.delta.rocks / unique-network / refs/commits / c9c7549767f0

1// Copyright 2019-2022 Unique Network (Gibraltar) Ltd.2// This file is part of Unique Network.34// Unique Network is free software: you can redistribute it and/or modify5// it under the terms of the GNU General Public License as published by6// the Free Software Foundation, either version 3 of the License, or7// (at your option) any later version.89// Unique Network is distributed in the hope that it will be useful,10// but WITHOUT ANY WARRANTY; without even the implied warranty of11// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the12// GNU General Public License for more details.1314// You should have received a copy of the GNU General Public License15// along with Unique Network. If not, see <http://www.gnu.org/licenses/>.1617//! # Fungible Pallet18//!19//! The Fungible pallet provides functionality for dealing with fungible assets.20//!21//! - [`CreateItemData`]22//! - [`Config`]23//! - [`FungibleHandle`]24//! - [`Pallet`]25//! - [`TotalSupply`]26//! - [`Balance`]27//! - [`Allowance`]28//! - [`Error`]29//!30//! ## Fungible tokens31//!32//! Fungible tokens or assets are divisible and non-unique. For instance,33//! fiat currencies like the dollar are fungible: A $1 bill34//! in New York City has the same value as a $1 bill in Miami.35//! A fungible token can also be a cryptocurrency like Bitcoin: 1 BTC is worth 1 BTC,36//! no matter where it is issued. Thus, the fungibility refers to a specific currency’s37//! ability to maintain one standard value. As well, it needs to have uniform acceptance.38//! This means that a currency’s history should not be able to affect its value,39//! and this is due to the fact that each piece that is a part of the currency is equal40//! in value when compared to every other piece of that exact same currency.41//! In the world of cryptocurrencies, this is essentially a coin or a token42//! that can be replaced by another identical coin or token, and they are43//! both mutually interchangeable. A popular implementation of fungible tokens is44//! the ERC-20 token standard.45//!46//! ### ERC-2047//!48//! The [ERC-20](https://ethereum.org/en/developers/docs/standards/tokens/erc-20/) (Ethereum Request for Comments 20), proposed by Fabian Vogelsteller in November 2015,49//! is a Token Standard that implements an API for tokens within Smart Contracts.50//!51//! Example functionalities ERC-20 provides:52//!53//! * transfer tokens from one account to another54//! * get the current token balance of an account55//! * get the total supply of the token available on the network56//! * approve whether an amount of token from an account can be spent by a third-party account57//!58//! ## Overview59//!60//! The module provides functionality for asset management of fungible asset, supports ERC-20 standart, includes:61//!62//! * Asset Issuance63//! * Asset Transferal64//! * Asset Destruction65//! * Delegated Asset Transfers66//!67//! **NOTE:** The created fungible asset always has `token_id` = 0.68//! So `tokenA` and `tokenB` will have different `collection_id`.69//!70//! ### Implementations71//!72//! The Fungible pallet provides implementations for the following traits.73//!74//! - [`WithRecorder`](pallet_evm_coder_substrate::WithRecorder): Trait for EVM support75//! - [`CommonCollectionOperations`](pallet_common::CommonCollectionOperations): Functions for dealing with collections76//!	- [`CommonWeightInfo`](pallet_common::CommonWeightInfo): Functions for retrieval of transaction weight77//! - [`CommonEvmHandler`](pallet_common::erc::CommonEvmHandler): Function for handling EVM runtime calls7879#![cfg_attr(not(feature = "std"), no_std)]8081use core::ops::Deref;82use evm_coder::ToLog;83use frame_support::{ensure};84use pallet_evm::account::CrossAccountId;85use up_data_structs::{86	AccessMode, CollectionId, CollectionFlags, TokenId, CreateCollectionData,87	mapping::TokenAddressMapping, budget::Budget,88};89use pallet_common::{90	Error as CommonError, Event as CommonEvent, Pallet as PalletCommon,91	eth::collection_id_to_address,92};93use pallet_evm::Pallet as PalletEvm;94use pallet_structure::Pallet as PalletStructure;95use pallet_evm_coder_substrate::WithRecorder;96use sp_core::H160;97use sp_runtime::{ArithmeticError, DispatchError, DispatchResult};98use sp_std::{collections::btree_map::BTreeMap, vec::Vec};99100pub use pallet::*;101102use crate::erc::ERC20Events;103#[cfg(feature = "runtime-benchmarks")]104pub mod benchmarking;105pub mod common;106pub mod erc;107pub mod weights;108109pub type CreateItemData<T> = (<T as pallet_evm::account::Config>::CrossAccountId, u128);110pub(crate) type SelfWeightOf<T> = <T as Config>::WeightInfo;111112#[frame_support::pallet]113pub mod pallet {114	use frame_support::{Blake2_128, Blake2_128Concat, Twox64Concat, pallet_prelude::*, storage::Key};115	use up_data_structs::CollectionId;116	use super::weights::WeightInfo;117118	#[pallet::error]119	pub enum Error<T> {120		/// Not Fungible item data used to mint in Fungible collection.121		NotFungibleDataUsedToMintFungibleCollectionToken,122		/// Fungible tokens hold no ID, and the default value of TokenId for Fungible collection is 0.123		FungibleItemsHaveNoId,124		/// Tried to set data for fungible item.125		FungibleItemsDontHaveData,126		/// Fungible token does not support nesting.127		FungibleDisallowsNesting,128		/// Setting item properties is not allowed.129		SettingPropertiesNotAllowed,130	}131132	#[pallet::config]133	pub trait Config:134		frame_system::Config + pallet_common::Config + pallet_structure::Config + pallet_evm::Config135	{136		type WeightInfo: WeightInfo;137	}138139	#[pallet::pallet]140	#[pallet::generate_store(pub(super) trait Store)]141	pub struct Pallet<T>(_);142143	/// Total amount of fungible tokens inside a collection.144	#[pallet::storage]145	pub type TotalSupply<T: Config> =146		StorageMap<Hasher = Twox64Concat, Key = CollectionId, Value = u128, QueryKind = ValueQuery>;147148	/// Amount of tokens owned by an account inside a collection.149	#[pallet::storage]150	pub type Balance<T: Config> = StorageNMap<151		Key = (152			Key<Twox64Concat, CollectionId>,153			Key<Blake2_128Concat, T::CrossAccountId>,154		),155		Value = u128,156		QueryKind = ValueQuery,157	>;158159	/// Storage for assets delegated to a limited extent to other users.160	#[pallet::storage]161	pub type Allowance<T: Config> = StorageNMap<162		Key = (163			Key<Twox64Concat, CollectionId>,164			Key<Blake2_128, T::CrossAccountId>,165			Key<Blake2_128Concat, T::CrossAccountId>,166		),167		Value = u128,168		QueryKind = ValueQuery,169	>;170}171172/// Wrapper around untyped collection handle, asserting inner collection is of fungible type.173/// Required for interaction with Fungible collections, type safety and implementation [`solidity_interface`][`evm_coder::solidity_interface`].174pub struct FungibleHandle<T: Config>(pallet_common::CollectionHandle<T>);175176/// Implementation of methods required for dispatching during runtime.177impl<T: Config> FungibleHandle<T> {178	/// Casts [`CollectionHandle`][`pallet_common::CollectionHandle`] into [`FungibleHandle`].179	pub fn cast(inner: pallet_common::CollectionHandle<T>) -> Self {180		Self(inner)181	}182183	/// Casts [`FungibleHandle`] into [`CollectionHandle`][`pallet_common::CollectionHandle`].184	pub fn into_inner(self) -> pallet_common::CollectionHandle<T> {185		self.0186	}187	/// Returns a mutable reference to the internal [`CollectionHandle`][`pallet_common::CollectionHandle`].188	pub fn common_mut(&mut self) -> &mut pallet_common::CollectionHandle<T> {189		&mut self.0190	}191}192impl<T: Config> WithRecorder<T> for FungibleHandle<T> {193	fn recorder(&self) -> &pallet_evm_coder_substrate::SubstrateRecorder<T> {194		self.0.recorder()195	}196	fn into_recorder(self) -> pallet_evm_coder_substrate::SubstrateRecorder<T> {197		self.0.into_recorder()198	}199}200impl<T: Config> Deref for FungibleHandle<T> {201	type Target = pallet_common::CollectionHandle<T>;202203	fn deref(&self) -> &Self::Target {204		&self.0205	}206}207208/// Pallet implementation for fungible assets209impl<T: Config> Pallet<T> {210	/// Initializes the collection. Returns [CollectionId] on success, [DispatchError] otherwise.211	pub fn init_collection(212		owner: T::CrossAccountId,213		payer: T::CrossAccountId,214		data: CreateCollectionData<T::AccountId>,215	) -> Result<CollectionId, DispatchError> {216		<PalletCommon<T>>::init_collection(owner, payer, data, CollectionFlags::default())217	}218219	/// Initializes the collection with ForeignCollection flag. Returns [CollectionId] on success, [DispatchError] otherwise.220	pub fn init_foreign_collection(221		owner: T::CrossAccountId,222		payer: T::CrossAccountId,223		data: CreateCollectionData<T::AccountId>,224	) -> Result<CollectionId, DispatchError> {225		let id = <PalletCommon<T>>::init_collection(226			owner,227			payer,228			data,229			CollectionFlags {230				foreign: true,231				..Default::default()232			},233		)?;234		Ok(id)235	}236237	/// Destroys a collection.238	pub fn destroy_collection(239		collection: FungibleHandle<T>,240		sender: &T::CrossAccountId,241	) -> DispatchResult {242		let id = collection.id;243244		if Self::collection_has_tokens(id) {245			return Err(<CommonError<T>>::CantDestroyNotEmptyCollection.into());246		}247248		// =========249250		PalletCommon::destroy_collection(collection.0, sender)?;251252		<TotalSupply<T>>::remove(id);253		let _ = <Balance<T>>::clear_prefix((id,), u32::MAX, None);254		let _ = <Allowance<T>>::clear_prefix((id,), u32::MAX, None);255		Ok(())256	}257258	///Checks if collection has tokens. Return `true` if it has.259	fn collection_has_tokens(collection_id: CollectionId) -> bool {260		<TotalSupply<T>>::get(collection_id) != 0261	}262263	/// Burns the specified amount of the token. If the token balance264	/// or total supply is less than the given value,265	/// it will return [DispatchError].266	pub fn burn(267		collection: &FungibleHandle<T>,268		owner: &T::CrossAccountId,269		amount: u128,270	) -> DispatchResult {271		let total_supply = <TotalSupply<T>>::get(collection.id)272			.checked_sub(amount)273			.ok_or(<CommonError<T>>::TokenValueTooLow)?;274275		let balance = <Balance<T>>::get((collection.id, owner))276			.checked_sub(amount)277			.ok_or(<CommonError<T>>::TokenValueTooLow)?;278279		// Foreign collection check280		ensure!(!collection.flags.foreign, <CommonError<T>>::NoPermission);281282		if collection.permissions.access() == AccessMode::AllowList {283			collection.check_allowlist(owner)?;284		}285286		// =========287288		if balance == 0 {289			<Balance<T>>::remove((collection.id, owner));290			<PalletStructure<T>>::unnest_if_nested(owner, collection.id, TokenId::default());291		} else {292			<Balance<T>>::insert((collection.id, owner), balance);293		}294		<TotalSupply<T>>::insert(collection.id, total_supply);295296		<PalletEvm<T>>::deposit_log(297			ERC20Events::Transfer {298				from: *owner.as_eth(),299				to: H160::default(),300				value: amount.into(),301			}302			.to_log(collection_id_to_address(collection.id)),303		);304		<PalletCommon<T>>::deposit_event(CommonEvent::ItemDestroyed(305			collection.id,306			TokenId::default(),307			owner.clone(),308			amount,309		));310		Ok(())311	}312313	/// Burns the specified amount of the token.314	pub fn burn_foreign(315		collection: &FungibleHandle<T>,316		owner: &T::CrossAccountId,317		amount: u128,318	) -> DispatchResult {319		let total_supply = <TotalSupply<T>>::get(collection.id)320			.checked_sub(amount)321			.ok_or(<CommonError<T>>::TokenValueTooLow)?;322323		let balance = <Balance<T>>::get((collection.id, owner))324			.checked_sub(amount)325			.ok_or(<CommonError<T>>::TokenValueTooLow)?;326		// =========327328		if balance == 0 {329			<Balance<T>>::remove((collection.id, owner));330			<PalletStructure<T>>::unnest_if_nested(owner, collection.id, TokenId::default());331		} else {332			<Balance<T>>::insert((collection.id, owner), balance);333		}334		<TotalSupply<T>>::insert(collection.id, total_supply);335336		<PalletEvm<T>>::deposit_log(337			ERC20Events::Transfer {338				from: *owner.as_eth(),339				to: H160::default(),340				value: amount.into(),341			}342			.to_log(collection_id_to_address(collection.id)),343		);344		<PalletCommon<T>>::deposit_event(CommonEvent::ItemDestroyed(345			collection.id,346			TokenId::default(),347			owner.clone(),348			amount,349		));350		Ok(())351	}352353	/// Transfers the specified amount of tokens. Will check that354	/// the transfer is allowed for the token.355	///356	/// - `from`: Owner of tokens to transfer.357	/// - `to`: Recepient of transfered tokens.358	/// - `amount`: Amount of tokens to transfer.359	/// - `collection`: Collection that contains the token360	pub fn transfer(361		collection: &FungibleHandle<T>,362		from: &T::CrossAccountId,363		to: &T::CrossAccountId,364		amount: u128,365		nesting_budget: &dyn Budget,366	) -> DispatchResult {367		ensure!(368			collection.limits.transfers_enabled(),369			<CommonError<T>>::TransferNotAllowed,370		);371372		if collection.permissions.access() == AccessMode::AllowList {373			collection.check_allowlist(from)?;374			collection.check_allowlist(to)?;375		}376		<PalletCommon<T>>::ensure_correct_receiver(to)?;377378		let balance_from = <Balance<T>>::get((collection.id, from))379			.checked_sub(amount)380			.ok_or(<CommonError<T>>::TokenValueTooLow)?;381		let balance_to = if from != to {382			Some(383				<Balance<T>>::get((collection.id, to))384					.checked_add(amount)385					.ok_or(ArithmeticError::Overflow)?,386			)387		} else {388			None389		};390391		// =========392393		<PalletStructure<T>>::nest_if_sent_to_token(394			from.clone(),395			to,396			collection.id,397			TokenId::default(),398			nesting_budget,399		)?;400401		if let Some(balance_to) = balance_to {402			// from != to403			if balance_from == 0 {404				<Balance<T>>::remove((collection.id, from));405				<PalletStructure<T>>::unnest_if_nested(from, collection.id, TokenId::default());406			} else {407				<Balance<T>>::insert((collection.id, from), balance_from);408			}409			<Balance<T>>::insert((collection.id, to), balance_to);410		}411412		<PalletEvm<T>>::deposit_log(413			ERC20Events::Transfer {414				from: *from.as_eth(),415				to: *to.as_eth(),416				value: amount.into(),417			}418			.to_log(collection_id_to_address(collection.id)),419		);420		<PalletCommon<T>>::deposit_event(CommonEvent::Transfer(421			collection.id,422			TokenId::default(),423			from.clone(),424			to.clone(),425			amount,426		));427		Ok(())428	}429430	/// Minting tokens for multiple IDs.431	/// It is a utility function used in [`create_multiple_items`][`Pallet::create_multiple_items`]432	/// and [`create_multiple_items_foreign`][`Pallet::create_multiple_items_foreign`]433	pub fn create_multiple_items_common(434		collection: &FungibleHandle<T>,435		sender: &T::CrossAccountId,436		data: BTreeMap<T::CrossAccountId, u128>,437		nesting_budget: &dyn Budget,438	) -> DispatchResult {439		let total_supply = data440			.iter()441			.map(|(_, v)| *v)442			.try_fold(<TotalSupply<T>>::get(collection.id), |acc, v| {443				acc.checked_add(v)444			})445			.ok_or(ArithmeticError::Overflow)?;446447		for (to, _) in data.iter() {448			<PalletStructure<T>>::check_nesting(449				sender.clone(),450				to,451				collection.id,452				TokenId::default(),453				nesting_budget,454			)?;455		}456457		let updated_balances = data458			.into_iter()459			.map(|(user, amount)| {460				let updated_balance = <Balance<T>>::get((collection.id, &user))461					.checked_add(amount)462					.ok_or(ArithmeticError::Overflow)?;463				Ok((user, amount, updated_balance))464			})465			.collect::<Result<Vec<_>, DispatchError>>()?;466467		// =========468469		<TotalSupply<T>>::insert(collection.id, total_supply);470		for (user, amount, updated_balance) in updated_balances {471			<Balance<T>>::insert((collection.id, &user), updated_balance);472			<PalletStructure<T>>::nest_if_sent_to_token_unchecked(473				&user,474				collection.id,475				TokenId::default(),476			);477			<PalletEvm<T>>::deposit_log(478				ERC20Events::Transfer {479					from: H160::default(),480					to: *user.as_eth(),481					value: amount.into(),482				}483				.to_log(collection_id_to_address(collection.id)),484			);485			<PalletCommon<T>>::deposit_event(CommonEvent::ItemCreated(486				collection.id,487				TokenId::default(),488				user.clone(),489				amount,490			));491		}492493		Ok(())494	}495496	/// Minting tokens for multiple IDs.497	/// See [`create_item`][`Pallet::create_item`] for more details.498	pub fn create_multiple_items(499		collection: &FungibleHandle<T>,500		sender: &T::CrossAccountId,501		data: BTreeMap<T::CrossAccountId, u128>,502		nesting_budget: &dyn Budget,503	) -> DispatchResult {504		// Foreign collection check505		ensure!(!collection.flags.foreign, <CommonError<T>>::NoPermission);506507		if !collection.is_owner_or_admin(sender) {508			ensure!(509				collection.permissions.mint_mode(),510				<CommonError<T>>::PublicMintingNotAllowed511			);512			collection.check_allowlist(sender)?;513514			for (owner, _) in data.iter() {515				collection.check_allowlist(owner)?;516			}517		}518519		Self::create_multiple_items_common(collection, sender, data, nesting_budget)520	}521522	/// Minting tokens for multiple IDs.523	/// See [`create_item_foreign`][`Pallet::create_item_foreign`] for more details.524	pub fn create_multiple_items_foreign(525		collection: &FungibleHandle<T>,526		sender: &T::CrossAccountId,527		data: BTreeMap<T::CrossAccountId, u128>,528		nesting_budget: &dyn Budget,529	) -> DispatchResult {530		Self::create_multiple_items_common(collection, sender, data, nesting_budget)531	}532533	fn set_allowance_unchecked(534		collection: &FungibleHandle<T>,535		owner: &T::CrossAccountId,536		spender: &T::CrossAccountId,537		amount: u128,538	) {539		if amount == 0 {540			<Allowance<T>>::remove((collection.id, owner, spender));541		} else {542			<Allowance<T>>::insert((collection.id, owner, spender), amount);543		}544545		<PalletEvm<T>>::deposit_log(546			ERC20Events::Approval {547				owner: *owner.as_eth(),548				spender: *spender.as_eth(),549				value: amount.into(),550			}551			.to_log(collection_id_to_address(collection.id)),552		);553		<PalletCommon<T>>::deposit_event(CommonEvent::Approved(554			collection.id,555			TokenId(0),556			owner.clone(),557			spender.clone(),558			amount,559		));560	}561562	/// Set allowance for the spender to `transfer` or `burn` owner's tokens.563	///564	/// - `collection`: Collection that contains the token565	/// - `owner`: Owner of tokens that sets the allowance.566	/// - `spender`: Recipient of the allowance rights.567	/// - `amount`: Amount of tokens the spender is allowed to `transfer` or `burn`.568	pub fn set_allowance(569		collection: &FungibleHandle<T>,570		owner: &T::CrossAccountId,571		spender: &T::CrossAccountId,572		amount: u128,573	) -> DispatchResult {574		if collection.permissions.access() == AccessMode::AllowList {575			collection.check_allowlist(owner)?;576			collection.check_allowlist(spender)?;577		}578579		if <Balance<T>>::get((collection.id, owner)) < amount {580			ensure!(581				collection.ignores_owned_amount(owner),582				<CommonError<T>>::CantApproveMoreThanOwned583			);584		}585586		// =========587588		Self::set_allowance_unchecked(collection, owner, spender, amount);589		Ok(())590	}591592	/// Checks if a non-owner has (enough) allowance from the owner to perform operations on the tokens.593	/// Returns the expected remaining allowance - it should be set manually if the transaction proceeds.594	///595	/// - `collection`: Collection that contains the token.596	/// - `spender`: CrossAccountId who has the allowance rights.597	/// - `from`: The owner of the tokens who sets the allowance.598	/// - `amount`: Amount of tokens by which the allowance sholud be reduced.599	fn check_allowed(600		collection: &FungibleHandle<T>,601		spender: &T::CrossAccountId,602		from: &T::CrossAccountId,603		amount: u128,604		nesting_budget: &dyn Budget,605	) -> Result<Option<u128>, DispatchError> {606		if spender.conv_eq(from) {607			return Ok(None);608		}609		if collection.permissions.access() == AccessMode::AllowList {610			// `from`, `to` checked in [`transfer`]611			collection.check_allowlist(spender)?;612		}613		if let Some(source) = T::CrossTokenAddressMapping::address_to_token(from) {614			// TODO: should collection owner be allowed to perform this transfer?615			ensure!(616				<PalletStructure<T>>::check_indirectly_owned(617					spender.clone(),618					source.0,619					source.1,620					None,621					nesting_budget622				)?,623				<CommonError<T>>::ApprovedValueTooLow,624			);625			return Ok(None);626		}627		let allowance = <Allowance<T>>::get((collection.id, from, spender)).checked_sub(amount);628		if allowance.is_none() {629			ensure!(630				collection.ignores_allowance(spender),631				<CommonError<T>>::ApprovedValueTooLow632			);633		}634635		Ok(allowance)636	}637638	/// Transfer fungible tokens from one account to another.639	/// Same as the [`transfer`][`Pallet::transfer`] but spender doesn't needs to be an owner of the token pieces.640	/// The owner should set allowance for the spender to transfer pieces.641	///	See [`set_allowance`][`Pallet::set_allowance`] for more details.642643	pub fn transfer_from(644		collection: &FungibleHandle<T>,645		spender: &T::CrossAccountId,646		from: &T::CrossAccountId,647		to: &T::CrossAccountId,648		amount: u128,649		nesting_budget: &dyn Budget,650	) -> DispatchResult {651		let allowance = Self::check_allowed(collection, spender, from, amount, nesting_budget)?;652653		// =========654655		Self::transfer(collection, from, to, amount, nesting_budget)?;656		if let Some(allowance) = allowance {657			Self::set_allowance_unchecked(collection, from, spender, allowance);658		}659		Ok(())660	}661662	/// Burn fungible tokens from the account.663	///664	/// Same as the [`burn`][`Pallet::burn`] but spender doesn't need to be an owner of the tokens. The `from` should665	/// set allowance for the spender to burn tokens.666	/// See [`set_allowance`][`Pallet::set_allowance`] for more details.667	pub fn burn_from(668		collection: &FungibleHandle<T>,669		spender: &T::CrossAccountId,670		from: &T::CrossAccountId,671		amount: u128,672		nesting_budget: &dyn Budget,673	) -> DispatchResult {674		let allowance = Self::check_allowed(collection, spender, from, amount, nesting_budget)?;675676		// =========677678		Self::burn(collection, from, amount)?;679		if let Some(allowance) = allowance {680			Self::set_allowance_unchecked(collection, from, spender, allowance);681		}682		Ok(())683	}684685	///	Creates fungible token.686	///687	/// The sender should be the owner/admin of the collection or collection should be configured688	/// to allow public minting.689	///690	/// - `data`: Contains user who will become the owners of the tokens and amount691	///   of tokens he will receive.692	pub fn create_item(693		collection: &FungibleHandle<T>,694		sender: &T::CrossAccountId,695		data: CreateItemData<T>,696		nesting_budget: &dyn Budget,697	) -> DispatchResult {698		Self::create_multiple_items(699			collection,700			sender,701			[(data.0, data.1)].into_iter().collect(),702			nesting_budget,703		)704	}705706	///	Creates fungible token.707	///708	/// - `data`: Contains user who will become the owners of the tokens and amount709	///   of tokens he will receive.710	pub fn create_item_foreign(711		collection: &FungibleHandle<T>,712		sender: &T::CrossAccountId,713		data: CreateItemData<T>,714		nesting_budget: &dyn Budget,715	) -> DispatchResult {716		Self::create_multiple_items_foreign(717			collection,718			sender,719			[(data.0, data.1)].into_iter().collect(),720			nesting_budget,721		)722	}723724	/// Returns 10 tokens owners in no particular order725	///726	/// There is no direct way to get token holders in ascending order,727	/// since `iter_prefix` returns values in no particular order.728	/// Therefore, getting the 10 largest holders with a large value of holders729	/// can lead to impact memory allocation + sorting with  `n * log (n)`.730	pub fn token_owners(731		collection: CollectionId,732		_token: TokenId,733	) -> Option<Vec<T::CrossAccountId>> {734		let res: Vec<T::CrossAccountId> = <Balance<T>>::iter_prefix((collection,))735			.map(|(owner, _amount)| owner)736			.take(10)737			.collect();738739		if res.is_empty() {740			None741		} else {742			Some(res)743		}744	}745}