uniffi-boundary

Use an Object for state and behavior such as engine handles, sessions, or connection pools; it crosses as an opaque reference-counted handle and must be Send + Sync. Use a Record for messages and data like requests or results; it becomes a Kotlin data class or Swift struct copied by value.

Declare the trait with #[uniffi::export(foreign)], require Send + Sync, and give every method a Result return whose error type implements From<uniffi::UnexpectedUniFFICallbackError>. Take it as Arc<dyn Trait>, pass arguments by value, and keep the trait coarse with one on_progress method.

This E0425 error means uniffi::setup_scaffolding!() is missing or placed in a submodule. Call it exactly once at the top of the crate root lib.rs; calling it twice causes duplicate ffi symbol definitions.

Yes, since UniFFI 0.32 a &[u8] argument avoids copying and becomes a direct java.nio.ByteBuffer in Kotlin and Data in Swift. It cannot be returned, used in async exports, or used in foreign-trait methods, and Kotlin callers must pass a direct buffer at position 0.

Kotlin receives an InternalException outside the declared error type, and a Swift non-throwing function ends the app with a fatal error. Every fallible export must return Result with a uniffi::Error type, and non-throwing exports must be panic-free by construction.

Yes, but the future must be Send, and exports using Tokio resources need #[uniffi::export(async_runtime = "tokio")] or the first poll panics with no reactor running. Async primary constructors are rejected by Kotlin bindgen, so use a named async constructor instead.