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Events

The last thing our contract will need at this point is an event handler.

What are Events for Smart Contracts?​

Events are important for smart contracts because they facilitate communication between the contract itself and the user interface. In traditional Web2 development, a server response is provided in a callback to the frontend. In Web3, when a transaction is executed, smart contracts emit events to the blockchain that the frontend is able to process.

Minting Event​

In our contract, an event should be emitted when a token is minted. One could expect that by calling the Openbrush _mint_to() function an event will be emitted, but upon closer examination we can see that _emit_transfer_event() has an empty default implementation. This grants developers flexibility to create events that are suitable for their own needs.

default fn _emit_transfer_event(&self, _from: Option<AccountId>, _to: Option<AccountId>, _id: Id) {}

Let's define two events that are required for token handling, Transfer and Approve, in the contracts's lib.rs file. Please note that there is no Mint event, as it's covered by the Transfer event, in which case from will be the contract address.

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

/// Event emitted when a token transfer occurs.
#[ink(event)]
pub struct Transfer {
    #[ink(topic)]
    from: Option<AccountId>,
    #[ink(topic)]
    to: Option<AccountId>,
    #[ink(topic)]
    id: Id,
}

/// Event emitted when a token approve occurs.
#[ink(event)]
pub struct Approval {
    #[ink(topic)]
    from: AccountId,
    #[ink(topic)]
    to: AccountId,
    #[ink(topic)]
    id: Option<Id>,
    approved: bool,
}

Override the default event emission function:

#[overrider(psp34::Internal)]
fn _emit_transfer_event(&self, from: Option<AccountId>, to: Option<AccountId>, id: Id) {
    self.env().emit_event(Transfer { from, to, id });
}

#[overrider(psp34::Internal)]
fn _emit_approval_event(&self, from: AccountId, to: AccountId, id: Option<Id>, approved: bool) {
    self.env().emit_event(Approval {
        from,
        to,
        id,
        approved,
    });
}

Update Unit Test​

As a final check let's add an event check at the end of our unit test. Since our test minted 5 tokens, we should expect 5 events to be emitted.

assert_eq!(5, ink::env::test::recorded_events().count());

Format your code with:

cargo fmt --all

Run unit test:

cargo test

At this stage, your code should look something like this.

Next Step​

Congratulations! You've made it through all the steps required to build your NFT Contract!

As a next step, review the code in the main branch for the repository used for this tutorial. There you can enhance your knowledge about:

  • Improving the unit test coverage.
  • Adding new useful functions.
  • End-to-end testing.
  • Improving error handling.