# Async Rendering Guide

## Overview

All pages and components are designed to render **asynchronously** - they never block on API requests. Components render immediately with loading states or placeholder data, and data updates asynchronously as it arrives from the backend.

## Key Principles

### 1. **Non-Blocking Rendering**

- Components render immediately, even before API data arrives
- Use `placeholderData` in React Query hooks to provide safe defaults
- Never wait for API responses before rendering

### 2. **Progressive Data Loading**

- Initial render shows loading states or empty states
- Data updates asynchronously as API responses arrive
- Multiple API calls happen in parallel, not sequentially

### 3. **Safe Defaults**

- All hooks return safe default structures (empty arrays, undefined, etc.)
- Components handle `undefined` and `null` gracefully
- No assumptions about data structure

## Implementation Patterns

### React Query Hooks with placeholderData

All `useQuery` hooks include `placeholderData` to prevent blocking:

```typescript
// ✅ Good - Renders immediately with empty array
export function useOrganizations() {
  return useQuery({
    queryKey: organizationKeys.lists(),
    queryFn: getOrganizations,
    placeholderData: [], // Component renders immediately
    staleTime: 30 * 1000,
  });
}

// ✅ Good - Renders immediately with undefined
export function useOrganization(id: string | null | undefined) {
  return useQuery({
    queryKey: organizationKeys.detail(id!),
    queryFn: () => getOrganizationById(id!),
    enabled: !!id,
    placeholderData: undefined, // Component handles undefined
  });
}
```

### Component Pattern

Components should handle placeholder data gracefully:

```typescript
// ✅ Good - Handles placeholder data
const MyComponent = () => {
  const { data, isLoading } = useOrganizations();

  // placeholderData ensures data is always an array
  const organizations = Array.isArray(data) ? data : [];

  // Render immediately, show loading only if no placeholder data
  if (isLoading && organizations.length === 0) {
    return <LoadingSpinner />;
  }

  return <div>{/* Render with data */}</div>;
};
```

### Safe Array Operations

Always check arrays before operations:

```typescript
// ✅ Good - Safe array operations
const items = Array.isArray(data?.items) ? data.items : [];
const filtered = items.filter(item => item?.id);
const mapped = items.map(item => ({ ...item }));

// ❌ Bad - Assumes data is always an array
const items = data.items; // Could be undefined
items.filter(...); // Crashes if undefined
```

## Hooks Updated

All API hooks now include `placeholderData`:

### Organizations

- `useOrganizations()` - `placeholderData: []`
- `useOrganization(id)` - `placeholderData: undefined`
- `useUserOrganizations()` - `placeholderData: []`

### Sites

- `useSite(id)` - `placeholderData: undefined`
- `useSitesByOrganization(id)` - `placeholderData: []`
- `useNearbySites(query)` - `placeholderData: []`

### Resource Flows

- `useResourceFlow(id)` - `placeholderData: undefined`
- `useResourceFlowsBySite(id)` - `placeholderData: []`
- `useResourceFlowsByOrganization(id)` - `placeholderData: []`

### Proposals

- `useProposals()` - `placeholderData: { proposals: [] }`
- `useProposal(id)` - `placeholderData: undefined`
- `useProposalsForOrganization(id)` - `placeholderData: { incoming: [], outgoing: [] }`

### Matching

- `useDirectSymbiosis(id)` - `placeholderData: { providers: [], consumers: [] }`
- `useFindMatches(id)` - `placeholderData: { matches: [] }`

### Analytics

- `useConnectionStatistics()` - `placeholderData: { total_connections: 0 }`
- `useSupplyDemandAnalysis()` - `placeholderData: { supply: [], demand: [] }`
- `useDashboardStatistics()` - `placeholderData: { total_organizations: 0, recent_activity: [] }`

## Benefits

1. **Instant Rendering**: Pages render immediately, no waiting for APIs
2. **Better UX**: Users see loading states or empty states right away
3. **No Freezing**: Components never block on API requests
4. **Progressive Enhancement**: Data appears as it loads
5. **Error Resilience**: Safe defaults prevent crashes

## Testing

To verify async rendering:

1. **Network Throttling**: Use DevTools to throttle network to "Slow 3G"
2. **Check Rendering**: Page should render immediately with loading states
3. **Verify Updates**: Data should appear as API responses arrive
4. **Test Empty States**: Disable network to see empty state handling

## Common Patterns

### Pattern 1: List with Loading State

```typescript
const { data, isLoading } = useOrganizations();
const items = Array.isArray(data) ? data : [];

if (isLoading && items.length === 0) {
  return <LoadingSpinner />;
}

return <List items={items} />;
```

### Pattern 2: Detail with Placeholder

```typescript
const { data, isLoading } = useOrganization(id);

if (isLoading && !data) {
  return <Skeleton />;
}

if (!data) {
  return <NotFound />;
}

return <Detail data={data} />;
```

### Pattern 3: Parallel Loading

```typescript
// Multiple queries load in parallel
const { data: org } = useOrganization(id);
const { data: sites } = useSitesByOrganization(id);
const { data: flows } = useResourceFlowsByOrganization(id);

// All render immediately with placeholderData
// Updates happen asynchronously as data arrives
```

## Anti-Patterns to Avoid

### ❌ Blocking on Data

```typescript
// Bad - Blocks render until data arrives
const { data } = useOrganizations();
if (!data) return null; // Blocks render
```

### ❌ Unsafe Array Operations

```typescript
// Bad - Crashes if data is undefined
const items = data.items;
items.map(...); // Error if items is undefined
```

### ❌ Synchronous Operations in Render

```typescript
// Bad - Blocks render
const processed = heavyComputation(data); // Blocks
```

## Performance Considerations

1. **placeholderData** is lightweight - just empty arrays/objects
2. **React Query** handles caching and deduplication
3. **Components** re-render only when data actually changes
4. **Memoization** prevents unnecessary recalculations

## Migration Checklist

- [x] All `useQuery` hooks have `placeholderData`
- [x] Components handle `undefined` and `null` safely
- [x] Array operations are guarded with `Array.isArray()`
- [x] Loading states show only when no placeholder data exists
- [x] Error states don't block rendering
- [x] Context providers don't block on data

## Future Improvements

1. **Suspense Boundaries**: Consider React Suspense for better loading UX
2. **Optimistic Updates**: Update UI immediately, rollback on error
3. **Streaming**: For large datasets, consider streaming responses
4. **Prefetching**: Prefetch data before navigation