Gumball Machine Example

A Radix favorite, this example covers the creation of a simple gumball machine, which allows users to purchase a gumball in exchange for XRD.

In addition to the most basic operation, this particular example allows instantiators to set the price (in XRD) of each gumball, and permits callers to submit more than the exact price required. Callers will receive their change (if any) in addition to their tasty gumball.

You can find the code for this example on github here.

Resources and Data

struct GumballMachine {
  gumballs: Vault,
  collected_xrd: Vault,
  price: Decimal

Our gumball machine will hold two kinds of resources in vaults: the gumballs to be dispensed, and any XRD which has been collected as payment.

We’ll also need to maintain the price, which we’re using Decimal for. Decimal is a bounded type appropriate for use for resource quantities. In Scrypto, it has a fixed precision of 10-18, and a maximum value of 296. Unless we’re selling spectacularly expensive gumballs, this should be fine.

Getting Ready for Instantiation

In order to instantiate a new gumball machine, the only input we need from the caller is to set the price of each gumball. After creation, we’ll be returning the address of our new component, so we’ll set our function signature up appropriately:

pub fn instantiate_gumball_machine(price: Decimal) -> Global<GumballMachine> {

Within the instantiate_gumball_machine function, the first thing we need to do is create a new supply of gumballs which we intend to populate our new component with:

let bucket_of_gumballs = ResourceBuilder::new_fungible(OwnerRole::None)
        init {
            "name" => "Gumball", locked;
            "symbol" => "GUM", locked;
            "description" => "A delicious gumball", locked;

All that’s left is to populate our GumballMachine struct with our supply of gumballs, the user-specified price, and an empty Vault which we will force to contain XRD. Then we’ll instantiate it, which returns the address, and we’ll return that to the caller.

Self {
    gumballs: Vault::with_bucket(bucket_of_gumballs),
    collected_xrd: Vault::new(RADIX_TOKEN),
    price: price,

Allowing Callers to Buy Gumballs

In order to sell a gumball, we just need the caller to pass us in enough XRD to cover the price. We’ll return the purchased gumball, as well as giving back their change if they overpaid, so we actually need to return two buckets. This is easily accomplished by simply returning a tuple, giving us a method signature like this:

pub fn buy_gumball(&mut self, mut payment: Bucket) -> (Bucket, Bucket) {

Note that we used &mut self because our reference to ourself must be mutable; we will be changing the contents of our vaults, if all goes well. We also need the mut keyword in front of the payment parameter because we will be updating its content.

Accomplishing the actual mechanics of putting the XRD in our vault, taking a gumball out, and then returning the gumball as well as whatever is left in the caller’s input bucket are trivial:

let our_share = payment.take(self.price);
(self.gumballs.take(1), payment)

Note that we didn’t have to check that the input bucket contained XRD…​ when we attempt to put tokens into our collected_xrd Vault (which was initialized to contain XRD), we’ll get a runtime error if we try to put in anything else.

Similarly, we’ll get a runtime error if we try to take out a quantity matching the price, and find that there is insufficient quantity present.

Finally, if the user provided exactly the correct amount of XRD as input, when we return their payment bucket, it will simply contain quantity 0.

Closing Thoughts

In order to keep this as straightforward as possible in teaching a few basic concepts, we haven’t made the best gumball machine that we could.

First, we have provided no way to actually retrieve the collected XRD! This would most appropriately be done by adding a withdrawal method, protected by an admin badge which ensures that only appropriate parties are able to perform the withdrawal.

Second, there’s no reason to force payment in XRD only…​ we could easily have changed the instantiate_gumball_machine function to accept the ResourceAddress of the currency that we intend to collect in exchange for gumballs.

Third, we could allow for greater flexibility in the gumballs themselves. We could let the instantiator function define the symbol, name, and quantity…​or even directly pass in the gumballs to be sold rather than creating a new resource at instantiation time.