git.delta.rocks / unique-network / refs/commits / 9d5e6bdc297d

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		flags: CollectionFlags,216	) -> Result<CollectionId, DispatchError> {217		<PalletCommon<T>>::init_collection(owner, payer, data, flags)218	}219220	/// Initializes the collection with ForeignCollection flag. Returns [CollectionId] on success, [DispatchError] otherwise.221	pub fn init_foreign_collection(222		owner: T::CrossAccountId,223		payer: T::CrossAccountId,224		data: CreateCollectionData<T::AccountId>,225	) -> Result<CollectionId, DispatchError> {226		let id = <PalletCommon<T>>::init_collection(227			owner,228			payer,229			data,230			CollectionFlags {231				foreign: true,232				..Default::default()233			},234		)?;235		Ok(id)236	}237238	/// Destroys a collection.239	pub fn destroy_collection(240		collection: FungibleHandle<T>,241		sender: &T::CrossAccountId,242	) -> DispatchResult {243		let id = collection.id;244245		if Self::collection_has_tokens(id) {246			return Err(<CommonError<T>>::CantDestroyNotEmptyCollection.into());247		}248249		// =========250251		PalletCommon::destroy_collection(collection.0, sender)?;252253		<TotalSupply<T>>::remove(id);254		let _ = <Balance<T>>::clear_prefix((id,), u32::MAX, None);255		let _ = <Allowance<T>>::clear_prefix((id,), u32::MAX, None);256		Ok(())257	}258259	///Checks if collection has tokens. Return `true` if it has.260	fn collection_has_tokens(collection_id: CollectionId) -> bool {261		<TotalSupply<T>>::get(collection_id) != 0262	}263264	/// Burns the specified amount of the token. If the token balance265	/// or total supply is less than the given value,266	/// it will return [DispatchError].267	pub fn burn(268		collection: &FungibleHandle<T>,269		owner: &T::CrossAccountId,270		amount: u128,271	) -> DispatchResult {272		let total_supply = <TotalSupply<T>>::get(collection.id)273			.checked_sub(amount)274			.ok_or(<CommonError<T>>::TokenValueTooLow)?;275276		let balance = <Balance<T>>::get((collection.id, owner))277			.checked_sub(amount)278			.ok_or(<CommonError<T>>::TokenValueTooLow)?;279280		// Foreign collection check281		ensure!(!collection.flags.foreign, <CommonError<T>>::NoPermission);282283		if collection.permissions.access() == AccessMode::AllowList {284			collection.check_allowlist(owner)?;285		}286287		// =========288289		if balance == 0 {290			<Balance<T>>::remove((collection.id, owner));291			<PalletStructure<T>>::unnest_if_nested(owner, collection.id, TokenId::default());292		} else {293			<Balance<T>>::insert((collection.id, owner), balance);294		}295		<TotalSupply<T>>::insert(collection.id, total_supply);296297		<PalletEvm<T>>::deposit_log(298			ERC20Events::Transfer {299				from: *owner.as_eth(),300				to: H160::default(),301				value: amount.into(),302			}303			.to_log(collection_id_to_address(collection.id)),304		);305		<PalletCommon<T>>::deposit_event(CommonEvent::ItemDestroyed(306			collection.id,307			TokenId::default(),308			owner.clone(),309			amount,310		));311		Ok(())312	}313314	/// Burns the specified amount of the token.315	pub fn burn_foreign(316		collection: &FungibleHandle<T>,317		owner: &T::CrossAccountId,318		amount: u128,319	) -> DispatchResult {320		let total_supply = <TotalSupply<T>>::get(collection.id)321			.checked_sub(amount)322			.ok_or(<CommonError<T>>::TokenValueTooLow)?;323324		let balance = <Balance<T>>::get((collection.id, owner))325			.checked_sub(amount)326			.ok_or(<CommonError<T>>::TokenValueTooLow)?;327		// =========328329		if balance == 0 {330			<Balance<T>>::remove((collection.id, owner));331			<PalletStructure<T>>::unnest_if_nested(owner, collection.id, TokenId::default());332		} else {333			<Balance<T>>::insert((collection.id, owner), balance);334		}335		<TotalSupply<T>>::insert(collection.id, total_supply);336337		<PalletEvm<T>>::deposit_log(338			ERC20Events::Transfer {339				from: *owner.as_eth(),340				to: H160::default(),341				value: amount.into(),342			}343			.to_log(collection_id_to_address(collection.id)),344		);345		<PalletCommon<T>>::deposit_event(CommonEvent::ItemDestroyed(346			collection.id,347			TokenId::default(),348			owner.clone(),349			amount,350		));351		Ok(())352	}353354	/// Transfers the specified amount of tokens. Will check that355	/// the transfer is allowed for the token.356	///357	/// - `from`: Owner of tokens to transfer.358	/// - `to`: Recepient of transfered tokens.359	/// - `amount`: Amount of tokens to transfer.360	/// - `collection`: Collection that contains the token361	pub fn transfer(362		collection: &FungibleHandle<T>,363		from: &T::CrossAccountId,364		to: &T::CrossAccountId,365		amount: u128,366		nesting_budget: &dyn Budget,367	) -> DispatchResult {368		ensure!(369			collection.limits.transfers_enabled(),370			<CommonError<T>>::TransferNotAllowed,371		);372373		if collection.permissions.access() == AccessMode::AllowList {374			collection.check_allowlist(from)?;375			collection.check_allowlist(to)?;376		}377		<PalletCommon<T>>::ensure_correct_receiver(to)?;378379		let balance_from = <Balance<T>>::get((collection.id, from))380			.checked_sub(amount)381			.ok_or(<CommonError<T>>::TokenValueTooLow)?;382		let balance_to = if from != to {383			Some(384				<Balance<T>>::get((collection.id, to))385					.checked_add(amount)386					.ok_or(ArithmeticError::Overflow)?,387			)388		} else {389			None390		};391392		// =========393394		<PalletStructure<T>>::nest_if_sent_to_token(395			from.clone(),396			to,397			collection.id,398			TokenId::default(),399			nesting_budget,400		)?;401402		if let Some(balance_to) = balance_to {403			// from != to404			if balance_from == 0 {405				<Balance<T>>::remove((collection.id, from));406				<PalletStructure<T>>::unnest_if_nested(from, collection.id, TokenId::default());407			} else {408				<Balance<T>>::insert((collection.id, from), balance_from);409			}410			<Balance<T>>::insert((collection.id, to), balance_to);411		}412413		<PalletEvm<T>>::deposit_log(414			ERC20Events::Transfer {415				from: *from.as_eth(),416				to: *to.as_eth(),417				value: amount.into(),418			}419			.to_log(collection_id_to_address(collection.id)),420		);421		<PalletCommon<T>>::deposit_event(CommonEvent::Transfer(422			collection.id,423			TokenId::default(),424			from.clone(),425			to.clone(),426			amount,427		));428		Ok(())429	}430431	/// Minting tokens for multiple IDs.432	/// It is a utility function used in [`create_multiple_items`][`Pallet::create_multiple_items`]433	/// and [`create_multiple_items_foreign`][`Pallet::create_multiple_items_foreign`]434	pub fn create_multiple_items_common(435		collection: &FungibleHandle<T>,436		sender: &T::CrossAccountId,437		data: BTreeMap<T::CrossAccountId, u128>,438		nesting_budget: &dyn Budget,439	) -> DispatchResult {440		let total_supply = data441			.iter()442			.map(|(_, v)| *v)443			.try_fold(<TotalSupply<T>>::get(collection.id), |acc, v| {444				acc.checked_add(v)445			})446			.ok_or(ArithmeticError::Overflow)?;447448		for (to, _) in data.iter() {449			<PalletStructure<T>>::check_nesting(450				sender.clone(),451				to,452				collection.id,453				TokenId::default(),454				nesting_budget,455			)?;456		}457458		let updated_balances = data459			.into_iter()460			.map(|(user, amount)| {461				let updated_balance = <Balance<T>>::get((collection.id, &user))462					.checked_add(amount)463					.ok_or(ArithmeticError::Overflow)?;464				Ok((user, amount, updated_balance))465			})466			.collect::<Result<Vec<_>, DispatchError>>()?;467468		// =========469470		<TotalSupply<T>>::insert(collection.id, total_supply);471		for (user, amount, updated_balance) in updated_balances {472			<Balance<T>>::insert((collection.id, &user), updated_balance);473			<PalletStructure<T>>::nest_if_sent_to_token_unchecked(474				&user,475				collection.id,476				TokenId::default(),477			);478			<PalletEvm<T>>::deposit_log(479				ERC20Events::Transfer {480					from: H160::default(),481					to: *user.as_eth(),482					value: amount.into(),483				}484				.to_log(collection_id_to_address(collection.id)),485			);486			<PalletCommon<T>>::deposit_event(CommonEvent::ItemCreated(487				collection.id,488				TokenId::default(),489				user.clone(),490				amount,491			));492		}493494		Ok(())495	}496497	/// Minting tokens for multiple IDs.498	/// See [`create_item`][`Pallet::create_item`] for more details.499	pub fn create_multiple_items(500		collection: &FungibleHandle<T>,501		sender: &T::CrossAccountId,502		data: BTreeMap<T::CrossAccountId, u128>,503		nesting_budget: &dyn Budget,504	) -> DispatchResult {505		// Foreign collection check506		ensure!(!collection.flags.foreign, <CommonError<T>>::NoPermission);507508		if !collection.is_owner_or_admin(sender) {509			ensure!(510				collection.permissions.mint_mode(),511				<CommonError<T>>::PublicMintingNotAllowed512			);513			collection.check_allowlist(sender)?;514515			for (owner, _) in data.iter() {516				collection.check_allowlist(owner)?;517			}518		}519520		Self::create_multiple_items_common(collection, sender, data, nesting_budget)521	}522523	/// Minting tokens for multiple IDs.524	/// See [`create_item_foreign`][`Pallet::create_item_foreign`] for more details.525	pub fn create_multiple_items_foreign(526		collection: &FungibleHandle<T>,527		sender: &T::CrossAccountId,528		data: BTreeMap<T::CrossAccountId, u128>,529		nesting_budget: &dyn Budget,530	) -> DispatchResult {531		Self::create_multiple_items_common(collection, sender, data, nesting_budget)532	}533534	fn set_allowance_unchecked(535		collection: &FungibleHandle<T>,536		owner: &T::CrossAccountId,537		spender: &T::CrossAccountId,538		amount: u128,539	) {540		if amount == 0 {541			<Allowance<T>>::remove((collection.id, owner, spender));542		} else {543			<Allowance<T>>::insert((collection.id, owner, spender), amount);544		}545546		<PalletEvm<T>>::deposit_log(547			ERC20Events::Approval {548				owner: *owner.as_eth(),549				spender: *spender.as_eth(),550				value: amount.into(),551			}552			.to_log(collection_id_to_address(collection.id)),553		);554		<PalletCommon<T>>::deposit_event(CommonEvent::Approved(555			collection.id,556			TokenId(0),557			owner.clone(),558			spender.clone(),559			amount,560		));561	}562563	/// Set allowance for the spender to `transfer` or `burn` owner's tokens.564	///565	/// - `collection`: Collection that contains the token566	/// - `owner`: Owner of tokens that sets the allowance.567	/// - `spender`: Recipient of the allowance rights.568	/// - `amount`: Amount of tokens the spender is allowed to `transfer` or `burn`.569	pub fn set_allowance(570		collection: &FungibleHandle<T>,571		owner: &T::CrossAccountId,572		spender: &T::CrossAccountId,573		amount: u128,574	) -> DispatchResult {575		if collection.permissions.access() == AccessMode::AllowList {576			collection.check_allowlist(owner)?;577			collection.check_allowlist(spender)?;578		}579580		if <Balance<T>>::get((collection.id, owner)) < amount {581			ensure!(582				collection.ignores_owned_amount(owner),583				<CommonError<T>>::CantApproveMoreThanOwned584			);585		}586587		// =========588589		Self::set_allowance_unchecked(collection, owner, spender, amount);590		Ok(())591	}592593	/// Checks if a non-owner has (enough) allowance from the owner to perform operations on the tokens.594	/// Returns the expected remaining allowance - it should be set manually if the transaction proceeds.595	///596	/// - `collection`: Collection that contains the token.597	/// - `spender`: CrossAccountId who has the allowance rights.598	/// - `from`: The owner of the tokens who sets the allowance.599	/// - `amount`: Amount of tokens by which the allowance sholud be reduced.600	fn check_allowed(601		collection: &FungibleHandle<T>,602		spender: &T::CrossAccountId,603		from: &T::CrossAccountId,604		amount: u128,605		nesting_budget: &dyn Budget,606	) -> Result<Option<u128>, DispatchError> {607		if spender.conv_eq(from) {608			return Ok(None);609		}610		if collection.permissions.access() == AccessMode::AllowList {611			// `from`, `to` checked in [`transfer`]612			collection.check_allowlist(spender)?;613		}614		if let Some(source) = T::CrossTokenAddressMapping::address_to_token(from) {615			// TODO: should collection owner be allowed to perform this transfer?616			ensure!(617				<PalletStructure<T>>::check_indirectly_owned(618					spender.clone(),619					source.0,620					source.1,621					None,622					nesting_budget623				)?,624				<CommonError<T>>::ApprovedValueTooLow,625			);626			return Ok(None);627		}628		let allowance = <Allowance<T>>::get((collection.id, from, spender)).checked_sub(amount);629		if allowance.is_none() {630			ensure!(631				collection.ignores_allowance(spender),632				<CommonError<T>>::ApprovedValueTooLow633			);634		}635636		Ok(allowance)637	}638639	/// Transfer fungible tokens from one account to another.640	/// Same as the [`transfer`][`Pallet::transfer`] but spender doesn't needs to be an owner of the token pieces.641	/// The owner should set allowance for the spender to transfer pieces.642	///	See [`set_allowance`][`Pallet::set_allowance`] for more details.643644	pub fn transfer_from(645		collection: &FungibleHandle<T>,646		spender: &T::CrossAccountId,647		from: &T::CrossAccountId,648		to: &T::CrossAccountId,649		amount: u128,650		nesting_budget: &dyn Budget,651	) -> DispatchResult {652		let allowance = Self::check_allowed(collection, spender, from, amount, nesting_budget)?;653654		// =========655656		Self::transfer(collection, from, to, amount, nesting_budget)?;657		if let Some(allowance) = allowance {658			Self::set_allowance_unchecked(collection, from, spender, allowance);659		}660		Ok(())661	}662663	/// Burn fungible tokens from the account.664	///665	/// Same as the [`burn`][`Pallet::burn`] but spender doesn't need to be an owner of the tokens. The `from` should666	/// set allowance for the spender to burn tokens.667	/// See [`set_allowance`][`Pallet::set_allowance`] for more details.668	pub fn burn_from(669		collection: &FungibleHandle<T>,670		spender: &T::CrossAccountId,671		from: &T::CrossAccountId,672		amount: u128,673		nesting_budget: &dyn Budget,674	) -> DispatchResult {675		let allowance = Self::check_allowed(collection, spender, from, amount, nesting_budget)?;676677		// =========678679		Self::burn(collection, from, amount)?;680		if let Some(allowance) = allowance {681			Self::set_allowance_unchecked(collection, from, spender, allowance);682		}683		Ok(())684	}685686	///	Creates fungible token.687	///688	/// The sender should be the owner/admin of the collection or collection should be configured689	/// to allow public minting.690	///691	/// - `data`: Contains user who will become the owners of the tokens and amount692	///   of tokens he will receive.693	pub fn create_item(694		collection: &FungibleHandle<T>,695		sender: &T::CrossAccountId,696		data: CreateItemData<T>,697		nesting_budget: &dyn Budget,698	) -> DispatchResult {699		Self::create_multiple_items(700			collection,701			sender,702			[(data.0, data.1)].into_iter().collect(),703			nesting_budget,704		)705	}706707	///	Creates fungible token.708	///709	/// - `data`: Contains user who will become the owners of the tokens and amount710	///   of tokens he will receive.711	pub fn create_item_foreign(712		collection: &FungibleHandle<T>,713		sender: &T::CrossAccountId,714		data: CreateItemData<T>,715		nesting_budget: &dyn Budget,716	) -> DispatchResult {717		Self::create_multiple_items_foreign(718			collection,719			sender,720			[(data.0, data.1)].into_iter().collect(),721			nesting_budget,722		)723	}724725	/// Returns 10 tokens owners in no particular order726	///727	/// There is no direct way to get token holders in ascending order,728	/// since `iter_prefix` returns values in no particular order.729	/// Therefore, getting the 10 largest holders with a large value of holders730	/// can lead to impact memory allocation + sorting with  `n * log (n)`.731	pub fn token_owners(732		collection: CollectionId,733		_token: TokenId,734	) -> Option<Vec<T::CrossAccountId>> {735		let res: Vec<T::CrossAccountId> = <Balance<T>>::iter_prefix((collection,))736			.map(|(owner, _amount)| owner)737			.take(10)738			.collect();739740		if res.is_empty() {741			None742		} else {743			Some(res)744		}745	}746}