gecko-dev/docs/rust-components/uniffi-bindgen-gecko-js-dev-guide/js-callback-interfaces.md

JavaScript Callback Interfaces

Background

• UniFFI general documentation
• Threading concerns
• Lifting and Lowering

JavaScript layer

The generated JavaScript bindings create a UniFFICallbackHandler for each callback interface. This stores the callback interface implementations that are manually written by Firefox engineers. These are stored in a map, where the key is an integer handle for the callback interface.

UniFFICallbackHandler.callAsync is used by the C++ layer to invoke callback interface methods. See below for why we only currently have callAsync(). UniFFICallbackHandler.callAsync() inputs:

• The object handle
• The method index
• Each argument for the callback method, after being lowered by JavaScript.

UniFFICallbackHandler.callAsync() returns a UniFFIScaffoldingCallResult. Like with Rust calls, this is a UniffiCallStatus combined with a return value.

For each callback interface, the JavaScript layer calls UniFFIScaffolding.registerCallbackHandler() with the UniFFICallbackHandler for that interface. Like with Rust calls, the bindings code generates a unique ID to identify each callback interface.

C++ layer

The C++ layer acts as a bridge between the generated Rust code and the generated JavaScript code. It registers a vtable with the Rust code where each field points to a generated C function that:

• Looks up the UniFFICallbackHandler registered with UniFFIScaffolding.registerCallbackHandler()
• Lifts all passed arguments and passes them to the UniFFICallbackHandler.
• For fire-and-forget calls:
  • Calls UniFFICallbackHandler.callAsync() with the lifted arguments then discards the returned Promise.
  • Note: sync calls are currently always wrapped to be "fire-and-forget" callbacks
• For async calls:
  • Calls UniFFICallbackHandler.callAsync() with the lifted arguments getting back a Promise object.
  • Appends a PromiseNativeHandler to promise object.
  • The PromiseNativeHandler completes the promise by calling the complete callback as described in the UniFFI FFI internals doc.
  • The PromiseNativeHandler also has code to handle a rejected promise by calling the complete callback with RustCallStatusCode::UnexpectedError.

Freeing Callback Interface Objects

Each VTable also has a uniffi_free method. When the Rust code drops the callback interface object, the generated UniFFI code arranges for uniffi_free to be called. When this happens, the C++ generated function calls UniFFICallbackHandler.destroy(). The generated JavaScript handles that by removing the entry from the callback interface map.