use near_sdk::json_types::U128;
use near_sdk::{AccountId, PromiseOrValue, ext_contract};

#[ext_contract(ext_ft_receiver)]
pub trait FungibleTokenReceiver {
    /// Called by fungible token contract after `ft_transfer_call` was initiated by
    /// `sender_id` of the given `amount` with the transfer message given in `msg` field.
    /// The `amount` of tokens were already transferred to this contract account and ready to be used.
    ///
    /// The method must return the amount of tokens that are *not* used/accepted by this contract from the transferred
    /// amount. Examples:
    /// - The transferred amount was `500`, the contract completely takes it and must return `0`.
    /// - The transferred amount was `500`, but this transfer call only needs `450` for the action passed in the `msg`
    ///   field, then the method must return `50`.
    /// - The transferred amount was `500`, but the action in `msg` field has expired and the transfer must be
    ///   cancelled. The method must return `500` or panic.
    ///
    /// Arguments:
    /// - `sender_id` - the account ID that initiated the transfer.
    /// - `amount` - the amount of tokens that were transferred to this account in a decimal string representation.
    /// - `msg` - a string message that was passed with this transfer call.
    ///
    /// Returns the amount of unused tokens that should be returned to sender, in a decimal string representation.
    fn ft_on_transfer(
        &mut self,
        sender_id: AccountId,
        amount: U128,
        msg: String,
    ) -> PromiseOrValue<U128>;
}