Background State

Background State

Add background state persistence to an Even Hub plugin by analyzing its code and inserting setBackgroundState + onBackgroundRestore calls from @evenrealities/even_hub_sdk.

Background

The Even Hub host app uses a Headless WebView migration strategy when the phone goes to the background:

1. inactive — host snapshots current JS state via window.__getStateSnapshot()
2. paused — host creates a new HeadlessInAppWebView, loads the same plugin URL, and calls window.__restoreState(snapshot) to replay the saved state
3. Background — the headless WebView runs invisibly, continuing to push frames to the glasses
4. resumed — snapshot is injected into the foreground WebView before it wakes up, then the headless WebView is destroyed

If a plugin does not register any state exporters, __getStateSnapshot() returns '{}' and the headless WebView starts from scratch — causing the plugin to reset to its initial state every time the phone comes back from the background.

When to Use

Apply this skill when a plugin:

• Shows counters, scores, timers, or sequence numbers that advance over time
• Tracks a selected item, current page, or active mode
• Maintains a data buffer or history that should not reset
• Has any variable that changes during plugin execution and must be consistent on foreground return

API Reference

import { setBackgroundState, onBackgroundRestore } from '@evenrealities/even_hub_sdk'

// Register state exporter — called by host before going to background
setBackgroundState('myKey', () => ({ ...myState }))

// Register state restorer — called by host after headless WebView loads
onBackgroundRestore('myKey', (saved) => {
  const s = saved as typeof myState
  myState = { ...myState, ...s }
})

Rules:

• key must be the same string in both calls
• The exporter must return a plain JSON-serializable object (no class instances, no functions, no Maps, no Sets)
• The restorer receives Record<string, unknown> — always cast before use
• Both calls must run at module init time (top level or inside the startup function, before bridge.onEvenHubEvent)
• Call setBackgroundState before any code that modifies the state variables, so the initial snapshot is always valid

Workflow

Step 1 — Read the source

If $ARGUMENTS points to a file, read it directly. If it points to a directory, glob for *.ts and *.tsx files and read the most relevant ones (typically main.ts, index.ts, or the file containing waitForEvenAppBridge).

Step 2 — Identify snapshot-worthy state

Look for variables that:

Key heuristic: a variable is snapshot-worthy if losing it on background entry would cause the plugin to behave differently than if it had never gone to the background.

Step 3 — Choose snapshot keys

Use one key per logical state group. Avoid one key per variable (too granular) and one key for everything (too coarse). A single plugin typically needs 1–2 keys.

Examples:

• 'counter' for a frame counter
• 'appState' for the main mutable state object
• 'selection' for the currently selected item index + related data

Step 4 — Generate and insert the code

Find the correct insertion point — after state variable declarations, before bridge.onEvenHubEvent(...). Insert:

// Background state persistence — survives host background/foreground migration
setBackgroundState('myKey', () => ({ /* snapshot of mutable state */ }))
onBackgroundRestore('myKey', (saved) => {
  const s = saved as { /* type annotation */ }
  // restore each field with nullish fallback
})

Also add the import if not already present:

import { setBackgroundState, onBackgroundRestore } from '@evenrealities/even_hub_sdk'

Step 5 — Verify correctness

After inserting, confirm:

• Import line present, no duplicate imports
• Both calls use the same key string
• Exporter spreads a snapshot copy ({ ...state }, not a live reference)
• Restorer reassigns the live variable (not just reads from saved)
• No class instances, Dates, Maps, or Sets in the exported object
• Every field in the restorer uses ?? fallback to its current value

Complete Example

Before (loses all state on background transition):

import { waitForEvenAppBridge } from '@evenrealities/even_hub_sdk'

let counter = 0
let lastEvent = ''

const bridge = await waitForEvenAppBridge()
await bridge.createStartUpPageContainer(container)

setInterval(() => {
  counter++
  bridge.rebuildPageContainer(buildPage(counter))
}, 500)

bridge.onEvenHubEvent(event => {
  if (event.sysEvent) {
    lastEvent = `sys:${event.sysEvent.eventType ?? 0}`
  }
})

After (state survives background):

import { waitForEvenAppBridge, setBackgroundState, onBackgroundRestore } from '@evenrealities/even_hub_sdk'

let counter = 0
let lastEvent = ''

// Background state persistence — survives host background/foreground migration
setBackgroundState('appState', () => ({ counter, lastEvent }))
onBackgroundRestore('appState', (saved) => {
  const s = saved as { counter: number; lastEvent: string }
  counter = s.counter ?? counter
  lastEvent = s.lastEvent ?? lastEvent
})

const bridge = await waitForEvenAppBridge()
await bridge.createStartUpPageContainer(container)

setInterval(() => {
  counter++
  bridge.rebuildPageContainer(buildPage(counter))
}, 500)

bridge.onEvenHubEvent(event => {
  if (event.sysEvent) {
    lastEvent = `sys:${event.sysEvent.eventType ?? 0}`
  }
})

Common Mistakes

Capturing a reference instead of a snapshot

// ❌ Wrong — exporter returns the live object reference; value changes by the time host reads it
setBackgroundState('state', () => myState)

// ✅ Correct — spread creates a snapshot copy at the moment of export
setBackgroundState('state', () => ({ ...myState }))

Restorer reads but does not reassign

// ❌ Wrong — local const, live variable unchanged
onBackgroundRestore('state', (saved) => {
  const s = saved as typeof myState
  console.log(s.counter) // myState.counter still 0
})

// ✅ Correct — reassign the live variable
onBackgroundRestore('state', (saved) => {
  const s = saved as typeof myState
  myState = { ...myState, ...s }
})

Non-serializable values in snapshot

// ❌ Wrong — Map and Date do not survive JSON round-trip
setBackgroundState('state', () => ({
  items: new Map([['a', 1]]),
  timestamp: new Date(),
}))

// ✅ Correct — convert to plain types
setBackgroundState('state', () => ({
  items: Object.fromEntries(itemsMap),
  timestamp: Date.now(),
}))

Missing nullish guard in restorer

// ❌ Wrong — crashes or resets to 0 if field missing in snapshot
onBackgroundRestore('state', (saved) => {
  const s = saved as { counter: number }
  counter = s.counter
})

// ✅ Correct — keep current value as fallback when field absent
onBackgroundRestore('state', (saved) => {
  const s = saved as { counter: number }
  counter = s.counter ?? counter
})

Registering inside a conditional or event handler

// ❌ Wrong — if this code path isn't hit before background, state is not saved
bridge.onEvenHubEvent(event => {
  if (event.sysEvent?.eventType === 4) {
    setBackgroundState('state', () => ({ counter })) // too late
  }
})

// ✅ Correct — always register at module init time
setBackgroundState('state', () => ({ counter }))
bridge.onEvenHubEvent(event => { /* ... */ })

Task

Analyze the plugin code at $ARGUMENTS. Identify all snapshot-worthy state variables following the table above. Then insert setBackgroundState + onBackgroundRestore at the correct location. Report a summary of:

1. State variables identified and why they are snapshot-worthy
2. What was skipped and why
3. The key(s) chosen and the exact code inserted