Functions, Constructors, Methods

Functions are exported to the namespace with the #[uniffi::export] attribute

#[uniffi::export]
fn hello_world() -> String {
    "Hello World!".to_owned()
}

All our owned types can be used as arguments and return types.

Arguments and receivers can also be references to these types, for example:

// Input data types as references
#[uniffi::export]
fn process_data(a: &MyRecord, b: &MyEnum, c: &Option<MyRecord>) {
    ...
}

To export methods of an interface you can use the #[uniffi::export] attribute on an impl block.

Default values

Exported functions/methods can have default values using the default argument of the attribute macro that wraps them. default inputs a comma-separated list of either [name] items (where the type's foreign default value should be used) or [name]=[value] items (where the default value is specified as a literal).

#[uniffi::export(default(text = " ", max_splits))]
pub fn split(
    text: String, // will have a default of " "
    sep: String,
    max_splits: Option<u32>, // Will have a default of `None`
) -> Vec<String> {
  ...
}

#[derive(uniffi::Object)]
pub struct TextSplitter { ... }

#[uniffi::export]
impl TextSplitter {
    #[uniffi::constructor(default(ignore_unicode_errors = false))]
    // Because we want the default value for the bool to be `false` it's not necessary to specify it, so
    // we could have written this as:
    // #[uniffi::constructor(default(ignore_unicode_errors))]
    fn new(ignore_unicode_errors: boolean) -> Self {
        ...
    }

    #[uniffi::method(default(text = " ", max_splits))]
    fn split(
        text: String,
        sep: String,
        max_splits: Option<u32>,
    ) -> Vec<String> {
      ...
    }
}

Supported defaults without a literal:

Type	Default value
Option	None
String	Empty string
Vec<T>	Empty list
HashMap<T>	Empty map
Builtin numeric types	0
bool	false
Records	Record with all fields set to their default value (only valid if they all have defaults)
Objects	Primary constructor called with 0 arguments
Custom Types	The default value of "bridge" type

Supported literal values:

• String, integer, float, and boolean literals
• Option<T> allows either None or Some(T)
• Custom type, which its "bridge" type listed above

Renaming functions, methods and constructors

A single exported function can specify an alternate name to be used by the bindings by specifying a name attribute.

#[uniffi::export(name = "something")]
fn do_something() {
}

will be exposed to foreign bindings as a namespace function something()

You can also rename constructors and methods:

#[uniffi::export]
impl Something {
    // Set this as the default constructor by naming it `new`
    #[uniffi::constructor(name = "new")]
    fn make_new() -> Arc<Self> { ... }

    // Expose this as `obj.something()`
    #[uniffi::method(name = "something")]
    fn do_something(&self) { }
}