Model parallel regions

This guide shows you how to model independent playback and volume modes inside one active player state. It assumes you already have a schema-bound machine builder.

Add the region slots to state

Represent each independently active mode as its own tagged union, then store both values in the parent state:

const Playback = Schema.TaggedUnion({ Playing: {}, Paused: { position: Schema.Number } })
const Volume = Schema.TaggedUnion({ Audible: {}, Muted: {} })

const State = Schema.TaggedUnion({
  Idle: {},
  Active: {
    trackId: Schema.String,
    playback: Playback,
    volume: Volume,
  },
  Stopped: {},
})

A transition entering Active supplies the initial configuration explicitly:

Play: {
  target: "Active",
  reduce: ({ event }) => ({
    trackId: event.trackId,
    playback: { _tag: "Playing" },
    volume: { _tag: "Audible" },
  }),
}

Declare the live regions

Use regions on the Active key. Each slot record is exhaustive over that slot's tagged union:

Active: player.regions(
  {
    playback: {
      Playing: {
        Pause: { target: "Paused", reduce: ({ event }) => ({ position: event.position }) },
      },
      Paused: {
        Resume: { target: "Playing", reduce: () => ({}) },
      },
    },
    volume: {
      Audible: {
        Mute: { target: "Muted", reduce: () => ({}) },
      },
      Muted: {
        Unmute: { target: "Audible", reduce: () => ({}) },
      },
    },
  },
  {
    Stop: { target: "Stopped", reduce: () => ({}) },
  },
)

An event is offered to the active child in every slot before the parent handler. If active children in several slots handle the same event, their reducers read the same pre-event parent snapshot and their destination values commit together. An explicit child ignore also suppresses the parent fallback.

Add child work, timers, or completion

Use player.region.invoke(...) for Effect work owned by a region child. Pass { after } as the third argument to give that child an entry-owned timer. Use { final: true } for a final child. When every slot is final, the parent's optional onComplete transition is selected.

Inspect the parallel macrostep

Select Play track-42, then Toggle both regions. Studio marks both active children and highlights the two sibling edges selected from the same pre-event snapshot. The History panel records their atomic commit as one macrostep.

The goal is complete when each region slot is exhaustive, every entry into the parent supplies its active child values, and parent-only events remain in the second regions argument. Studio displays the slots as labeled boundaries and groups sibling transitions into one macrostep.