Implement PSP22 for Pair

Please check out this branch and open it in your IDE.

Pair contract implements an ERC-20 (slightly modified as uint256::MAX does not decrease allowance). In BalanceAI the standard for fungible tokens is PSP22. We will use the OpenBrush PSP22 implementation.

1. Implement Basic PSP22 in our Contract.

In the Cargo.toml file, import crates from ink!, scale, and Openbrush (with feature "psp22").

[package]
name = "pair_contract"
version = "0.1.0"
authors = ["Stake Technologies <devops@stake.co.jp>"]
edition = "2021"

[dependencies]
ink = { version = "4.0.0", default-features = false}

scale = { package = "parity-scale-codec", version = "3", default-features = false, features = ["derive"] }
scale-info = { version = "2.3", default-features = false, features = ["derive"], optional = true }

openbrush = { git = "https://github.com/727-Ventures/openbrush-contracts", version = "3.0.0", default-features = false, features = ["psp22"] }

[lib]
name = "pair_contract"
path = "lib.rs"
crate-type = [
    "cdylib"
]

[features]
default = ["std"]
std = [
    "ink/std",
    "scale/std",
    "scale-info/std",
    "openbrush/std",
]
ink-as-dependency = []

[profile.dev]
overflow-checks = false

[profile.release]
overflow-checks = false

contracts/pair/Cargo.toml

In the lib.rs file in the contract crate import everything (with *) from openbrush::contracts::psp22 as well as the Storage trait and SpreadAllocate from ink!

As reminder the #![cfg_attr(not(feature = "std"), no_std)] attribute is for conditional compilation and the #![feature(min_specialization)] is the feature needed to enable specialization:

#![cfg_attr(not(feature = "std"), no_std)]
#![feature(min_specialization)]

#[openbrush::contract]
pub mod pair {
    use openbrush::{
        contracts::{
            psp22::{
                Internal,
                *,
            },
        },
        traits::Storage,
    };
    
}

Add the storage struct and add the psp22 field:

#[ink(storage)]
#[derive(Default, Storage)]
pub struct PairContract {
    #[storage_field]
    psp22: psp22::Data,
}

implement PSP22 trait into your contract struct:

    impl PSP22 for PairContract {}

Add an impl block for the contract and add the constructor:

impl PairContract {
    #[ink(constructor)]
    pub fn new() -> Self {
        Self { psp22: Default::default() }
    }
}

Your contract should look like the following, and build if you run:

cargo contract build

#![cfg_attr(not(feature = "std"), no_std)]
#![feature(min_specialization)]

#[openbrush::contract]
pub mod pair {
    use openbrush::{
        contracts::{
            psp22::{
                Internal,
                *,
            },
        },
        traits::Storage,
    };

    #[ink(storage)]
    #[derive(Default, Storage)]
    pub struct PairContract {
        #[storage_field]
        psp22: psp22::Data,
    }

    impl PSP22 for PairContract {}

    impl PairContract {
        #[ink(constructor)]
        pub fn new() -> Self {
            Self { psp22: Default::default() }
        }
    }
}

contracts/pair/lib.rs

2. Add Events

You should add an events struct to your contract and also override the event emission methods from the PSP22 implementation. Import what's needed for editing events:

use ink::{
    codegen::{
        EmitEvent,
        Env,
    }
};

PSP22 emits Transfer and Approval events. An event is a struct with #[ink(event)] attribute. Some fields can be marked with #[ink(topic)] attribute which acts as indexed in Solidity:

#[ink(event)]
pub struct Transfer {
    #[ink(topic)]
    from: Option<AccountId>,
    #[ink(topic)]
    to: Option<AccountId>,
    value: Balance,
}

#[ink(event)]
pub struct Approval {
    #[ink(topic)]
    owner: AccountId,
    #[ink(topic)]
    spender: AccountId,
    value: Balance,
}

And finally, override the event emitting methods of the PSP22 Internal trait (Due to ink!'s design it is not possible to share event definitions between multiple contracts since events can only be defined in the ink! module scope directly.):

impl Internal for PairContract {
    fn _emit_transfer_event(
        &self,
        from: Option<AccountId>,
        to: Option<AccountId>,
        amount: Balance,
    ) {
        self.env().emit_event(Transfer {
            from,
            to,
            value: amount,
        });
    }

    fn _emit_approval_event(&self, owner: AccountId, spender: AccountId, amount: Balance) {
        self.env().emit_event(Approval {
            owner,
            spender,
            value: amount,
        });
    }
}

3. Override Generic Function of PSP22

The PSP22 OpenBrush implementation has a built-in check for a zero account in mint, burn, transfer_from and approve functions. But Uniswap V2 uses the zero address to lock tokens. The upside in our case is that we can override any functions of the generic implementation, by using the same function body, but removing the check for the zero address:

impl Internal for PairContract {
    // in uniswapv2 no check for zero account
    fn _mint_to(&mut self, account: AccountId, amount: Balance) -> Result<(), PSP22Error> {
        let mut new_balance = self._balance_of(&account);
        new_balance += amount;
        self.psp22.balances.insert(&account, &new_balance);
        self.psp22.supply += amount;
        self._emit_transfer_event(None, Some(account), amount);
        Ok(())
    }

    fn _burn_from(&mut self, account: AccountId, amount: Balance) -> Result<(), PSP22Error> {
        let mut from_balance = self._balance_of(&account);

        if from_balance < amount {
            return Err(PSP22Error::InsufficientBalance)
        }

        from_balance -= amount;
        self.psp22.balances.insert(&account, &from_balance);
        self.psp22.supply -= amount;
        self._emit_transfer_event(Some(account), None, amount);
        Ok(())
    }

    fn _approve_from_to(
        &mut self,
        owner: AccountId,
        spender: AccountId,
        amount: Balance,
    ) -> Result<(), PSP22Error> {
        self.psp22.allowances.insert(&(&owner, &spender), &amount);
        self._emit_approval_event(owner, spender, amount);
        Ok(())
    }

    fn _transfer_from_to(
        &mut self,
        from: AccountId,
        to: AccountId,
        amount: Balance,
        _data: Vec<u8>,
    ) -> Result<(), PSP22Error> {
        let from_balance = self._balance_of(&from);

        if from_balance < amount {
            return Err(PSP22Error::InsufficientBalance)
        }

        self.psp22.balances.insert(&from, &(from_balance - amount));
        let to_balance = self._balance_of(&to);
        self.psp22.balances.insert(&to, &(to_balance + amount));

        self._emit_transfer_event(Some(from), Some(to), amount);
        Ok(())
    }
    ...

Also in Uniswap V2 max allowance will not decrease allowance. For this, we need to override transfer_from and not decrease allowance if it's u128::MAX. Important here: please note that #[ink(message)] is needed in order to compile correctly. Inside the existing impl PSP22 block, add:

impl PSP22 for PairContract {
    #[ink(message)]
    fn transfer_from(
        &mut self,
        from: AccountId,
        to: AccountId,
        value: Balance,
        data: Vec<u8>,
    ) -> Result<(), PSP22Error> {
        let caller = self.env().caller();
        let allowance = self._allowance(&from, &caller);

        // In uniswapv2 max allowance never decrease
        if allowance != u128::MAX {
            if allowance < value {
                return Err(PSP22Error::InsufficientAllowance)
            }

            self._approve_from_to(from, caller, allowance - value)?;
        }
        self._transfer_from_to(from, to, value, data)?;
        Ok(())
    }
}

Import Vec from ink::prelude:

 use ink::prelude::vec::Vec;

And that's it! You implemented PSP22, its event and overrode its default implementation. Check your Pair contract (run in contract folder):

cargo contract build

It should now look like this branch.