NFT Rental Example

NFT renting is a mechanism that allows individuals without ownership or possession of a specific NFT to temporarily utilize or experience it. The implementation of this process leverages the Kiosk Apps standard to establish an infrastructure for rental transactions. This approach closely aligns with the Ethereum ERC-4907 renting standard, making it a suitable choice for Solidity-based use cases intended for implementation on Sui.

The NFT Rental example satisfies the following project requirements:

• Enable a lender to offer their assets for renting for a specified period of time (list for renting).
• Enable a lender to define the rental duration.
  • Borrower has to comply with the renting period.
• Borrower can gain mutable or immutable access to the NFT.
  • Immutable access is read-only.
  • Mutable, the lender should consider downgrade and upgrade operations and include them in the renting fee.
• After the renting period has finished, the item can be sold normally.
• Creator-defined royalties are respected by encompassing transfer policy rules.

Use cases

Some use cases for real-world NFT rental example include:

• Gaming
• Ticketing
• Virtual land
• Temporary assets and subscriptions

Gaming

There are multiple cases in gaming where renting NFTs can be beneficial to user experience:

• In-game assets: NFTs can represent unique in-game items, characters, skins, or accessories. Players can rent these assets securely.
• Ownership and authenticity: NFTs provide a transparent and immutable record of ownership, ensuring that players who truly own their in-game items can rent them and receive back the item under rent after the renting period expires. This can combat issues like fraud and counterfeiting.
• Cross-game integration: Renting NFTs can work across multiple games, allowing players to carry and rent their unique items or characters from one game to another, fostering interoperability.
• Gaming collectibles: NFTs can represent digital collectibles within games, creating a digital asset ecosystem where players can rent unique items.

Ticketing

In the realm of ticketing, NFTs play a pivotal role in enhancing transferability. These digital assets facilitate a secure and traceable transfer, resale, or rental of tickets, mitigating the risk of counterfeit tickets within the secondary market. The blockchain-based nature of NFTs ensures transparency and authenticity in each transaction, providing users with a reliable and fraud-resistant means to engage in ticket-related activities. This innovation not only simplifies the process for ticket holders but also contributes to a more trustworthy and efficient secondary ticket market.

Virtual land

Renting virtual lands and offices in the metaverse provides businesses with flexible solutions, enabling event companies to host gatherings without the commitment of permanent acquisitions and facilitating remote work through virtual offices. This approach not only offers cost-effective alternatives but also aligns with the evolving dynamics of digital business operations.

Temporary assets and subscriptions

Temporary assets and subscriptions are notable applications of rental NFTs, offering accessibility to virtual experiences like high-end virtual casinos or curated digital fashion. These NFTs cater to diverse budgets, broadening audience reach. Subscription rentals extend to pools of digital assets, allowing users to pay monthly for a set number of items, fostering accessibility, user retention, and acquisition. Holders can rent out unused subscriptions, ensuring no loss for them, potential customer gains for the protocol, and a commitment-free trial for temporary holders. This showcases the adaptability and user-centric appeal of rental NFTs in diverse scenarios.

Smart contract design

warning

Transferring kiosks might result in unexpected behaviors while an asset is being rented. If you want to disallow kiosk transferring all together, consider using personal kiosks.

The rental smart contract uses the Kiosk Apps standard. Both the lender and borrower must install a Kiosk extension to take part, and the creator of the borrowed asset type must create a rental policy and ProtectedTP object to allow the extension to manage rentals while enforcing royalties.

info

This implementation is charging a rental fee based on days. You can re-purpose and update the logic to support charging per hour, or even seconds.

Move modules

The NFT Rental example uses a single module, nft_rental.move. You can find the source for this file hosted in the sui repository in the examples directory. The source code includes extensive comments to help you follow the example's logic and structure.

nft_rental

The nft_rental module provides an API that facilitates lending or borrowing through the following operations:

• List for renting
• Delist from renting
• Rent
• Borrow by reference and borrow by value
• Reclaim for the lender

Structs

The object model of the nft_rental module provides the structure of the app, beginning with the Rentables object. The struct has only the drop ability and acts as the extension key for the kiosk Rentables extension.

public struct Rentables has drop {}

The Rented struct represents a rented item. The only field the struct includes is the ID of the object. It is used as the dynamic field key in the borrower's Bag entry when someone is actively borrowing an item. The struct has store, copy, and drop abilities because they are necessary for all dynamic field keys.

public struct Rented has store, copy, drop { id: ID }

The Listed struct represents a listed item. The only field the struct includes is the ID of the object. It is used as the dynamic field key in the renter's Bag entry after an item is listed for renting. Like Rented, this struct has store, copy, and drop abilities because they are necessary for all dynamic field keys.

public struct Listed has store, copy, drop { id: ID }

The Promise struct is created for borrowing by value. The Promise operates as the hot potato (a struct that has no capabilities that you can only pack and unpack in its module) that can only be resolved by returning the item back to the extension's Bag.

The Promise field lacks the store ability as it shouldn't be wrapped inside other objects. It also lacks the drop ability because only the return_val function can consume it.

public struct Promise {
  item: Rented,
  duration: u64,
  start_date: u64,
  price_per_day: u64,
  renter_kiosk: address,
  borrower_kiosk: ID
}

The Rentable struct is as a wrapper object that holds an asset that is being rented. Contains information relevant to the rental period, cost, and renter. This struct requires the store ability because it stores a value T that definitely also has store.

public struct Rentable<T: key + store> has store {
  object: T,
  /// Total amount of time offered for renting in days.
  duration: u64,
  /// Initially undefined, is updated once someone rents it.
  start_date: Option<u64>,
  price_per_day: u64,
  /// The kiosk id that the object was taken from.
  kiosk_id: ID,
}

The RentalPolicy struct is a shared object that every creator mints. The struct defines the royalties the creator receives from each rent invocation.

public struct RentalPolicy<phantom T> has key, store {
  id: UID,
  balance: Balance<SUI>,
  /// Note: Move does not support float numbers.
  ///
  /// If you need to represent a float, you need to determine the desired
  /// precision and use a larger integer representation.
  ///
  /// For example, percentages can be represented using basis points:
  /// 10000 basis points represent 100% and 100 basis points represent 1%.
  amount_bp: u64
}

The ProtectedTP object is a shared object that creators mint to enable renting. The object provides authorized access to an empty TransferPolicy. This is in part required because of the restrictions that Kiosk imposes around royalty enforced items and their tradability. Additionally it allows the rental module to operate within the Extension framework while maintaining the guarantee that the assets handled will always be tradable.

A protected empty transfer policy is required to facilitate the rental process so that the extension can transfer the asset without any additional rules to resolve (like lock rule, loyalty rule, and so on). If creators want to enforce royalties on rentals, they can use the RentalPolicy detailed previously.

public struct ProtectedTP<phantom T> has key, store {
  id: UID,
  transfer_policy: TransferPolicy<T>,
  policy_cap: TransferPolicyCap<T>
}

Function signatures

The NFT Rental example includes the following functions that define the project's logic.

The install function enables installation of the Rentables extension in a kiosk. The party facilitating the rental process is responsible for making sure that the user installs the extension in the their kiosk.

public fun install(
  kiosk: &mut Kiosk,
  cap: &KioskOwnerCap,
  ctx: &mut TxContext
){
  kiosk_extension::add(Rentables {}, kiosk, cap, PERMISSIONS, ctx);
}

The remove function enables the owner (and only the owner) of the kiosk to remove the extension. The extension storage must be empty for the transaction to succeed. The extension storage empties after the user is no longer borrowing or renting any items. The kiosk_extension::remove function performs the ownership check before executing.

public fun remove(kiosk: &mut Kiosk, cap: &KioskOwnerCap, _ctx: &mut TxContext){
  kiosk_extension::remove<Rentables>(kiosk, cap);
}

The setup_renting function mints and shares a ProtectedTP and a RentalPolicy object for type T. The publisher of type T is the only entity that can perform the action.

public fun setup_renting<T>(publisher: &Publisher, amount_bp: u64, ctx: &mut TxContext) {
  // Creates an empty TP and shares a ProtectedTP<T> object.
  // This can be used to bypass the lock rule under specific conditions.
  // Storing inside the cap the ProtectedTP with no way to access it
  // as we do not want to modify this policy
  let (transfer_policy, policy_cap) = transfer_policy::new<T>(publisher, ctx);

  let protected_tp = ProtectedTP {
    id: object::new(ctx),
    transfer_policy,
    policy_cap,
  };

  let rental_policy = RentalPolicy<T> {
    id: object::new(ctx),
    balance: balance::zero<SUI>(),
    amount_bp,
  };

  transfer::share_object(protected_tp);
  transfer::share_object(rental_policy);
}

The list function enables listing of an asset within the Rentables extension's bag, creating a bag entry with the asset's ID as the key and a Rentable wrapper object as the value. Requires the existence of a ProtectedTP transfer policy that only the creator of type T can create. The function assumes an item is already placed (and optionally locked) in a kiosk.

public fun list<T: key + store>(
  kiosk: &mut Kiosk,
  cap: &KioskOwnerCap,
  protected_tp: &ProtectedTP<T>,
  item_id: ID,
  duration: u64,
  price_per_day: u64,
  ctx: &mut TxContext,
) {
    
  // Aborts if Rentables extension is not installed.
  assert!(kiosk_extension::is_installed<Rentables>(kiosk), EExtensionNotInstalled);

  // Sets the kiosk owner to the transaction sender to keep metadata fields up to date.
  // This is also crucial to ensure the correct person receives the payment.
  // Prevents unexpected results in cases where the kiosk could have been transferred 
  // between users without the owner being updated.
  kiosk.set_owner(cap, ctx);

  // Lists the item for zero SUI.
  kiosk.list<T>(cap, item_id, 0);

  // Constructs a zero coin.
  let coin = coin::zero<SUI>(ctx);
  // Purchases the item with 0 SUI.
  let (object, request) = kiosk.purchase<T>(item_id, coin);

  // Resolves the TransferRequest with the empty TransferPolicy which is protected and accessible only via this module.
  let (_item, _paid, _from) = protected_tp.transfer_policy.confirm_request(request);

  // Wraps the item in the Rentable struct along with relevant metadata.
  let rentable = Rentable {
    object,
    duration,
    start_date: option::none<u64>(),
    price_per_day,
    kiosk_id: object::id(kiosk),
  };

  // Places the rentable as listed in the extension's bag (place_in_bag is a helper method defined in nft_rental.move file).
  place_in_bag<T, Listed>(kiosk, Listed { id: item_id }, rentable);
}

The delist function allows the renter to delist an item, as long as it's not currently being rented. The function also places (or locks, if a lock rule is present) the object back to owner's kiosk. You should mint an empty TransferPolicy even if you don't want to apply any royalties. If at some point you do want to enforce royalties, you can always update the existing TransferPolicy.

public fun delist<T: key + store>(
  kiosk: &mut Kiosk,
  cap: &KioskOwnerCap,
  transfer_policy: &TransferPolicy<T>,
  item_id: ID,
  _ctx: &mut TxContext,
) {

  // Aborts if the cap doesn't match the Kiosk.
  assert!(kiosk.has_access(cap), ENotOwner);

  // Removes the rentable item from the extension's Bag (take_from_bag is a helper method defined in nft_rental.move file). 
  let rentable = take_from_bag<T, Listed>(kiosk, Listed { id: item_id });

  // Deconstructs the Rentable object.
  let Rentable {
    object,
    duration: _,
    start_date: _,
    price_per_day: _,
    kiosk_id: _,
  } = rentable;

  // Respects the lock rule, if present, by re-locking the asset in the owner's Kiosk.
  if (has_rule<T, LockRule>(transfer_policy)) {
    kiosk.lock(cap, transfer_policy, object);
  } else {
    kiosk.place(cap, object);
  };
}

The rent function enables renting a listed Rentable. It permits anyone to borrow an item on behalf of another user, provided they have the Rentables extension installed. The rental_policy defines the portion of the coin that is retained as fees and added to the rental policy's balance.

public fun rent<T: key + store>(
  renter_kiosk: &mut Kiosk,
  borrower_kiosk: &mut Kiosk,
  rental_policy: &mut RentalPolicy<T>,
  item_id: ID,
  mut coin: Coin<SUI>,
  clock: &Clock,
  ctx: &mut TxContext,
) {

  // Aborts if Rentables extension is not installed.
  assert!(kiosk_extension::is_installed<Rentables>(borrower_kiosk), EExtensionNotInstalled);

  let mut rentable = take_from_bag<T, Listed>(renter_kiosk, Listed { id: item_id });

  // Calculates the price of the rental based on the days it was rented for by ensuring the outcome can be stored as a u64.
  let max_price_per_day = MAX_VALUE_U64 / rentable.duration;
  assert!(rentable.price_per_day <= max_price_per_day, ETotalPriceOverflow);
  let total_price = rentable.price_per_day * rentable.duration;

  // Accepts only exact balance for the payment and does not give change.
  let coin_value = coin.value();
  assert!(coin_value == total_price, ENotEnoughCoins);

  // Calculate fees_amount using the given basis points amount (percentage), ensuring the
  // result fits into a 64-bit unsigned integer.
  let mut fees_amount = coin_value as u128;
  fees_amount = fees_amount * (rental_policy.amount_bp as u128);
  fees_amount = fees_amount / (MAX_BASIS_POINTS as u128);

  // Calculate fees_amount using the given basis points amount (percentage), ensuring the result fits into a 64-bit unsigned integer.
  let fees = coin.split(fees_amount as u64, ctx);

  // Merges the fee balance of the given coin with the RentalPolicy balance.
  coin::put(&mut rental_policy.balance, fees);
  // Transfers the payment to the renter.
  transfer::public_transfer(coin, renter_kiosk.owner());
  rentable.start_date.fill(clock.timestamp_ms());

  place_in_bag<T, Rented>(borrower_kiosk, Rented { id: item_id }, rentable);
}

The borrow function enables the borrower to acquire the Rentable by reference from their bag.

public fun borrow<T: key + store>(
  kiosk: &mut Kiosk,
  cap: &KioskOwnerCap,
  item_id: ID,
  _ctx: &mut TxContext,
): &T {
  // Aborts if the cap doesn't match the Kiosk.
  assert!(kiosk.has_access(cap), ENotOwner);
  let ext_storage_mut = kiosk_extension::storage_mut(Rentables {}, kiosk);
  let rentable: &Rentable<T> = &ext_storage_mut[Rented { id: item_id }];
  &rentable.object
}

The borrow_val function enables the borrower to temporarily acquire the Rentable with an agreement or promise to return it. The Promise stores all the information about the Rentable, facilitating the reconstruction of the Rentable upon object return.

public fun borrow_val<T: key + store>(
  kiosk: &mut Kiosk,
  cap: &KioskOwnerCap,
  item_id: ID,
  _ctx: &mut TxContext,
): (T, Promise) {
  // Aborts if the cap doesn't match the Kiosk.
  assert!(kiosk.has_access(cap), ENotOwner);
  let borrower_kiosk = object::id(kiosk);

  let rentable = take_from_bag<T, Rented>(kiosk, Rented { id: item_id });

  // Construct a Promise struct containing the Rentable's metadata.
  let promise = Promise {
    item: Rented { id: item_id },
    duration: rentable.duration,
    start_date: *option::borrow(&rentable.start_date),
    price_per_day: rentable.price_per_day,
    renter_kiosk: rentable.kiosk_id,
    borrower_kiosk
  };

  // Deconstructs the rentable and returns the promise along with the wrapped item T.
  let Rentable {
    object,
    duration: _,
    start_date: _,
    price_per_day: _,
    kiosk_id: _,
  } = rentable;

  (object, promise)
}

The return_val function enables the borrower to return the borrowed item.

public fun return_val<T: key + store>(
  kiosk: &mut Kiosk,
  object: T,
  promise: Promise,
  _ctx: &mut TxContext,
) {
  assert!(kiosk_extension::is_installed<Rentables>(kiosk), EExtensionNotInstalled);

  let Promise {
    item,
    duration,
    start_date,
    price_per_day,
    renter_kiosk,
    borrower_kiosk,
  } = promise;

  let kiosk_id = object::id(kiosk);
  assert!(kiosk_id == borrower_kiosk, EInvalidKiosk);

  let rentable = Rentable {
    object,
    duration,
    start_date: option::some(start_date),
    price_per_day,
    kiosk_id: renter_kiosk,
  };

  place_in_bag(kiosk, item, rentable);
}

note

The reclaim functionality is manually invoked and the rental service provider is responsible for ensuring that the renter is reminded to reclaim. As such, this can cause the borrower to hold the asset for longer than the rental period. This can be mitigated through modification of the current contract by adding an assertion in the borrow and borrow_val functions to check if the rental period has expired.

The reclaim function enables an owner to claim back their asset after the rental period is over and place it inside their kiosk. If a lock rule is present, the example also locks the item inside the owner kiosk.

public fun reclaim<T: key + store>(
  renter_kiosk: &mut Kiosk,
  borrower_kiosk: &mut Kiosk,
  transfer_policy: &TransferPolicy<T>,
  clock: &Clock,
  item_id: ID,
  _ctx: &mut TxContext,
) {

  // Aborts if Rentables extension is not installed.
  assert!(kiosk_extension::is_installed<Rentables>(renter_kiosk), EExtensionNotInstalled);

  let rentable = take_from_bag<T, Rented>(borrower_kiosk, Rented { id: item_id });

  // Destructures the Rentable struct to place it back to the renter's Kiosk.
  let Rentable {
    object,
    duration,
    start_date,
    price_per_day: _,
    kiosk_id,
  } = rentable;

  // Aborts if provided kiosk is different that the initial kiosk the item was borrowed from.
  assert!(object::id(renter_kiosk) == kiosk_id, EInvalidKiosk);

  let start_date_ms = *option::borrow(&start_date);
  let current_timestamp = clock.timestamp_ms();
  let final_timestamp = start_date_ms + duration * SECONDS_IN_A_DAY;

  // Aborts if rental duration has not elapsed.
  assert!(current_timestamp > final_timestamp, ERentingPeriodNotOver);

  // Respects the lock rule, if present, by re-locking the asset in the owner's kiosk.
  if (transfer_policy.has_rule<T, LockRule>()) {
    kiosk_extension::lock<Rentables, T>(
      Rentables {},
      renter_kiosk,
      object,
      transfer_policy,
    );
  } else {
    kiosk_extension::place<Rentables, T>(
      Rentables {},
      renter_kiosk,
      object,
      transfer_policy,
    );
  };
}

Sequence diagrams

note

This implementation assumes that each creator, as an enabling action, creates a TransferPolicy even if empty, so that the Rentables extension can operate. This is a requirement in addition to invoking the setup_renting method.

Initialize

The initialization process is part of the flow but only happens once for each entity:

• For a new type that a creator would like to allow to be rented
  • Involves invoking setup_renting and TransferPolicy creation with optional lock rule
• For a Borrower that has never borrowed before using this framework
  • If no kiosk exists for the user, one should be created
  • Involves installing the extension in their kiosk
• For a Renter that has never rented before using this framework
  • If no kiosk exists for the user, one should be created
  • Involves installing the extension in their kiosk

List-Rent-Borrow-Reclaim

List-Delist

Deployment

info

See Publish a Package for a more detailed guide on publishing packages or Sui Client CLI for a complete reference of client commands in the Sui CLI.

Before publishing your code, you must first initialize the Sui Client CLI, if you haven't already. To do so, in a terminal or console at the root directory of the project enter sui client. If you receive the following response, complete the remaining instructions:

Config file ["<FILE-PATH>/.sui/sui_config/client.yaml"] doesn't exist, do you want to connect to a Sui Full node server [y/N]?

Enter y to proceed. You receive the following response:

Sui Full node server URL (Defaults to Sui Devnet if not specified) :

Leave this blank (press Enter). You receive the following response:

Select key scheme to generate keypair (0 for ed25519, 1 for secp256k1, 2: for secp256r1):

Select 0. Now you should have a Sui address set up.

Before being able to publish your package to Testnet, you need Testnet SUI tokens. To get some, join the Sui Discord, complete the verification steps, enter the #testnet-faucet channel and type !faucet <WALLET ADDRESS>. For other ways to get SUI in your Testnet account, see Get SUI Tokens.

Now that you have an account with some Testnet SUI, you can deploy your contracts. To publish your package, use the following command in the same terminal or console:

sui client publish --gas-budget <GAS-BUDGET>

For the gas budget, use a standard value such as 20000000.

Related links

• NFT Rental example: The source code that this document references.
• Sui Kiosk: Kiosk is a decentralized system for commerce applications on Sui.
• Kiosk Apps: The Kiosk Apps standard extends the functionality of Sui Kiosk.
• Custom Transfer Rules: Custom transfer rules are used in this example to define rental policies.