uniffi-bindgen for React Native

Records

Uniffi records are data objects whose fields are serialized and passed over the FFI i.e. pass by value. They do not have methods.

In UDL, they may be specified with the dictionary keyword:

dictionary MyRecord {
  string mandatory_property;
  string defaulted_property = "Specified by UDL or Rust";
};

Alternatively, they are specified using a Rust proc-macro:

#![allow(unused)]
fn main() {
#[derive(uniffi::Record)]
struct MyRecord {
    mandatory_property: String,
    #[uniffi(default = "Specified by UDL or Rust")]
    defaulted_property: String,
}
}

They are implemented as bare objects in Javascript, with a type declaration in Typescript.

type MyRecord = {
    mandatoryProperty: string,
    defaultedProperty: string,
};

Using this scheme alone however, Typescript cannot represent the default values provided by the UDL, or Rust.

To correct this, uniffi-bindgen-react-native generates a companion factory object.

const MyRecord = {
    create(fields: Missing<MyRecord>) { … },
    defaults(): Partial<MyRecord> { … },
    new: create // a synonym for `create`.
};

The Missing<MyRecord> type above is a little bit hand-wavy, but it’s defined as the union of non-defaulted fields, and the partial of the defaulted fields.

So, continuing with our example, the factory will be minimally happy with:

const myRecord = MyRecord.create({
    mandatoryProperty: "Specified in Typescript"
});

assert(myRecord.mandatoryProperty === "Specified in Typescript");
assert(myRecord.defaultProperty === "Specified by UDL or Rust");