Structs ​

Uniffi can expose Rust structs and their associated fields.

Suppose a Rust struct is defined as such:

pub struct CalendarOptions {
    name: String,
    year: u32,
    private: bool,
}

We expose it in the UDL language using the dictionnary keyword like so:

namespace calendar {};

dictionary CalendarOptions {
  string name;
  u32 year;
  boolean private;
};

And the resulting Kotlin class would look like this:

data class CalendarOptions (
    var name: String, 
    var year: UInt, 
    var private: Boolean
)

Fields with object references ​

If a dictionary contains a field whose type is an interface, then that field will hold a reference to an underlying instance of a Rust struct. The Rust code for working with such fields must store them as an Arc in order to help properly manage the lifetime of the instance. So if the UDL interface looked like this:

interface User {
  // Some sort of "user" object that can own todo items
};

dictionary TodoEntry {
    User owner;
    string text;
}

Then the corresponding Rust code would need to look like this:

struct TodoEntry {
    owner: Arc<User>,
    text: String,
}