Private Methods

When using callbacks

Usually, when a contract has to have a callback for a remote cross-contract call, this callback method should only be called by the contract itself. It's to avoid someone else calling it and messing the state. Pretty common pattern is to have an assertion that validates that the direct caller (predecessor account ID) matches to the contract's account (current account ID). Macro #[private] simplifies it, by making it a single line macro instead and improves readability.

Use this annotation within the near_bindgen macro as follows:

#[private]
pub fn my_method(&mut self) {
    …
}

Which is equivalent to:

pub fn my_method(&mut self ) {
    if near_sdk::env::current_account_id() != near_sdk::env::predecessor_account_id() {
        near_sdk::env::panic_str("Method method is private");
    }
...
}

Now with this annotation, only the account of the contract itself can call this method, either directly or through a promise.

Writing internal methods

Not all functions need to be exposed publicly. It may be beneficial to write private methods for helper or utility functions, for instance. There are three approaches to write internal methods:

1. Using fn instead of pub fn

   fn helper_method(a: u8, b: u8) {
     …
   }

2. Using pub(crate) fn. This may be helpful when an internal method is in a different module.

   // Function that can be called in another Rust file
   pub(crate) fn get_first_name(account: Account) {
     …
   }

   More information from the official Rust docs regarding public/private methods.

3. Separate impl block

   Another way of not exporting methods is by having a separate impl Contract section, that is not marked with #[near_bindgen].

   #[near_bindgen]
   impl Contract {
       pub fn increment(&mut self) {
           self.internal_increment();
       }
   }
   impl Contract {
       /// This methods is still not exported.
       pub fn internal_increment(&mut self) {
           self.counter += 1;
       }
   }