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Testing a transfer function

A basic overview to help you write out your checks and balances.

Goal#

Learn how to write tests and improve the correctness of a transfer function.

Use cases#

Testing a custom transfer function.

Overview#

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

Steps#

1. 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>::Transfered(sender, to, amount))
Ok(().into())
}

2. 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(Origin::signed(1), 2, 42));
// Send 50 coins to account 3.
asset_noop!(RewardCoin::transfer(Origin::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.

3. Check that sending account doesn't go below minimum balance#

4. Check that both tests work together#

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

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

5. 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#

Resources#

Rust docs#

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