Icon LinkDefining the ABI

Next, we will define our ABI. ABI stands for application binary interface. In a Sway contract, it's an outline of all of the functions in the contract. For each function, you must specify its name, input types, return types, and level of storage access.

Our contract's ABI will look like this:

abi SwayStore {
	// a function to list an item for sale
	// takes the price and metadata as args
	#[storage(read, write)]
	fn list_item(price: u64, metadata: String);
 
	// a function to buy an item
	// takes the item id as the arg
	#[storage(read, write), payable]
	fn buy_item(item_id: u64);
 
	// a function to get a certain item
	#[storage(read)]
	fn get_item(item_id: u64) -> Item;
 
	// a function to set the contract owner
	#[storage(read, write)]
	fn initialize_owner() -> Identity;
 
	// a function to withdraw contract funds
	#[storage(read)]
	fn withdraw_funds();
 
	// return the number of items listed
	#[storage(read)]
	fn get_count() -> u64;
}

Icon LinkFunctions

A function is defined with the fn keyword. Sway uses snake case, so instead of naming a function myFunction, you would use my_function.

You must define the return type using a skinny arrow if the function returns anything. If there are any parameters, the types must also be defined for those. Semicolons are required at the end of each line.

If any function reads from or writes to storage, you must define that level of access above the function with either #[storage(read)] or #[storage(read, write)].

If you expect funds to be sent when a function is called, like the buy_item function, you must use the #[payable] annotation.