Test a Transfer Transaction

Testing each function is an important part of developing pallets for production. This guide steps you through best practices for writing test cases for a basic transfer function.

Outline the transfer function

A transfer function has two key elements: subtracting a balance from an account and adding that balance to another account. Here, we'll start by outlining this function:

#[pallet::weight(10_000)]
pub (super) fn transfer(
  origin: OriginFor<T>,
  to: T::AccountId,
  #[pallet::compact] amount: T::Balance,
  ) -> DispatchResultWithPostInfo {
    let sender = ensure_signed(origin)?;

    Accounts::<T>::mutate(&sender, |bal| {
      *bal = bal.saturating_sub(amount);
      });
      Accounts::<T>::mutate(&to, |bal| {
        *bal = bal.saturating_add(amount);
        });

    Self::deposit_event(Event::<T>::Transferred(sender, to, amount))
    Ok(().into())
}

Check that the sender has enough balance

The first thing to verify, is whether the sender has enough balance. In a separate tests.rs file, write out this first test case:

#[test]
fn transfer_works() {
  new_test_ext().execute_with(|| {
    MetaDataStore::<Test>::put(MetaData {
			issuance: 0,
			minter: 1,
			burner: 1,
		});
    // Mint 42 coins to account 2.
    assert_ok!(RewardCoin::mint(RuntimeOrigin::signed(1), 2, 42));
    // Send 50 coins to account 3.
    asset_noop!(RewardCoin::transfer(RuntimeOrigin::signed(2), 3, 50), Error::<T>::InsufficientBalance);

Configure error handling

To implement some error check, replace mutate with try_mutate to use ensure!. This will check whether bal is greater or equal to amount and throw an error message if not:

Accounts::<T>::try_mutate(&sender, |bal| {
  ensure!(bal >= amount, Error::<T>::InsufficientBalance);
  *bal = bal.saturating_sub(amount);
  Ok(())
});

Run cargo test from your pallet's directory.

Check that sending account doesn't go below minimum balance

Inside your transfer_works function:

assert_noop!(RewardCoin::transfer(RuntimeOrigin::signed(2), 3, 50), Error::<Test>::InsufficientBalance);

Check that both tests work together

Use #[transactional] to generate a wrapper around both checks:

#[transactional]
pub(super) fn transfer(
/*--snip--*/

Handle dust accounts

Make sure that sending and receiving accounts aren't dust accounts. Use T::MinBalance::get():

/*--snip--*/
let new_bal = bal.checked_sub(&amount).ok_or(Error::<T>::InsufficientBalance)?;
ensure!(new_bal >= T::MinBalance::get(), Error::<T>::BelowMinBalance);
/*--snip--*/

Examples

• Tests in the reward-coin example pallet.

Resources

• assert_ok!

• assert_noop!

• ensure!

• try_mutate