Non-fungible token traits (#8993)

* Non-fungible token traits

* Docs

* Fixes

* Implement non-fungible trait for Uniques

* Update frame/uniques/src/impl_nonfungibles.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update frame/uniques/src/impl_nonfungibles.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/frame/support/src/lib.rs

@@ -76,7 +76,7 @@ pub use self::hash::{
 pub use self::storage::{
 	StorageValue, StorageMap, StorageDoubleMap, StorageNMap, StoragePrefixedMap,
 	IterableStorageMap, IterableStorageDoubleMap, IterableStorageNMap, migration,
-	bounded_vec::BoundedVec, weak_bounded_vec::WeakBoundedVec,
+	bounded_vec::{BoundedVec, BoundedSlice}, weak_bounded_vec::WeakBoundedVec,
 };
 pub use self::dispatch::{Parameter, Callable};
 pub use sp_runtime::{self, ConsensusEngineId, print, traits::Printable};

substrate/frame/support/src/storage/bounded_vec.rs

@@ -20,7 +20,7 @@
 
 use sp_std::prelude::*;
 use sp_std::{convert::TryFrom, fmt, marker::PhantomData};
-use codec::{Encode, Decode};
+use codec::{Encode, Decode, EncodeLike};
 use core::{
 	ops::{Deref, Index, IndexMut},
 	slice::SliceIndex,
@@ -40,6 +40,33 @@ use crate::{
 #[derive(Encode)]
 pub struct BoundedVec<T, S>(Vec<T>, PhantomData<S>);
 
+/// A bounded slice.
+///
+/// Similar to a `BoundedVec`, but not owned and cannot be decoded.
+#[derive(Encode)]
+pub struct BoundedSlice<'a, T, S>(&'a [T], PhantomData<S>);
+
+// `BoundedSlice`s encode to something which will always decode into a `BoundedVec` or a `Vec`.
+impl<'a, T: Encode + Decode, S: Get<u32>> EncodeLike<BoundedVec<T, S>> for BoundedSlice<'a, T, S> {}
+impl<'a, T: Encode + Decode, S: Get<u32>> EncodeLike<Vec<T>> for BoundedSlice<'a, T, S> {}
+
+impl<'a, T, S: Get<u32>> TryFrom<&'a [T]> for BoundedSlice<'a, T, S> {
+	type Error = ();
+	fn try_from(t: &'a [T]) -> Result<Self, Self::Error> {
+		if t.len() < S::get() as usize {
+			Ok(BoundedSlice(t, PhantomData))
+		} else {
+			Err(())
+		}
+	}
+}
+
+impl<'a, T, S> From<BoundedSlice<'a, T, S>> for &'a [T] {
+	fn from(t: BoundedSlice<'a, T, S>) -> Self {
+		t.0
+	}
+}
+
 impl<T: Decode, S: Get<u32>> Decode for BoundedVec<T, S> {
 	fn decode<I: codec::Input>(input: &mut I) -> Result<Self, codec::Error> {
 		let inner = Vec::<T>::decode(input)?;
@@ -54,6 +81,9 @@ impl<T: Decode, S: Get<u32>> Decode for BoundedVec<T, S> {
 	}
 }
 
+// `BoundedVec`s encode to something which will always decode as a `Vec`.
+impl<T: Encode + Decode, S: Get<u32>> EncodeLike<Vec<T>> for BoundedVec<T, S> {}
+
 impl<T, S> BoundedVec<T, S> {
 	/// Create `Self` from `t` without any checks.
 	fn unchecked_from(t: Vec<T>) -> Self {

substrate/frame/support/src/traits/tokens.rs

@@ -21,6 +21,8 @@ pub mod fungible;
 pub mod fungibles;
 pub mod currency;
 pub mod imbalance;
+pub mod nonfungible;
+pub mod nonfungibles;
 mod misc;
 pub use misc::{
 	WithdrawConsequence, DepositConsequence, ExistenceRequirement, BalanceStatus, WithdrawReasons,

substrate/frame/support/src/traits/tokens/nonfungible.rs

@@ -0,0 +1,190 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2019-2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Traits for dealing with a single non-fungible asset class.
+//!
+//! This assumes a single level namespace identified by `Inspect::InstanceId`, and could
+//! reasonably be implemented by pallets which wants to expose a single collection of NFT-like
+//! objects.
+//!
+//! For an NFT API which has dual-level namespacing, the traits in `nonfungibles` are better to
+//! use.
+
+use codec::{Encode, Decode};
+use sp_std::prelude::*;
+use sp_runtime::TokenError;
+use crate::dispatch::DispatchResult;
+use crate::traits::Get;
+use super::nonfungibles;
+
+/// Trait for providing an interface to a read-only NFT-like set of asset instances.
+pub trait Inspect<AccountId> {
+	/// Type for identifying an asset instance.
+	type InstanceId;
+
+	/// Returns the owner of asset `instance`, or `None` if the asset doesn't exist or has no
+	/// owner.
+	fn owner(instance: &Self::InstanceId) -> Option<AccountId>;
+
+	/// Returns the attribute value of `instance` corresponding to `key`.
+	///
+	/// By default this is `None`; no attributes are defined.
+	fn attribute(_instance: &Self::InstanceId, _key: &[u8]) -> Option<Vec<u8>> { None }
+
+	/// Returns the strongly-typed attribute value of `instance` corresponding to `key`.
+	///
+	/// By default this just attempts to use `attribute`.
+	fn typed_attribute<K: Encode, V: Decode>(instance: &Self::InstanceId, key: &K) -> Option<V> {
+		key.using_encoded(|d| Self::attribute(instance, d))
+			.and_then(|v| V::decode(&mut &v[..]).ok())
+	}
+
+	/// Returns `true` if the asset `instance` may be transferred.
+	///
+	/// Default implementation is that all assets are transferable.
+	fn can_transfer(_instance: &Self::InstanceId) -> bool { true }
+}
+
+/// Interface for enumerating assets in existence or owned by a given account over a collection
+/// of NFTs.
+///
+/// WARNING: These may be a heavy operations. Do not use when execution time is limited.
+pub trait InspectEnumerable<AccountId>: Inspect<AccountId> {
+	/// Returns the instances of an asset `class` in existence.
+	fn instances() -> Vec<Self::InstanceId>;
+
+	/// Returns the asset instances of all classes owned by `who`.
+	fn owned(who: &AccountId) -> Vec<Self::InstanceId>;
+}
+
+/// Trait for providing an interface for NFT-like assets which may be minted, burned and/or have
+/// attributes set on them.
+pub trait Mutate<AccountId>: Inspect<AccountId> {
+	/// Mint some asset `instance` to be owned by `who`.
+	///
+	/// By default, this is not a supported operation.
+	fn mint_into(_instance: &Self::InstanceId, _who: &AccountId) -> DispatchResult {
+		Err(TokenError::Unsupported.into())
+	}
+
+	/// Burn some asset `instance`.
+	///
+	/// By default, this is not a supported operation.
+	fn burn_from(_instance: &Self::InstanceId) -> DispatchResult {
+		Err(TokenError::Unsupported.into())
+	}
+
+	/// Set attribute `value` of asset `instance`'s `key`.
+	///
+	/// By default, this is not a supported operation.
+	fn set_attribute(_instance: &Self::InstanceId, _key: &[u8], _value: &[u8]) -> DispatchResult {
+		Err(TokenError::Unsupported.into())
+	}
+
+	/// Attempt to set the strongly-typed attribute `value` of `instance`'s `key`.
+	///
+	/// By default this just attempts to use `set_attribute`.
+	fn set_typed_attribute<K: Encode, V: Encode>(
+		instance: &Self::InstanceId,
+		key: &K,
+		value: &V,
+	) -> DispatchResult {
+		key.using_encoded(|k| value.using_encoded(|v| Self::set_attribute(instance, k, v)))
+	}
+}
+
+/// Trait for providing a non-fungible set of assets which can only be transferred.
+pub trait Transfer<AccountId>: Inspect<AccountId> {
+	/// Transfer asset `instance` into `destination` account.
+	fn transfer(instance: &Self::InstanceId, destination: &AccountId) -> DispatchResult;
+}
+
+/// Convert a `fungibles` trait implementation into a `fungible` trait implementation by identifying
+/// a single item.
+pub struct ItemOf<
+	F: nonfungibles::Inspect<AccountId>,
+	A: Get<<F as nonfungibles::Inspect<AccountId>>::ClassId>,
+	AccountId,
+>(
+	sp_std::marker::PhantomData<(F, A, AccountId)>
+);
+
+impl<
+	F: nonfungibles::Inspect<AccountId>,
+	A: Get<<F as nonfungibles::Inspect<AccountId>>::ClassId>,
+	AccountId,
+> Inspect<AccountId> for ItemOf<F, A, AccountId> {
+	type InstanceId = <F as nonfungibles::Inspect<AccountId>>::InstanceId;
+	fn owner(instance: &Self::InstanceId) -> Option<AccountId> {
+		<F as nonfungibles::Inspect<AccountId>>::owner(&A::get(), instance)
+	}
+	fn attribute(instance: &Self::InstanceId, key: &[u8]) -> Option<Vec<u8>> {
+		<F as nonfungibles::Inspect<AccountId>>::attribute(&A::get(), instance, key)
+	}
+	fn typed_attribute<K: Encode, V: Decode>(instance: &Self::InstanceId, key: &K) -> Option<V> {
+		<F as nonfungibles::Inspect<AccountId>>::typed_attribute(&A::get(), instance, key)
+	}
+	fn can_transfer(instance: &Self::InstanceId) -> bool {
+		<F as nonfungibles::Inspect<AccountId>>::can_transfer(&A::get(), instance)
+	}
+}
+
+impl<
+	F: nonfungibles::InspectEnumerable<AccountId>,
+	A: Get<<F as nonfungibles::Inspect<AccountId>>::ClassId>,
+	AccountId,
+> InspectEnumerable<AccountId> for ItemOf<F, A, AccountId> {
+	fn instances() -> Vec<Self::InstanceId> {
+		<F as nonfungibles::InspectEnumerable<AccountId>>::instances(&A::get())
+	}
+	fn owned(who: &AccountId) -> Vec<Self::InstanceId> {
+		<F as nonfungibles::InspectEnumerable<AccountId>>::owned_in_class(&A::get(), who)
+	}
+}
+
+impl<
+	F: nonfungibles::Mutate<AccountId>,
+	A: Get<<F as nonfungibles::Inspect<AccountId>>::ClassId>,
+	AccountId,
+> Mutate<AccountId> for ItemOf<F, A, AccountId> {
+	fn mint_into(instance: &Self::InstanceId, who: &AccountId) -> DispatchResult {
+		<F as nonfungibles::Mutate<AccountId>>::mint_into(&A::get(), instance, who)
+	}
+	fn burn_from(instance: &Self::InstanceId) -> DispatchResult {
+		<F as nonfungibles::Mutate<AccountId>>::burn_from(&A::get(), instance)
+	}
+	fn set_attribute(instance: &Self::InstanceId, key: &[u8], value: &[u8]) -> DispatchResult {
+		<F as nonfungibles::Mutate<AccountId>>::set_attribute(&A::get(), instance, key, value)
+	}
+	fn set_typed_attribute<K: Encode, V: Encode>(
+		instance: &Self::InstanceId,
+		key: &K,
+		value: &V,
+	) -> DispatchResult {
+		<F as nonfungibles::Mutate<AccountId>>::set_typed_attribute(&A::get(), instance, key, value)
+	}
+}
+
+impl<
+	F: nonfungibles::Transfer<AccountId>,
+	A: Get<<F as nonfungibles::Inspect<AccountId>>::ClassId>,
+	AccountId,
+> Transfer<AccountId> for ItemOf<F, A, AccountId> {
+	fn transfer(instance: &Self::InstanceId, destination: &AccountId) -> DispatchResult {
+		<F as nonfungibles::Transfer<AccountId>>::transfer(&A::get(), instance, destination)
+	}
+}

substrate/frame/support/src/traits/tokens/nonfungibles.rs

@@ -0,0 +1,194 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2019-2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Traits for dealing with multiple collections of non-fungible assets.
+//!
+//! This assumes a dual-level namespace identified by `Inspect::InstanceId`, and could
+//! reasonably be implemented by pallets which want to expose multiple independent collections of
+//! NFT-like objects.
+//!
+//! For an NFT API which has single-level namespacing, the traits in `nonfungible` are better to
+//! use.
+//!
+//! Implementations of these traits may be converted to implementations of corresponding
+//! `nonfungible` traits by using the `nonfungible::ItemOf` type adapter.
+
+use sp_std::prelude::*;
+use codec::{Encode, Decode};
+use sp_runtime::TokenError;
+use crate::dispatch::DispatchResult;
+
+/// Trait for providing an interface to many read-only NFT-like sets of asset instances.
+pub trait Inspect<AccountId> {
+	/// Type for identifying an asset instance.
+	type InstanceId;
+
+	/// Type for identifying an asset class (an identifier for an independent collection of asset
+	/// instances).
+	type ClassId;
+
+	/// Returns the owner of asset `instance` of `class`, or `None` if the asset doesn't exist (or
+	/// somehow has no owner).
+	fn owner(class: &Self::ClassId, instance: &Self::InstanceId) -> Option<AccountId>;
+
+	/// Returns the owner of the asset `class`, if there is one. For many NFTs this may not make
+	/// any sense, so users of this API should not be surprised to find an asset class results in
+	/// `None` here.
+	fn class_owner(_class: &Self::ClassId) -> Option<AccountId> { None }
+
+	/// Returns the attribute value of `instance` of `class` corresponding to `key`.
+	///
+	/// By default this is `None`; no attributes are defined.
+	fn attribute(_class: &Self::ClassId, _instance: &Self::InstanceId, _key: &[u8])
+		-> Option<Vec<u8>>
+	{
+		None
+	}
+
+	/// Returns the strongly-typed attribute value of `instance` of `class` corresponding to `key`.
+	///
+	/// By default this just attempts to use `attribute`.
+	fn typed_attribute<K: Encode, V: Decode>(
+		class: &Self::ClassId,
+		instance: &Self::InstanceId,
+		key: &K,
+	) -> Option<V> {
+		key.using_encoded(|d| Self::attribute(class, instance, d))
+			.and_then(|v| V::decode(&mut &v[..]).ok())
+	}
+
+	/// Returns the attribute value of `class` corresponding to `key`.
+	///
+	/// By default this is `None`; no attributes are defined.
+	fn class_attribute(_class: &Self::ClassId, _key: &[u8]) -> Option<Vec<u8>> { None }
+
+	/// Returns the strongly-typed attribute value of `class` corresponding to `key`.
+	///
+	/// By default this just attempts to use `class_attribute`.
+	fn typed_class_attribute<K: Encode, V: Decode>(
+		class: &Self::ClassId,
+		key: &K,
+	) -> Option<V> {
+		key.using_encoded(|d| Self::class_attribute(class, d))
+			.and_then(|v| V::decode(&mut &v[..]).ok())
+	}
+
+	/// Returns `true` if the asset `instance` of `class` may be transferred.
+	///
+	/// Default implementation is that all assets are transferable.
+	fn can_transfer(_class: &Self::ClassId, _instance: &Self::InstanceId) -> bool { true }
+}
+
+/// Interface for enumerating assets in existence or owned by a given account over many collections
+/// of NFTs.
+///
+/// WARNING: These may be a heavy operations. Do not use when execution time is limited.
+pub trait InspectEnumerable<AccountId>: Inspect<AccountId> {
+	/// Returns the asset classes in existence.
+	fn classes() -> Vec<Self::ClassId>;
+
+	/// Returns the instances of an asset `class` in existence.
+	fn instances(class: &Self::ClassId) -> Vec<Self::InstanceId>;
+
+	/// Returns the asset instances of all classes owned by `who`.
+	fn owned(who: &AccountId) -> Vec<(Self::ClassId, Self::InstanceId)>;
+
+	/// Returns the asset instances of `class` owned by `who`.
+	fn owned_in_class(class: &Self::ClassId, who: &AccountId) -> Vec<Self::InstanceId>;
+}
+
+/// Trait for providing an interface for multiple classes of NFT-like assets which may be minted,
+/// burned and/or have attributes set on them.
+pub trait Mutate<AccountId>: Inspect<AccountId> {
+	/// Mint some asset `instance` of `class` to be owned by `who`.
+	///
+	/// By default, this is not a supported operation.
+	fn mint_into(
+		_class: &Self::ClassId,
+		_instance: &Self::InstanceId,
+		_who: &AccountId,
+	) -> DispatchResult {
+		Err(TokenError::Unsupported.into())
+	}
+
+	/// Burn some asset `instance` of `class`.
+	///
+	/// By default, this is not a supported operation.
+	fn burn_from(_class: &Self::ClassId, _instance: &Self::InstanceId) -> DispatchResult {
+		Err(TokenError::Unsupported.into())
+	}
+
+	/// Set attribute `value` of asset `instance` of `class`'s `key`.
+	///
+	/// By default, this is not a supported operation.
+	fn set_attribute(
+		_class: &Self::ClassId,
+		_instance: &Self::InstanceId,
+		_key: &[u8],
+		_value: &[u8],
+	) -> DispatchResult {
+		Err(TokenError::Unsupported.into())
+	}
+
+	/// Attempt to set the strongly-typed attribute `value` of `instance` of `class`'s `key`.
+	///
+	/// By default this just attempts to use `set_attribute`.
+	fn set_typed_attribute<K: Encode, V: Encode>(
+		class: &Self::ClassId,
+		instance: &Self::InstanceId,
+		key: &K,
+		value: &V,
+	) -> DispatchResult {
+		key.using_encoded(|k| value.using_encoded(|v|
+			Self::set_attribute(class, instance, k, v)
+		))
+	}
+
+	/// Set attribute `value` of asset `class`'s `key`.
+	///
+	/// By default, this is not a supported operation.
+	fn set_class_attribute(
+		_class: &Self::ClassId,
+		_key: &[u8],
+		_value: &[u8],
+	) -> DispatchResult {
+		Err(TokenError::Unsupported.into())
+	}
+
+	/// Attempt to set the strongly-typed attribute `value` of `class`'s `key`.
+	///
+	/// By default this just attempts to use `set_attribute`.
+	fn set_typed_class_attribute<K: Encode, V: Encode>(
+		class: &Self::ClassId,
+		key: &K,
+		value: &V,
+	) -> DispatchResult {
+		key.using_encoded(|k| value.using_encoded(|v|
+			Self::set_class_attribute(class, k, v)
+		))
+	}
+}
+
+/// Trait for providing a non-fungible sets of assets which can only be transferred.
+pub trait Transfer<AccountId>: Inspect<AccountId> {
+	/// Transfer asset `instance` of `class` into `destination` account.
+	fn transfer(
+		class: &Self::ClassId,
+		instance: &Self::InstanceId,
+		destination: &AccountId,
+	) -> DispatchResult;
+}

substrate/frame/uniques/src/functions.rs

@@ -0,0 +1,115 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2017-2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Various pieces of common functionality.
+
+use super::*;
+use frame_support::{ensure, traits::Get};
+use sp_runtime::{DispatchResult, DispatchError};
+
+impl<T: Config<I>, I: 'static> Pallet<T, I> {
+	pub(crate) fn do_transfer(
+		class: T::ClassId,
+		instance: T::InstanceId,
+		dest: T::AccountId,
+		with_details: impl FnOnce(
+			&ClassDetailsFor<T, I>,
+			&mut InstanceDetailsFor<T, I>,
+		) -> DispatchResult,
+	) -> DispatchResult {
+		let class_details = Class::<T, I>::get(&class).ok_or(Error::<T, I>::Unknown)?;
+		ensure!(!class_details.is_frozen, Error::<T, I>::Frozen);
+
+		let mut details = Asset::<T, I>::get(&class, &instance).ok_or(Error::<T, I>::Unknown)?;
+		ensure!(!details.is_frozen, Error::<T, I>::Frozen);
+		with_details(&class_details, &mut details)?;
+
+		Account::<T, I>::remove((&details.owner, &class, &instance));
+		Account::<T, I>::insert((&dest, &class, &instance), ());
+		let origin = details.owner;
+		details.owner = dest;
+		Asset::<T, I>::insert(&class, &instance, &details);
+
+		Self::deposit_event(Event::Transferred(class, instance, origin, details.owner));
+		Ok(())
+	}
+
+	pub(super) fn do_mint(
+		class: T::ClassId,
+		instance: T::InstanceId,
+		owner: T::AccountId,
+		with_details: impl FnOnce(
+			&ClassDetailsFor<T, I>,
+		) -> DispatchResult,
+	) -> DispatchResult {
+		ensure!(!Asset::<T, I>::contains_key(class, instance), Error::<T, I>::AlreadyExists);
+
+		Class::<T, I>::try_mutate(&class, |maybe_class_details| -> DispatchResult {
+			let class_details = maybe_class_details.as_mut().ok_or(Error::<T, I>::Unknown)?;
+
+			with_details(&class_details)?;
+
+			let instances = class_details.instances.checked_add(1)
+				.ok_or(ArithmeticError::Overflow)?;
+			class_details.instances = instances;
+
+			let deposit = match class_details.free_holding {
+				true => Zero::zero(),
+				false => T::InstanceDeposit::get(),
+			};
+			T::Currency::reserve(&class_details.owner, deposit)?;
+			class_details.total_deposit += deposit;
+
+			let owner = owner.clone();
+			Account::<T, I>::insert((&owner, &class, &instance), ());
+			let details = InstanceDetails { owner, approved: None, is_frozen: false, deposit};
+			Asset::<T, I>::insert(&class, &instance, details);
+			Ok(())
+		})?;
+
+		Self::deposit_event(Event::Issued(class, instance, owner));
+		Ok(())
+	}
+
+	pub(super) fn do_burn(
+		class: T::ClassId,
+		instance: T::InstanceId,
+		with_details: impl FnOnce(
+			&ClassDetailsFor<T, I>,
+			&InstanceDetailsFor<T, I>,
+		) -> DispatchResult,
+	) -> DispatchResult {
+		let owner = Class::<T, I>::try_mutate(&class, |maybe_class_details| -> Result<T::AccountId, DispatchError> {
+			let class_details = maybe_class_details.as_mut().ok_or(Error::<T, I>::Unknown)?;
+			let details = Asset::<T, I>::get(&class, &instance)
+				.ok_or(Error::<T, I>::Unknown)?;
+			with_details(&class_details, &details)?;
+
+			// Return the deposit.
+			T::Currency::unreserve(&class_details.owner, details.deposit);
+			class_details.total_deposit.saturating_reduce(details.deposit);
+			class_details.instances.saturating_dec();
+			Ok(details.owner)
+		})?;
+
+		Asset::<T, I>::remove(&class, &instance);
+		Account::<T, I>::remove((&owner, &class, &instance));
+
+		Self::deposit_event(Event::Burned(class, instance, owner));
+		Ok(())
+	}
+}

substrate/frame/uniques/src/impl_nonfungibles.rs

@@ -0,0 +1,108 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2017-2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Implementations for `nonfungibles` traits.
+
+use super::*;
+use sp_std::convert::TryFrom;
+use frame_support::traits::tokens::nonfungibles::{Inspect, Mutate, Transfer};
+use frame_support::BoundedSlice;
+use sp_runtime::DispatchResult;
+
+impl<T: Config<I>, I: 'static> Inspect<<T as SystemConfig>::AccountId> for Pallet<T, I> {
+	type InstanceId = T::InstanceId;
+	type ClassId = T::ClassId;
+
+	fn owner(
+		class: &Self::ClassId,
+		instance: &Self::InstanceId,
+	) -> Option<<T as SystemConfig>::AccountId> {
+		Asset::<T, I>::get(class, instance).map(|a| a.owner)
+	}
+
+	fn class_owner(class: &Self::ClassId) -> Option<<T as SystemConfig>::AccountId> {
+		Class::<T, I>::get(class).map(|a| a.owner)
+	}
+
+	/// Returns the attribute value of `instance` of `class` corresponding to `key`.
+	///
+	/// When `key` is empty, we return the instance metadata value.
+	///
+	/// By default this is `None`; no attributes are defined.
+	fn attribute(class: &Self::ClassId, instance: &Self::InstanceId, key: &[u8])
+		-> Option<Vec<u8>>
+	{
+		if key.is_empty() {
+			// We make the empty key map to the instance metadata value.
+			InstanceMetadataOf::<T, I>::get(class, instance).map(|m| m.data.into())
+		} else {
+			let key = BoundedSlice::<_, _>::try_from(key).ok()?;
+			Attribute::<T, I>::get((class, Some(instance), key)).map(|a| a.0.into())
+		}
+	}
+
+	/// Returns the attribute value of `instance` of `class` corresponding to `key`.
+	///
+	/// When `key` is empty, we return the instance metadata value.
+	///
+	/// By default this is `None`; no attributes are defined.
+	fn class_attribute(class: &Self::ClassId, key: &[u8])
+		-> Option<Vec<u8>>
+	{
+		if key.is_empty() {
+			// We make the empty key map to the instance metadata value.
+			ClassMetadataOf::<T, I>::get(class).map(|m| m.data.into())
+		} else {
+			let key = BoundedSlice::<_, _>::try_from(key).ok()?;
+			Attribute::<T, I>::get((class, Option::<T::InstanceId>::None, key)).map(|a| a.0.into())
+		}
+	}
+
+	/// Returns `true` if the asset `instance` of `class` may be transferred.
+	///
+	/// Default implementation is that all assets are transferable.
+	fn can_transfer(class: &Self::ClassId, instance: &Self::InstanceId) -> bool {
+		match (Class::<T, I>::get(class), Asset::<T, I>::get(class, instance)) {
+			(Some(cd), Some(id)) if !cd.is_frozen && !id.is_frozen => true,
+			_ => false,
+		}
+	}
+}
+
+impl<T: Config<I>, I: 'static> Mutate<<T as SystemConfig>::AccountId> for Pallet<T, I> {
+	fn mint_into(
+		class: &Self::ClassId,
+		instance: &Self::InstanceId,
+		who: &T::AccountId,
+	) -> DispatchResult {
+		Self::do_mint(class.clone(), instance.clone(), who.clone(), |_| Ok(()))
+	}
+
+	fn burn_from(class: &Self::ClassId, instance: &Self::InstanceId) -> DispatchResult {
+		Self::do_burn(class.clone(), instance.clone(), |_, _| Ok(()))
+	}
+}
+
+impl<T: Config<I>, I: 'static> Transfer<T::AccountId> for Pallet<T, I> {
+	fn transfer(
+		class: &Self::ClassId,
+		instance: &Self::InstanceId,
+		destination: &T::AccountId,
+	) -> DispatchResult {
+		Self::do_transfer(class.clone(), instance.clone(), destination.clone(), |_, _| Ok(()))
+	}
+}

substrate/frame/uniques/src/lib.rs

@@ -36,6 +36,8 @@ pub mod mock;
 mod tests;
 
 mod types;
+mod functions;
+mod impl_nonfungibles;
 pub use types::*;
 
 use sp_std::prelude::*;
@@ -448,32 +450,10 @@ pub mod pallet {
 			let origin = ensure_signed(origin)?;
 			let owner = T::Lookup::lookup(owner)?;
 
-			ensure!(!Asset::<T, I>::contains_key(class, instance), Error::<T, I>::AlreadyExists);
-
-			Class::<T, I>::try_mutate(&class, |maybe_class_details| -> DispatchResult {
-				let class_details = maybe_class_details.as_mut().ok_or(Error::<T, I>::Unknown)?;
+			Self::do_mint(class, instance, owner, |class_details| {
 				ensure!(class_details.issuer == origin, Error::<T, I>::NoPermission);
-
-				let instances = class_details.instances.checked_add(1)
-					.ok_or(ArithmeticError::Overflow)?;
-				class_details.instances = instances;
-
-				let deposit = match class_details.free_holding {
-					true => Zero::zero(),
-					false => T::InstanceDeposit::get(),
-				};
-				T::Currency::reserve(&class_details.owner, deposit)?;
-				class_details.total_deposit += deposit;
-
-				let owner = owner.clone();
-				Account::<T, I>::insert((&owner, &class, &instance), ());
-				let details = InstanceDetails { owner, approved: None, is_frozen: false, deposit};
-				Asset::<T, I>::insert(&class, &instance, details);
 				Ok(())
-			})?;
-
-			Self::deposit_event(Event::Issued(class, instance, owner));
-			Ok(())
+			})
 		}
 
 		/// Destroy a single asset instance.
@@ -499,27 +479,12 @@ pub mod pallet {
 			let origin = ensure_signed(origin)?;
 			let check_owner = check_owner.map(T::Lookup::lookup).transpose()?;
 
-			let owner = Class::<T, I>::try_mutate(&class, |maybe_class_details| -> Result<T::AccountId, DispatchError> {
-				let class_details = maybe_class_details.as_mut().ok_or(Error::<T, I>::Unknown)?;
-				let details = Asset::<T, I>::get(&class, &instance)
-					.ok_or(Error::<T, I>::Unknown)?;
+			Self::do_burn(class, instance, |class_details, details| {
 				let is_permitted = class_details.admin == origin || details.owner == origin;
 				ensure!(is_permitted, Error::<T, I>::NoPermission);
 				ensure!(check_owner.map_or(true, |o| o == details.owner), Error::<T, I>::WrongOwner);
-
-				// Return the deposit.
-				T::Currency::unreserve(&class_details.owner, details.deposit);
-				class_details.total_deposit.saturating_reduce(details.deposit);
-				class_details.instances.saturating_dec();
-				Ok(details.owner)
-			})?;
-
-
-			Asset::<T, I>::remove(&class, &instance);
-			Account::<T, I>::remove((&owner, &class, &instance));
-
-			Self::deposit_event(Event::Burned(class, instance, owner));
-			Ok(())
+				Ok(())
+			})
 		}
 
 		/// Move an asset from the sender account to another.
@@ -547,24 +512,13 @@ pub mod pallet {
 			let origin = ensure_signed(origin)?;
 			let dest = T::Lookup::lookup(dest)?;
 
-			let class_details = Class::<T, I>::get(&class).ok_or(Error::<T, I>::Unknown)?;
-			ensure!(!class_details.is_frozen, Error::<T, I>::Frozen);
-
-			let mut details = Asset::<T, I>::get(&class, &instance).ok_or(Error::<T, I>::Unknown)?;
-			ensure!(!details.is_frozen, Error::<T, I>::Frozen);
-			if details.owner != origin && class_details.admin != origin {
-				let approved = details.approved.take().map_or(false, |i| i == origin);
-				ensure!(approved, Error::<T, I>::NoPermission);
-			}
-
-			Account::<T, I>::remove((&details.owner, &class, &instance));
-			Account::<T, I>::insert((&dest, &class, &instance), ());
-			details.owner = dest;
-			Asset::<T, I>::insert(&class, &instance, &details);
-
-			Self::deposit_event(Event::Transferred(class, instance, origin, details.owner));
-
-			Ok(())
+			Self::do_transfer(class, instance, dest, |class_details, details| {
+				if details.owner != origin && class_details.admin != origin {
+					let approved = details.approved.take().map_or(false, |i| i == origin);
+					ensure!(approved, Error::<T, I>::NoPermission);
+				}
+				Ok(())
+			})
 		}
 
 		/// Reevaluate the deposits on some assets.

substrate/frame/uniques/src/types.rs

@@ -22,6 +22,11 @@ use frame_support::{traits::Get, BoundedVec};
 
 pub(super) type DepositBalanceOf<T, I = ()> =
 	<<T as Config<I>>::Currency as Currency<<T as SystemConfig>::AccountId>>::Balance;
+pub(super) type ClassDetailsFor<T, I> =
+	ClassDetails<<T as SystemConfig>::AccountId, DepositBalanceOf<T, I>>;
+pub(super) type InstanceDetailsFor<T, I> =
+	InstanceDetails<<T as SystemConfig>::AccountId, DepositBalanceOf<T, I>>;
+
 
 #[derive(Clone, Encode, Decode, Eq, PartialEq, RuntimeDebug)]
 pub struct ClassDetails<

substrate/primitives/runtime/src/lib.rs

@@ -544,6 +544,8 @@ pub enum TokenError {
 	UnknownAsset,
 	/// Funds exist but are frozen.
 	Frozen,
+	/// Operation is not supported by the asset.
+	Unsupported,
 }
 
 impl From<TokenError> for &'static str {
@@ -555,6 +557,7 @@ impl From<TokenError> for &'static str {
 			TokenError::CannotCreate => "Account cannot be created",
 			TokenError::UnknownAsset => "The asset in question is unknown",
 			TokenError::Frozen => "Funds exist but are frozen",
+			TokenError::Unsupported => "Operation is not supported by the asset",
 		}
 	}
 }