# `midi-in` handler

## Purpose

The `midiIn` handler is the binding-registry slot that hosts an inbound
MIDI subscription — a control bound this way reflects external CC traffic
rather than driving it. The default export is the "unwired" implementation
that throws `REACT_CONTROL_SURFACE.BINDING_HANDLER_MISSING`; consumers
replace it with a closure that captures a subscription created by
`useMidiInput(binding.portId)` and pumps the inbound value back into the
control via the runtime's `setValue` channel.

Throwing at the default site is the right call even for a "passive" inbound
binding: silently dropping inbound updates would leave the control's
visual state out of sync with the device.

## Features

- Strongly-typed signature `(binding: MidiInBinding, value: number) => never`.
- Error context preserves `portId`, `channel`, `cc`, and the most recent
  inbound value.

## Usage: Basic

```ts
import { BindingRegistry } from "@suaveplan/react-control-surface/runtime/binding-registry";

const registry = new BindingRegistry({
    midiIn: (binding, value) => surfaceValueStore.set(binding, value),
});
```

## Usage: Advanced

```ts
import { midiInHandler } from "@suaveplan/react-control-surface/runtime/binding-registry";

try {
    midiInHandler({ kind: "midiIn", portId: "p1", channel: 1, cc: 7 }, 0);
} catch (err) {
    // err.context.kind === "midiIn"
}
```

## API Reference

- `midiInHandler(binding: MidiInBinding, value: number): never`
  — always throws with code `REACT_CONTROL_SURFACE.BINDING_HANDLER_MISSING`.

## Implementation Notes

Because inbound bindings are an "inverse" of outbound ones, the same
handler signature is reused with the understanding that the production
handler treats the `value` argument as the most recent inbound observation
rather than a value to send. Keeping the signatures unified simplifies the
generic dispatcher and lets test spies swap inbound + outbound paths with
identical types.