Traits

Traits can be exported with the uniffi::export() macro.

#[uniffi::export]
pub trait Person {
    fn name() -> String;
    fn age() -> u32;
}

These traits can then be used in the type model in the same was as objects, but as usual, with dyn - ie, as Arc<dyn Person> - but see below for an important exception.

For traits implemented in Rust, these will appear to the foreign side exactly like an objects. If the trait is exported as shown above, only Rust implemented traits will be able to be used.

Foreign traits.

It's possible to export the trait as a "foreign" trait by using #[uniffi::export(with_foreign)]. This allows for the trait to also be implemented by foreign bindings - meaning the bindings can implement an object and store it in Records or Enums, or pass them into Rust functions etc.

#[uniffi::export(with_foreign)]
pub trait Person {
    fn name() -> String;
    fn age() -> u32;
}

See the foreign trait documentation documentation for more.

Callback Interfaces

Callback interfaces are very similar to, and pre-date Foreign traits. Due to backwards compatibility, they appear in all signatures and Record/Enum definitions as Box<dyn TraitName> instead of Arc<>. This makes them less useful in general - because Foreign traits offer all the same capabilities but don't have this limitation, you should consider callback interfaces soft deprecated and use Foreign Traits instead.

See their documentation for more

However, if you really do want to use them from proc-macros, you would use #[uniffi::export(callback_interface)]

#[uniffi::export(callback_interface)]
pub trait Person {
    fn name() -> String;
    fn age() -> u32;
}