Selectors

Selectors in ink! are a language agnostic way of identifying constructors and messages. They are four-byte hexadecimal strings which look something like: 0x633aa551.

You can find the selector of an ink! constructor or message in your contract metadata by looking for the selector field of the dispatchable you're interested in.

Here is an example of how you can grab the message name and selector from your contract metadata using jq.

cat target/ink/flipper.json | jq '.spec.messages[0] | "\(.label): \(.selector)"'
"flip: 0x633aa551"

Selector Calculation

If you do not have access to a contract's metadata, you can also calculate it yourself.

The algorithm ink! uses is fairly straightforward:

1. Get just the name of the constructor or message
2. Compute the BLAKE2 hash of the name
3. Take the first four bytes of the hash as the selector

Let's walk through a short example of what this looks like in practice. Consider the following message:

#[ink(message)]
fn frobinate(&mut self, fro: bool, bi: u32, nate: AccountId) -> bool {
    unimplemented!()
}

To calculate the selector we:

1. Grab the name of the message, frobinate
2. Compute BLAKE2("frobinate") = 0x8e39d7f22ef4f9f1404fe5200768179a8b4f2b67799082d7b39f6a8ca82da8f1
3. Grab the first four bytes, 0x8e39d7f2

Selector Calculation: ink! Traits

These rules change a bit if you define any messages using the [ink::trait_definition] macro. For our first step, instead of taking just the message name, we now also add the trait name to the selector calculation.

cat target/ink/trait-flipper.json | jq '.spec.messages[0] | "\(.label): \(.selector)"'
"Flip::flip: 0xaa97cade"

Let's see what this would look like in practice. Consider the following trait:

#[ink::trait_definition]
pub trait Frobinate {
    fn frobinate(&mut self, fro: bool, bi: u32, nate: AccountId) -> bool;
}

-- snip --

impl Frobinate for Contract {
    #[ink(message)]
    fn frobinate(&mut self, fro: bool, bi: u32, nate: AccountId) -> bool {
        unimplemented!()
    }
}

To calculate the selector we:

1. Grab the name of the trait and the name of the message, Frobinate::frobinate
2. Compute BLAKE2("Frobinate::frobinate") = 0x8915412ad772b2a116917cf75df4ba461b5808556a73f729bce582fb79200c5b
3. Grab the first four bytes, 0x8915412a

tip

Don't worry if you're not able to calculate the BLAKE2 hash of a string by hand. You can use Shawn's Substrate Utilities to do it for you!