Finally, it's time to compose our contract functions. Begin by copying and pasting the ABI we outlined earlier. It's crucial to ensure that the functions within the contract exactly align with the ABI; otherwise, the compiler will generate an error. Now, substitute the semicolons at the conclusion of each function with curly brackets. Also, modify abi SwayStore to impl SwayStore for Contract, as demonstrated below:
impl SwayStore for Contract {
#[storage(read, write)]
fn list_item(price: u64, metadata: str[20]){
}
#[storage(read, write), payable]
fn buy_item(item_id: u64) {
}
#[storage(read)]
fn get_item(item_id: u64) -> Item {
}
#[storage(read, write)]
fn initialize_owner() -> Identity {
}
#[storage(read)]
fn withdraw_funds(){
}
#[storage(read)]
fn get_count() -> u64{
}
}This guide will first show each of the completed functions above. Then, we'll break it down to explain each part, clarify specific syntax, and discuss fundamental concepts in Sway.
Our first function enables sellers to list an item for sale. They can specify the item's price and provide a string that references externally-stored data about the item.
#[storage(read, write)]
fn list_item(price: u64, metadata: str[20]) {
// increment the item counter
storage.item_counter.write(storage.item_counter.try_read().unwrap() + 1);
// get the message sender
let sender = msg_sender().unwrap();
// configure the item
let new_item: Item = Item {
id: storage.item_counter.try_read().unwrap(),
price: price,
owner: sender,
metadata: metadata,
total_bought: 0,
};
// save the new item to storage using the counter value
storage.item_map.insert(storage.item_counter.try_read().unwrap(), new_item);
} The initial step involves incrementing the item_counter in storage, which will serve as the item's ID. In Sway, all storage variables are contained within the storage keyword, ensuring clarity and preventing conflicts with other variable names. This also allows developers to easily track when and where storage is accessed or altered. The standard library in Sway provides read(), write(), and try_read() methods to access or manipulate contract storage. It's advisable to use try_read() when possible to prevent potential issues arising from accessing uninitialized storage. In this case, we read the current count of listed items, modify it, and then store the updated count back into storage, making use of the well-organized and conflict-free storage system.
When a function returns an Option or Result type, we can use unwrap() to access its inner value. For instance, try_read() returns an Option type. If it yields Some, we get the contained value; but if it returns None, the contract call is immediately halted.
// increment the item counter
storage.item_counter.write(storage.item_counter.try_read().unwrap() + 1); Next, we'll retrieve the Identity of the account listing the item.
To obtain the Identity, utilize the msg_sender function from the standard library. The msg_sender represents the address of the entity (be it a user address or another contract address) initiating the current function call.
This function yields a Result, which is an enum type that can either be OK or an error. Use the Result type when anticipating a value that might result in an error. For example in the case of msg_sender when an external caller is involved and the coin input owners differ, identifying the caller becomes impossible. In such edge cases, an Err(AuthError) is returned.
enum Result<T, E> {
Ok(T),
Err(E),
} In Sway, you can define a variable using either let or const.
// get the message sender
let sender = msg_sender().unwrap(); To retrieve the inner value, you can use the unwrap method. It returns the contained value if the Result is OK and triggers a panic if the result indicates an error.
You can instantiate a new item using the Item struct. Use the item_counter value from storage as the ID, set the price and metadata based on the input parameters, and initialize total_bought to 0.
Since the owner field requires an Identity type, you should utilize the sender value obtained from msg_sender().
// configure the item
let new_item: Item = Item {
id: storage.item_counter.try_read().unwrap(),
price: price,
owner: sender,
metadata: metadata,
total_bought: 0,
}; Lastly, add the item to the item_map within storage using the insert method. Utilize the same ID for the key and designate the item as its corresponding value.
// save the new item to storage using the counter value
storage.item_map.insert(storage.item_counter.try_read().unwrap(), new_item);Next, we aim to allow buyers to purchase listed items. To achieve this, we'll need to:
total_bought count for that item. #[storage(read, write), payable]
fn buy_item(item_id: u64) {
// get the asset id for the asset sent
let asset_id = msg_asset_id();
// require that the correct asset was sent
require(asset_id == BASE_ASSET_ID, InvalidError::IncorrectAssetId(asset_id));
// get the amount of coins sent
let amount = msg_amount();
// get the item to buy
let mut item = storage.item_map.get(item_id).try_read().unwrap();
// require that the amount is at least the price of the item
require(amount >= item.price, InvalidError::NotEnoughTokens(amount));
// update the total amount bought
item.total_bought += 1;
// update the item in the storage map
storage.item_map.insert(item_id, item);
// only charge commission if price is more than 0.1 ETH
if amount > 100_000_000 {
// keep a 5% commission
let commission = amount / 20;
let new_amount = amount - commission;
// send the payout minus commission to the seller
transfer(item.owner, asset_id, new_amount);
} else {
// send the full payout to the seller
transfer(item.owner, asset_id, amount);
}
} We can use the msg_asset_id function from the standard library to obtain the asset ID of the coins being transferred in the transaction.
let asset_id = msg_asset_id(); Next, we'll utilize the require statement to ensure the sent asset is the correct one.
The require statement accepts two arguments: a condition, and a value that's logged when the condition is false. Should the condition evaluate as false, the entire transaction is rolled back, leaving no changes.
In this case, the condition checks if the asset_id matches the BASE_ASSET_ID—the default asset associated with the base blockchain, imported from the standard library. For example, if the base blockchain is Ethereum, the base asset would be ETH.
If there's a mismatch in the asset, for instance, if someone attempts to purchase an item using a different coin, we'll trigger the custom error previously defined, passing along the asset_id.
require(asset_id == BASE_ASSET_ID, InvalidError::IncorrectAssetId(asset_id)); Next, we can use the msg_amount function from the standard library to retrieve the quantity of coins transmitted by the buyer within the transaction.
let amount = msg_amount(); To ensure the sent amount is not less than the item's price, we should retrieve the item details using the item_id parameter.
To obtain a value for a specific key in a storage map, the get method is handy, wherein the key value is passed. For mapping storage access, the try_read() method is utilized. As this method produces a Result type, the unwrap method can be applied to extract the item value.
let mut item = storage.item_map.get(item_id).try_read().unwrap(); In Sway, all variables are immutable by default, whether declared with let or const. To modify the value of any variable, it must be declared mutable using the mut keyword. Since we plan to update the item's total_bought value, it should be defined as mutable.
Additionally, it's essential to ensure that the quantity of coins sent for the item isn't less than the item's price.
require(amount >= item.price, InvalidError::NotEnoughTokens(amount)); We can increase the item's total_bought field value and subsequently reinsert it into the item_map. This action will replace the earlier value with the revised item.
// update the total amount bought
item.total_bought += 1;
// update the item in the storage map
storage.item_map.insert(item_id, item);Lastly, we can process the payment to the seller. It's recommended to transfer assets only after all storage modifications are completed to prevent reentrancy attacks .
For items reaching a specific price threshold, a fee can be deducted using a conditional if statement. The structure of if statements in Sway mirrors that in JavaScript except for the brackets ().
// only charge commission if price is more than 0.1 ETH
if amount > 100_000_000 {
// keep a 5% commission
let commission = amount / 20;
let new_amount = amount - commission;
// send the payout minus commission to the seller
transfer(item.owner, asset_id, new_amount);
} else {
// send the full payout to the seller
transfer(item.owner, asset_id, amount);
} In the aforementioned if-condition, we assess if the transmitted amount surpasses 100,000,000. For clarity in large numbers like 100000000, we can represent it as 100_000_000. If the foundational asset for this contract is ETH, this equates to 0.1 ETH given that Fuel uses a 9 decimal system.
Should the amount exceed 0.1 ETH, a commission is determined and then deducted from the total.
To facilitate the payment to the item's owner, the transfer function is utilized. This function, sourced from the standard library, requires three parameters: the Identity to which the coins are sent, the coin's asset ID, and the coin quantity for transfer.
To get the details for an item, we can create a read-only function that returns the Item struct for a given item ID.
#[storage(read)]
fn get_item(item_id: u64) -> Item {
// returns the item for the given item_id
return storage.item_map.get(item_id).try_read().unwrap();
} To return a value in a function, you can use the return keyword, similar to JavaScript. Alternatively, you can omit the semicolon in the last line to return that value like in Rust. Although both methods are effective.
fn my_function(num: u64) -> u64{
// returning the num variable
return num;
// this would also work:
num
} This method sets the owner's Identity for the contract but only once.
#[storage(read, write)]
fn initialize_owner() -> Identity {
let owner = storage.owner.try_read().unwrap();
// make sure the owner has NOT already been initialized
require(owner.is_none(), __to_str_array("owner already initialized"));
// get the identity of the sender
let sender = msg_sender().unwrap();
// set the owner to the sender's identity
storage.owner.write(Option::Some(sender));
// return the owner
return sender;
} To ensure that this function can only be called once, specifically right after the contract's deployment, it's imperative that the owner's value remains set to None. We can achieve this verification using the is_none method, which assesses if an Option type is None.
It's also important to note the potential risk of front running in this context this code has not been audited.
let owner = storage.owner.try_read().unwrap();
// make sure the owner has NOT already been initialized
require(owner.is_none(), __to_str_array("owner already initialized")); To assign the owner as the message sender, it's necessary to transform the Result type into an Option type.
// get the identity of the sender
let sender = msg_sender().unwrap();
// set the owner to the sender's identity
storage.owner.write(Option::Some(sender)); Finally, we'll return the Identity of the message sender.
// return the owner
return sender; The withdraw_funds function permits the owner to withdraw any accumulated funds from the contract.
#[storage(read)]
fn withdraw_funds() {
let owner = storage.owner.try_read().unwrap();
// make sure the owner has been initialized
require(owner.is_some(), __to_str_array("owner not initialized"));
let sender = msg_sender().unwrap();
// require the sender to be the owner
require(sender == owner.unwrap(), InvalidError::OnlyOwner(sender));
// get the current balance of this contract for the base asset
let amount = this_balance(BASE_ASSET_ID);
// require the contract balance to be more than 0
require(amount > 0, InvalidError::NotEnoughTokens(amount));
// send the amount to the owner
transfer(owner.unwrap(), BASE_ASSET_ID, amount);
}First, we'll ensure that the owner has been initialized to a specific address.
let owner = storage.owner.try_read().unwrap();
// make sure the owner has been initialized
require(owner.is_some(), __to_str_array("owner not initialized"));Next, we'll verify that the individual attempting to withdraw the funds is indeed the owner.
let sender = msg_sender().unwrap();
// require the sender to be the owner
require(sender == owner.unwrap(), InvalidError::OnlyOwner(sender)); Additionally, we can confirm the availability of funds for withdrawal using the this_balance function from the standard library. This function returns the current balance of the contract.
// get the current balance of this contract for the base asset
let amount = this_balance(BASE_ASSET_ID);
// require the contract balance to be more than 0
require(amount > 0, InvalidError::NotEnoughTokens(amount));Lastly, we'll transfer the entire balance of the contract to the owner.
// send the amount to the owner
transfer(owner.unwrap(), BASE_ASSET_ID, amount); The final function we'll introduce is get_count. This straightforward getter function returns the value of the item_counter variable stored in the contract's storage.
#[storage(read)]
fn get_count() -> u64 {
return storage.item_counter.try_read().unwrap();
} The SwayStore contract implementation in your main.sw should now look like this, following everything else we have previously written:
contract;
use std::{
auth::msg_sender,
call_frames::msg_asset_id,
constants::BASE_ASSET_ID,
context::{
msg_amount,
this_balance,
},
token::transfer,
hash::Hash
};
struct Item {
id: u64,
price: u64,
owner: Identity,
metadata: str[20],
total_bought: u64,
}
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: str[20]);
// 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;
}
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,
}
enum InvalidError {
IncorrectAssetId: AssetId,
NotEnoughTokens: u64,
OnlyOwner: Identity,
}
impl SwayStore for Contract {
#[storage(read, write)]
fn list_item(price: u64, metadata: str[20]) {
// increment the item counter
storage.item_counter.write(storage.item_counter.try_read().unwrap() + 1);
// get the message sender
let sender = msg_sender().unwrap();
// configure the item
let new_item: Item = Item {
id: storage.item_counter.try_read().unwrap(),
price: price,
owner: sender,
metadata: metadata,
total_bought: 0,
};
// save the new item to storage using the counter value
storage.item_map.insert(storage.item_counter.try_read().unwrap(), new_item);
}
#[storage(read, write), payable]
fn buy_item(item_id: u64) {
// get the asset id for the asset sent
let asset_id = msg_asset_id();
// require that the correct asset was sent
require(asset_id == BASE_ASSET_ID, InvalidError::IncorrectAssetId(asset_id));
// get the amount of coins sent
let amount = msg_amount();
// get the item to buy
let mut item = storage.item_map.get(item_id).try_read().unwrap();
// require that the amount is at least the price of the item
require(amount >= item.price, InvalidError::NotEnoughTokens(amount));
// update the total amount bought
item.total_bought += 1;
// update the item in the storage map
storage.item_map.insert(item_id, item);
// only charge commission if price is more than 0.1 ETH
if amount > 100_000_000 {
// keep a 5% commission
let commission = amount / 20;
let new_amount = amount - commission;
// send the payout minus commission to the seller
transfer(item.owner, asset_id, new_amount);
} else {
// send the full payout to the seller
transfer(item.owner, asset_id, amount);
}
}
#[storage(read)]
fn get_item(item_id: u64) -> Item {
// returns the item for the given item_id
return storage.item_map.get(item_id).try_read().unwrap();
}
#[storage(read, write)]
fn initialize_owner() -> Identity {
let owner = storage.owner.try_read().unwrap();
// make sure the owner has NOT already been initialized
require(owner.is_none(), __to_str_array("owner already initialized"));
// get the identity of the sender
let sender = msg_sender().unwrap();
// set the owner to the sender's identity
storage.owner.write(Option::Some(sender));
// return the owner
return sender;
}
#[storage(read)]
fn withdraw_funds() {
let owner = storage.owner.try_read().unwrap();
// make sure the owner has been initialized
require(owner.is_some(), __to_str_array("owner not initialized"));
let sender = msg_sender().unwrap();
// require the sender to be the owner
require(sender == owner.unwrap(), InvalidError::OnlyOwner(sender));
// get the current balance of this contract for the base asset
let amount = this_balance(BASE_ASSET_ID);
// require the contract balance to be more than 0
require(amount > 0, InvalidError::NotEnoughTokens(amount));
// send the amount to the owner
transfer(owner.unwrap(), BASE_ASSET_ID, amount);
}
#[storage(read)]
fn get_count() -> u64 {
return storage.item_counter.try_read().unwrap();
}
}