Callbacks ​

We build callbacks using the following UDL syntax:

callback interface Reminder {
  [Throws=CalendarError]
  string remind_me();

  [Throws=CalendarError]
  void ping_me(Event event);
};

With the corresponding Rust code:

pub trait Reminder: Send + Sync + std::fmt::Debug {
    fn remind_me(&self) -> Result<String, CalendarError>;

    fn ping_me(&self, event: Event) -> Result<(), CalendarError>;
}

Note that for callbacks you will need to implement the From<uniffi::UnexpectedUniFFICallbackError> trait on one of your error types in order to handle unexpected callback errors. See the Callback Interfaces section of the handbook for more info.

impl From<uniffi::UnexpectedUniFFICallbackError> for CalendarError {
    fn from(_: uniffi::UnexpectedUniFFICallbackError) -> Self { Self::EventError }
}

From the bindings we simply override the remind_me and ping_me methods:

class CustomReminder : Reminder {
    override fun remindMe(): String {
        return "Don't forget to call your mom!"
    }

    override fun pingMe(event: Event): Unit {
        println("Don't forget you have a $event coming up!")
    }
}

See the tests for more examples.