Struct near_sdk::Promise

pub struct Promise { /* fields omitted */ }

A structure representing a result of the scheduled execution on another contract.

Smart contract developers will explicitly use Promise in two situations:

• When they need to return Promise.

  In the following code if someone calls method ContractA::a they will internally cause an execution of method ContractB::b of bob_near account, and the return value of ContractA::a will be what ContractB::b returned.

#[ext_contract]
pub trait ContractB {
    fn b(&mut self);
}

#[near_bindgen]
#[derive(Default, BorshDeserialize, BorshSerialize)]
struct ContractA {}

#[near_bindgen]
impl ContractA {
    pub fn a(&self) -> Promise {
        contract_b::b(&"bob_near".to_string(), 0, 1_000)
    }
}

• When they need to create a transaction with one or many actions, e.g. the following code schedules a transaction that creates an account, transfers tokens, and assigns a public key:

Promise::new("bob_near".to_string())
  .create_account()
  .transfer(1000)
  .add_full_access_key(env::signer_account_pk());

Implementations

impl Promise

pub fn new(account_id: AccountId) -> Self

Create a promise that acts on the given account.

pub fn create_account(self) -> Self

Create account on which this promise acts.

pub fn deploy_contract(self, code: Vec<u8>) -> Self

Deploy a smart contract to the account on which this promise acts.

pub fn function_call(
    self,
    method_name: Vec<u8>,
    arguments: Vec<u8>,
    amount: Balance,
    gas: Gas
) -> Self

A low-level interface for making a function call to the account that this promise acts on.

pub fn transfer(self, amount: Balance) -> Self

Transfer tokens to the account that this promise acts on.

pub fn stake(self, amount: Balance, public_key: PublicKey) -> Self

Stake the account for the given amount of tokens using the given public key.

pub fn add_full_access_key(self, public_key: PublicKey) -> Self

Add full access key to the given account.

pub fn add_access_key(
    self,
    public_key: PublicKey,
    allowance: Balance,
    receiver_id: AccountId,
    method_names: Vec<u8>
) -> Self

Add an access key that is restricted to only calling a smart contract on some account using only a restricted set of methods. Here method_names is a comma separated list of methods, e.g. b"method_a,method_b".

pub fn delete_key(self, public_key: PublicKey) -> Self

Delete access key from the given account.

pub fn delete_account(self, beneficiary_id: AccountId) -> Self

Delete the given account.

pub fn and(self, other: Promise) -> Promise

Merge this promise with another promise, so that we can schedule execution of another smart contract right after all merged promises finish.

Note, once the promises are merged it is not possible to add actions to them, e.g. the following code will panic during the execution of the smart contract:

let p1 = Promise::new("bob_near".to_string()).create_account();
let p2 = Promise::new("carol_near".to_string()).create_account();
let p3 = p1.and(p2);
// p3.create_account();

pub fn then(self, other: Promise) -> Promise

Schedules execution of another promise right after the current promise finish executing.

In the following code bob_near and dave_near will be created concurrently. carol_near creation will wait for bob_near to be created, and eva_near will wait for both carol_near and dave_near to be created first.

let p1 = Promise::new("bob_near".to_string()).create_account();
let p2 = Promise::new("carol_near".to_string()).create_account();
let p3 = Promise::new("dave_near".to_string()).create_account();
let p4 = Promise::new("eva_near".to_string()).create_account();
p1.then(p2).and(p3).then(p4);

pub fn as_return(self) -> Self

A specialized, relatively low-level API method. Allows to mark the given promise as the one that should be considered as a return value.

In the below code a1 and a2 functions are equivalent.

#[ext_contract]
pub trait ContractB {
    fn b(&mut self);
}

#[near_bindgen]
#[derive(Default, BorshDeserialize, BorshSerialize)]
struct ContractA {}

#[near_bindgen]
impl ContractA {
    pub fn a1(&self) {
       contract_b::b(&"bob_near".to_string(), 0, 1_000).as_return();
    }

    pub fn a2(&self) -> Promise {
       contract_b::b(&"bob_near".to_string(), 0, 1_000)
    }
}

Trait Implementations

impl BorshSchema for Promise

Until we implement strongly typed promises we serialize them as unit struct.

fn add_definitions_recursively(
    definitions: &mut HashMap<Declaration, Definition>
)

Recursively, using DFS, add type definitions required for this type. For primitive types this is an empty map. Type definition explains how to serialize/deserialize a type.

fn declaration() -> Declaration

Get the name of the type without brackets.

fn add_definition(
    declaration: String,
    definition: Definition,
    definitions: &mut HashMap<String, Definition, RandomState>
)

Helper method to add a single type definition to the map.

fn schema_container() -> BorshSchemaContainer

impl Clone for Promise

fn clone(&self) -> Promise

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Drop for Promise

fn drop(&mut self)

Executes the destructor for this type.

impl<T> From<Promise> for PromiseOrValue<T>

fn from(promise: Promise) -> Self

Performs the conversion.

impl Serialize for Promise

fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where
    S: Serializer, 

Serialize this value into the given Serde serializer.