[O1] Reactive State Container

[cjpillsbury]

Description

Build the foundational reactive state management system based on the spike implementation. This is a lightweight state container with patch() for updating state, subscribe() for reactive listeners, batched updates via microtask flush, and immutable state snapshots. This is the #1 priority item - everything else depends on it.

Acceptance Criteria

• createState<T>(initial: T): WritableState<T> factory function
• WritableState interface with current, patch(), subscribe()
• State updates are immutable (frozen objects)
• Subscribers are notified on state changes
• Updates are batched via microtask (queueMicrotask)
• Multiple patches in same tick only trigger one notification
• flush() function for manual batching control
• Type-safe state access and updates
• Unit tests with ≥80% coverage

Dependencies

Depends on: None - START DAY 1

Blocks: O2, O3, O5, O6, O7, O8, O9, O12, O13, F1-F16 (40+ items)

Technical Notes

Reference: /Users/cpillsbury/dev/experiments/xstate-concepts-experiments/src/vjs/store/state.ts

Key Implementation:

• Use Object.freeze() for immutable snapshots
• Use Object.is() for change detection
• Use queueMicrotask() for batched flush
• Use Set<StateChange> for subscriber management

Files: packages/spf/src/core/state/create-state.ts, types.ts, tests/create-state.test.ts

Bundle Size Impact

Target: <1KB minified+gzipped (Core primitive, must be tiny)

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Parent Epic: #384