Icon LinkDefining The Storage Block

Next, we can add the storage block. The storage block is where you can store any state variables in your contract that you want to be persistent. Any Sway primitive type can be stored in the storage block.

Any variables declared inside a function and not saved in the storage block will be destroyed when the function finishes executing.

storage {
	// counter for total items listed
	item_counter: u64 = 0,
	// map of item IDs to Items
	item_map: StorageMap<u64, Item> = StorageMap {},
	// owner of the contract
	owner: Option<Identity> = Option::None,
}

The first variable we have stored is item_counter, a number initialized to 0. You can use this counter to track the total number of items listed.

Icon LinkStorageMap

A StorageMap is a special type that allows you to save key-value pairs inside a storage block.

To define a storage map, you must specify the type for the key and value. For example, below, the type for the key is u64, and the type for the value is an Item struct.

item_map: StorageMap<u64, Item> = StorageMap{}

Here, we are saving a mapping of the item's ID to the Item struct. With this, we can look up information about an item with the ID.

Icon LinkOptions

Here we are setting the variable owner as a variable that could be None or could store an Identity.

owner: Option<Identity> = Option::None

If you want a value to be null or undefined under certain conditions, you can use an Option type, which is an enum that can be either Some(value) or None. The keyword None represents that no value exists, while the keyword Some means there is some value stored.