rn-zustand-patterns

# Zustand Patterns for React Native

## Problem Statement

Zustand's simplicity hides important timing details. `set()` is synchronous, but React re-renders are batched. `getState()` escapes stale closures. Async actions in stores need careful handling. Understanding these internals prevents subtle bugs.

---

## Pattern: set() is Synchronous, Renders are Batched

**Problem:** Assuming state is "ready" for React immediately after `set()`.

```typescript
const useStore = create((set, get) => ({
  count: 0,
  
  increment: () => {
    set({ count: get().count + 1 });
    // State IS updated here (set is sync)
    console.log(get().count); // ✅ Shows new value
    
    // But React hasn't re-rendered yet
    // Component will see old value until next render cycle
  },
}));
```

**Key insight:** 
- `set()` updates the store synchronously
- `getState()` immediately reflects the new value
- React components re-render asynchronously (batched)

**When this matters:**
- Chaining multiple state updates
- Validating state after update
- Debugging "stale" component values

---

## Pattern: getState() Escapes Stale Closures

**Problem:** Callbacks and async functions capture state at creation time. Using `get()` or `getState()` always gets current state.

```typescript
const useStore = create((set, get) => ({
  answers: {},
  
  // WRONG - state captured at function creation
  saveAnswerBad: (questionId: string, value: number) => {
    setTimeout(() => {
      const answers = get().answers; // ❌ This is fine
      // But if someone passed `answers` as a parameter...
    }, 1000);
  },
  
  // CORRECT - always use get() for current state
  saveAnswer: async (questionId: string, value: number) => {
    await someAsyncOperation();
    // After await, use get() to ensure current state
    const currentAnswers = get().answers;
    set({ answers: { ...currentAnswers, [questionId]: value } });
  },
}));

// In components - same principle
function Component() {
  const answers = useStore((s) => s.answers);
  
  const handleSave = async () => {
    await delay(1000);
    // answers here is stale! Captured at render time
    
    // Use getState() for current value
    const current = useStore.getState().answers;
  };
}
```

**Rule:** After any `await`, use `get()` or `getState()` - never rely on closure-captured values.

---

## Pattern: Async Actions in Stores

**Problem:** Async actions need explicit `async/await` and careful state reads after awaits.

```typescript
const useStore = create((set, get) => ({
  loading: false,
  data: null,
  error: null,
  
  // WRONG - no async keyword, race condition prone
  fetchDataBad: (id: string) => {
    set({ loading: true });
    api.fetch(id).then((data) => {
      set({ data, loading: false });
    });
    // Returns immediately, caller can't await
  },
  
  // CORRECT - proper async action
  fetchData: async (id: string) => {
    set({ loading: true, error: null });
    
    try {
      const data = await api.fetch(id);
      // Re-read state after await if needed
      if (get().loading) { // Check we're still in loading state
        set({ data, loading: false });
      }
    } catch (error) {
      set({ error: error.message, loading: false });
    }
  },
}));

// Caller can properly await
await useStore.getState().fetchData('123');
```

---

## Pattern: Selector Stability

**Problem:** Selectors that create new objects cause unnecessary re-renders.

```typescript
// WRONG - creates new object every render
const data = useStore((state) => ({
  name: state.name,
  count: state.count,
}));

// CORRECT - use multiple selectors
const name = useStore((state) => state.name);
const count = useStore((state) => state.count);

// OR - use shallow comparison (Zustand 4.x)
import { shallow } from 'zustand/shallow';

const { name, count } = useStore(
  (state) => ({ name: state.name, count: state.count }),
  shallow
);

// Zustand 5.x - use useShallow hook
import { useShallow } from 'zustand/react/shallow';

const { name, count } = useStore(
  useShallow((state) => ({ name: state.name, count: state.count }))
);
```

---

## Pattern: Derived State

**Problem:** Computing derived values in selectors vs storing them.

```typescript
const useStore = create((set, get) => ({
  answers: {},
  
  // WRONG - storing derived state that can become stale
  totalAnswers: 0,
  updateTotalAnswers: () => {
    set({ totalAnswers: Object.keys(get().answers).length });
  },
  
  // CORRECT - compute in selector (always fresh)
  // answers: {},  // Just store the source data
}));

// Selector computes derived value
const totalAnswers = useStore((state) => Object.keys(state.answers).length);

// For expensive computations, memoize outside the store
import { useMemo } from 'react';

function Component() {
  const answers = useStore((state) => state.answers);
  const expensiveResult = useMemo(() => {
    return computeExpensiveAnalysis(answers);
  }, [answers]);
}
```

---

## Pattern: Store Subscriptions for Side Effects

**Problem:** Need to react to state changes outside React components.

```typescript
// Subscribe to specific state changes
const unsubscribe = useStore.subscribe(
  (state) => state.answers,
  (answers, prevAnswers) => {
    console.log('Answers changed:', { prev: prevAnswers, current: answers });
    // Persist to storage, send analytics, etc.
  },
  { equalityFn: shallow }
);

// In Zustand 4.x with subscribeWithSelector middleware
import { subscribeWithSelector } from 'zustand/middleware';

const useStore = create(
  subscribeWithSelector((set, get) => ({
    answers: {},
    // ...
  }))
);
```

---

## Pattern: Testing Zustand Stores

**Problem:** Tests need to reset store state and verify async flows.

```typescript
// Store with reset capability
const initialState = {
  answers: {},
  loading: false,
};

const useStore = create((set, get) => ({
  ...initialState,
  
  // Actions...
  
  // Reset for testing
  _reset: () => set(initialState),
}));

// Test
describe('Assessment Store', () => {
  beforeEach(() => {
    useStore.getState()._reset();
  });

  it('saves answers during retake flow', async () => {
    const store = useStore.getState();
    
    // Full async flow
    await store.loadCompletedAnswers(assessmentId);
    await store.enableSkillAreaRetake('fundamentals');
    
    // Verify state after async
    expect(store.getState().retakeAreas).toContain('fundamentals');
    
    // Continue flow
    await store.saveAnswer('q1', 4);
    
    // Verify final state
    expect(useStore.getState().userAnswers['q1']).toBe(4);
  });
});
```

---

## Pattern: Debugging State Changes

**Problem:** Tracking down when/where state changed unexpectedly.

```typescript
// Add logging middleware
import { devtools } from 'zustand/middleware';

const useStore = create(
  devtools(
    (set, get) => ({
      // ... your store
    }),
    { name: 'AssessmentStore' }
  )
);

// Manual logging for specific debugging
const useStore = create((set, get) => ({
  answers: {},
  
  saveAnswer: (questionId: string, value: number) => {
    console.log('[saveAnswer] Before:', {
      questionId,
      value,
      currentAnswers: get().answers,
      retakeAreas: get().retakeAreas,
    });
    
    set((state) => ({
      answers: { ...state.answers, [questionId]: value },
    }));
    
    console.log('[saveAnswer] After:', {
      answers: get().answers,
    });
  },
}));
```

---

## Common Pitfalls

| Pitfall | Solution |
|---------|----------|
| Stale closure after await | Use `get()` after every await |
| Selector returns new object | Use `shallow` or multiple selectors |
| Action not awaitable | Add `async` keyword, return promise |
| State seems stale in component | Component hasn't re-rendered yet - use `getState()` for immediate reads |
| Can't find when state changed | Add devtools middleware or manual logging |

---

## Zustand 5.x Migration Notes

If upgrading from 4.x:

```typescript
// 4.x - shallow from main package
import { shallow } from 'zustand/shallow';

// 5.x - useShallow hook for React
import { useShallow } from 'zustand/react/shallow';

// 4.x - type parameter often needed
const useStore = create<StoreType>()((set, get) => ({...}));

// 5.x - improved type inference
const useStore = create((set, get) => ({...}));
```