Use the Gumball Machine on Stokenet

In the previous Step-by-Step section we allowed our gumball machine to mint its own gumballs. The blueprint still isn't quite ready for us to publish on the ledger though. Currently anyone can mint gumballs, so let's look at restricting it to the component it's methods. We'll also add an icon for the gumball token, so it's easily identifiable in wallets and explorers.

Contents

• Virtual Badges
  • Address Reservation
  • Restricting Mint Roles
• Icons for Tokens
• Publishing the Package and Using Gumball Machine

Virtual Badges

The Owner badge, like other badges we might make, is a resource. We make it the required evidence of ownership in one of the instantiation steps. In some cases it makes more sense to use a component's own address instead of creating a badge. This is known as the Virtual Badge pattern.

Address Reservation

All components on the Radix ledger have a unique address assigned at instantiation, but the gumball token is defined inside the instantiate function. So we have access to the a component address inside the function before it's complete, we need to reserve it. We do so right at the start of instantiate_gumball_machine.

let (address_reservation, component_address) =
    Runtime::allocate_component_address(GumballMachine::blueprint_id());

This give us the component_address to use for the minter rule, and the
address_reservation to apply to the component in the last instantiation step.

Restricting Mint Roles

We update the minter rule to now require the component's own address to mint new gumballs. Only the component and therefore it's methods will be able to mint.

.mint_roles(mint_roles! {
    minter => rule!(require(global_caller(component_address)));
    minter_updater => rule!(deny_all);
})

The address_reservation is applied in the last instantiation step, giving our component the address we reserved and same address as used to mint new gumballs.

    .instantiate()
    .prepare_to_globalize(OwnerRole::Fixed(rule!(require(
        owner_badge.resource_address()
    ))))
    // Apply the address reservation
    .with_address(address_reservation)
    .globalize();

Icons for Tokens

We now have one last thing to do before we publish the gumball machine. Outside of resim the token will be displayed in a variety of ways in dapps and wallets. So these can be more visually appealing we need to add an icon for the gumball token. This is done with just an extra metadata field called icon_url.

   "icon_url" => Url::of("https://assets.radixdlt.com/icons/icon-gumball-pink.png"), locked;

URLs and strings are not treated the same in the Radix ledger and so we need to use the Url type. The locked keyword, as before, is used to prevent the icon URL from being changed after the component is instantiated.

We now have a blueprint we can publish on the ledger.

Publishing the Package and Using the Gumball Machine