# Frontend Design Patterns

## Overview

This document outlines the design patterns to be used in the Heritage Custodian Frontend application. These patterns ensure consistency, maintainability, and scalability across the codebase.

## Core Design Patterns

### 1. Component Composition Pattern

**Problem**: Need to build complex UI from simple, reusable components.

**Solution**: Use composition over inheritance with React components.

```typescript
// Base component
interface CardProps {
  children: React.ReactNode;
  className?: string;
}

export const Card: React.FC<CardProps> = ({ children, className = '' }) => {
  return (
    <div className={`rounded-lg shadow-md p-4 ${className}`}>
      {children}
    </div>
  );
};

// Composed components
export const CardHeader: React.FC<{ children: React.ReactNode }> = ({ children }) => {
  return <div className="border-b pb-2 mb-3">{children}</div>;
};

export const CardBody: React.FC<{ children: React.ReactNode }> = ({ children }) => {
  return <div className="text-gray-700">{children}</div>;
};

// Usage
<Card>
  <CardHeader>
    <h3>Rijksmuseum</h3>
  </CardHeader>
  <CardBody>
    <p>National museum of the Netherlands</p>
  </CardBody>
</Card>
```

### 2. Container/Presenter Pattern

**Problem**: Mixing business logic with presentation logic makes components hard to test and reuse.

**Solution**: Separate containers (smart components) from presenters (dumb components).

```typescript
// Presenter component (dumb)
interface CustodianListViewProps {
  custodians: CustodianObservation[];
  isLoading: boolean;
  error?: Error;
  onSelect: (custodian: CustodianObservation) => void;
}

export const CustodianListView: React.FC<CustodianListViewProps> = ({
  custodians,
  isLoading,
  error,
  onSelect,
}) => {
  if (isLoading) return <LoadingSpinner />;
  if (error) return <ErrorMessage error={error} />;
  
  return (
    <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-4">
      {custodians.map((custodian) => (
        <CustodianCard
          key={custodian.id}
          custodian={custodian}
          onClick={() => onSelect(custodian)}
        />
      ))}
    </div>
  );
};

// Container component (smart)
export const CustodianListContainer: React.FC = () => {
  const { data, isLoading, error } = useQuery('custodians', fetchCustodians);
  const navigate = useNavigate();
  
  const handleSelect = (custodian: CustodianObservation) => {
    navigate(`/custodian/${custodian.id}`);
  };
  
  return (
    <CustodianListView
      custodians={data || []}
      isLoading={isLoading}
      error={error}
      onSelect={handleSelect}
    />
  );
};
```

### 3. Custom Hook Pattern

**Problem**: Reusing stateful logic across components.

**Solution**: Extract logic into custom hooks.

```typescript
// Custom hook for RDF operations
export function useRDF(initialFormat: RDFFormat = 'turtle') {
  const [store, setStore] = useState(() => new N3.Store());
  const [format, setFormat] = useState(initialFormat);
  const [isLoading, setIsLoading] = useState(false);
  const [error, setError] = useState<Error | null>(null);
  
  const loadRDF = useCallback(async (content: string) => {
    setIsLoading(true);
    setError(null);
    
    try {
      const parser = new N3.Parser({ format });
      const newStore = new N3.Store();
      
      parser.parse(content, (error, quad) => {
        if (error) throw error;
        if (quad) newStore.addQuad(quad);
      });
      
      setStore(newStore);
    } catch (err) {
      setError(err as Error);
    } finally {
      setIsLoading(false);
    }
  }, [format]);
  
  const query = useCallback((sparql: string) => {
    // SPARQL query execution
    const engine = new SPARQLEngine(store);
    return engine.execute(sparql);
  }, [store]);
  
  return {
    store,
    format,
    setFormat,
    loadRDF,
    query,
    isLoading,
    error,
  };
}

// Usage in component
function RDFExplorer() {
  const { store, loadRDF, query, isLoading } = useRDF();
  
  // Component logic using the hook
}
```

### 4. Render Props Pattern

**Problem**: Sharing component logic while maintaining flexibility in rendering.

**Solution**: Use render props for flexible component composition.

```typescript
interface DataFetcherProps<T> {
  url: string;
  children: (data: T | null, loading: boolean, error: Error | null) => React.ReactNode;
}

function DataFetcher<T>({ url, children }: DataFetcherProps<T>) {
  const [data, setData] = useState<T | null>(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState<Error | null>(null);
  
  useEffect(() => {
    fetch(url)
      .then(res => res.json())
      .then(setData)
      .catch(setError)
      .finally(() => setLoading(false));
  }, [url]);
  
  return <>{children(data, loading, error)}</>;
}

// Usage
<DataFetcher<CustodianObservation[]> url="/api/custodians">
  {(custodians, loading, error) => {
    if (loading) return <Spinner />;
    if (error) return <Error message={error.message} />;
    return <CustodianList custodians={custodians!} />;
  }}
</DataFetcher>
```

### 5. Higher-Order Component (HOC) Pattern

**Problem**: Adding common functionality to multiple components.

**Solution**: Wrap components with HOCs to inject functionality.

```typescript
// HOC for authentication
function withAuth<P extends object>(
  Component: React.ComponentType<P>
): React.FC<P> {
  return (props: P) => {
    const { user, isLoading } = useAuth();
    const navigate = useNavigate();
    
    useEffect(() => {
      if (!isLoading && !user) {
        navigate('/login');
      }
    }, [user, isLoading, navigate]);
    
    if (isLoading) return <LoadingSpinner />;
    if (!user) return null;
    
    return <Component {...props} />;
  };
}

// Usage
const ProtectedDashboard = withAuth(Dashboard);

// HOC for error boundary
function withErrorBoundary<P extends object>(
  Component: React.ComponentType<P>,
  fallback?: React.ComponentType<{ error: Error }>
): React.FC<P> {
  return class WithErrorBoundary extends React.Component<P, { hasError: boolean; error?: Error }> {
    constructor(props: P) {
      super(props);
      this.state = { hasError: false };
    }
    
    static getDerivedStateFromError(error: Error) {
      return { hasError: true, error };
    }
    
    render() {
      if (this.state.hasError) {
        const Fallback = fallback || DefaultErrorFallback;
        return <Fallback error={this.state.error!} />;
      }
      
      return <Component {...this.props} />;
    }
  };
}
```

### 6. Factory Pattern for Visualizations

**Problem**: Creating different types of visualizations based on data type.

**Solution**: Use factory pattern to instantiate appropriate visualization.

```typescript
// Visualization factory
type VisualizationType = 'network' | 'timeline' | 'map' | 'tree' | 'chart';

interface Visualization {
  render(container: HTMLElement, data: any): void;
  update(data: any): void;
  destroy(): void;
}

class NetworkVisualization implements Visualization {
  private simulation: d3.Simulation<any, any> | null = null;
  
  render(container: HTMLElement, data: { nodes: Node[], edges: Edge[] }): void {
    const svg = d3.select(container).append('svg');
    this.simulation = d3.forceSimulation(data.nodes)
      .force('link', d3.forceLink(data.edges))
      .force('charge', d3.forceManyBody())
      .force('center', d3.forceCenter());
    
    // Rendering logic
  }
  
  update(data: any): void {
    if (this.simulation) {
      this.simulation.nodes(data.nodes);
      this.simulation.alpha(1).restart();
    }
  }
  
  destroy(): void {
    if (this.simulation) {
      this.simulation.stop();
    }
  }
}

class TimelineVisualization implements Visualization {
  render(container: HTMLElement, data: Event[]): void {
    // Timeline rendering logic
  }
  
  update(data: Event[]): void {
    // Update timeline
  }
  
  destroy(): void {
    // Cleanup
  }
}

class VisualizationFactory {
  static create(type: VisualizationType): Visualization {
    switch (type) {
      case 'network':
        return new NetworkVisualization();
      case 'timeline':
        return new TimelineVisualization();
      case 'map':
        return new MapVisualization();
      case 'tree':
        return new TreeVisualization();
      case 'chart':
        return new ChartVisualization();
      default:
        throw new Error(`Unknown visualization type: ${type}`);
    }
  }
}

// Usage
const viz = VisualizationFactory.create('network');
viz.render(containerRef.current, graphData);
```

### 7. Observer Pattern (Pub/Sub)

**Problem**: Decoupling components that need to react to events.

**Solution**: Implement publish-subscribe pattern.

```typescript
type EventCallback = (data: any) => void;

class EventEmitter {
  private events: Map<string, Set<EventCallback>> = new Map();
  
  on(event: string, callback: EventCallback): () => void {
    if (!this.events.has(event)) {
      this.events.set(event, new Set());
    }
    this.events.get(event)!.add(callback);
    
    // Return unsubscribe function
    return () => this.off(event, callback);
  }
  
  off(event: string, callback: EventCallback): void {
    const callbacks = this.events.get(event);
    if (callbacks) {
      callbacks.delete(callback);
    }
  }
  
  emit(event: string, data?: any): void {
    const callbacks = this.events.get(event);
    if (callbacks) {
      callbacks.forEach(callback => callback(data));
    }
  }
  
  clear(): void {
    this.events.clear();
  }
}

// Singleton instance
export const eventBus = new EventEmitter();

// Usage in component
function CustodianMap() {
  useEffect(() => {
    const unsubscribe = eventBus.on('custodian:selected', (custodian) => {
      // Highlight custodian on map
      highlightMarker(custodian.id);
    });
    
    return unsubscribe; // Cleanup on unmount
  }, []);
  
  return <MapContainer />;
}

// Emit event from another component
function CustodianList() {
  const handleClick = (custodian: CustodianObservation) => {
    eventBus.emit('custodian:selected', custodian);
  };
  
  return (
    <div>
      {custodians.map(c => (
        <button onClick={() => handleClick(c)}>{c.name}</button>
      ))}
    </div>
  );
}
```

### 8. Builder Pattern for Complex Queries

**Problem**: Constructing complex SPARQL queries with many optional parts.

**Solution**: Use builder pattern for fluent query construction.

```typescript
class SPARQLQueryBuilder {
  private prefixes: Map<string, string> = new Map();
  private selectVars: string[] = [];
  private wherePatterns: string[] = [];
  private filterExpressions: string[] = [];
  private orderByClauses: string[] = [];
  private limitValue?: number;
  private offsetValue?: number;
  
  prefix(prefix: string, uri: string): this {
    this.prefixes.set(prefix, uri);
    return this;
  }
  
  select(...vars: string[]): this {
    this.selectVars.push(...vars);
    return this;
  }
  
  where(pattern: string): this {
    this.wherePatterns.push(pattern);
    return this;
  }
  
  filter(expression: string): this {
    this.filterExpressions.push(expression);
    return this;
  }
  
  orderBy(clause: string): this {
    this.orderByClauses.push(clause);
    return this;
  }
  
  limit(value: number): this {
    this.limitValue = value;
    return this;
  }
  
  offset(value: number): this {
    this.offsetValue = value;
    return this;
  }
  
  build(): string {
    let query = '';
    
    // Add prefixes
    for (const [prefix, uri] of this.prefixes) {
      query += `PREFIX ${prefix}: <${uri}>\n`;
    }
    
    // Add SELECT
    query += `\nSELECT ${this.selectVars.join(' ')}\n`;
    
    // Add WHERE
    query += 'WHERE {\n';
    query += this.wherePatterns.map(p => `  ${p}`).join('\n');
    
    if (this.filterExpressions.length > 0) {
      query += '\n' + this.filterExpressions.map(f => `  FILTER(${f})`).join('\n');
    }
    
    query += '\n}\n';
    
    // Add ORDER BY
    if (this.orderByClauses.length > 0) {
      query += `ORDER BY ${this.orderByClauses.join(' ')}\n`;
    }
    
    // Add LIMIT and OFFSET
    if (this.limitValue !== undefined) {
      query += `LIMIT ${this.limitValue}\n`;
    }
    if (this.offsetValue !== undefined) {
      query += `OFFSET ${this.offsetValue}\n`;
    }
    
    return query;
  }
}

// Usage
const query = new SPARQLQueryBuilder()
  .prefix('rdf', 'http://www.w3.org/1999/02/22-rdf-syntax-ns#')
  .prefix('hc', 'https://w3id.org/heritage/custodian/')
  .select('?custodian', '?name', '?type')
  .where('?custodian a hc:CustodianObservation .')
  .where('?custodian hc:name ?name .')
  .where('?custodian hc:institution_type ?type .')
  .filter('LANG(?name) = "en"')
  .orderBy('?name')
  .limit(50)
  .build();
```

### 9. Strategy Pattern for Export Formats

**Problem**: Supporting multiple export formats without complex conditionals.

**Solution**: Define strategy interface with concrete implementations for each format.

```typescript
interface ExportStrategy {
  export(data: CustodianObservation[]): string | Blob;
  getFileExtension(): string;
  getMimeType(): string;
}

class JSONExportStrategy implements ExportStrategy {
  export(data: CustodianObservation[]): string {
    return JSON.stringify(data, null, 2);
  }
  
  getFileExtension(): string {
    return 'json';
  }
  
  getMimeType(): string {
    return 'application/json';
  }
}

class CSVExportStrategy implements ExportStrategy {
  export(data: CustodianObservation[]): string {
    const header = ['ID', 'Name', 'Type', 'City', 'Country'];
    const rows = data.map(c => [
      c.id,
      c.name,
      c.institution_type,
      c.locations?.[0]?.city || '',
      c.locations?.[0]?.country || '',
    ]);
    
    return [header, ...rows]
      .map(row => row.map(cell => `"${cell}"`).join(','))
      .join('\n');
  }
  
  getFileExtension(): string {
    return 'csv';
  }
  
  getMimeType(): string {
    return 'text/csv';
  }
}

class RDFTurtleExportStrategy implements ExportStrategy {
  export(data: CustodianObservation[]): string {
    const transformer = new CustodianTransformer();
    const triples = data.flatMap(c => transformer.toRDF(c));
    return serializeTriples(triples, 'turtle');
  }
  
  getFileExtension(): string {
    return 'ttl';
  }
  
  getMimeType(): string {
    return 'text/turtle';
  }
}

class ExportService {
  private strategies: Map<string, ExportStrategy> = new Map([
    ['json', new JSONExportStrategy()],
    ['csv', new CSVExportStrategy()],
    ['turtle', new RDFTurtleExportStrategy()],
  ]);
  
  export(data: CustodianObservation[], format: string): void {
    const strategy = this.strategies.get(format);
    if (!strategy) {
      throw new Error(`Unsupported export format: ${format}`);
    }
    
    const content = strategy.export(data);
    const blob = new Blob([content], { type: strategy.getMimeType() });
    const url = URL.createObjectURL(blob);
    
    const a = document.createElement('a');
    a.href = url;
    a.download = `custodians.${strategy.getFileExtension()}`;
    a.click();
    
    URL.revokeObjectURL(url);
  }
}

// Usage
const exportService = new ExportService();
exportService.export(selectedCustodians, 'csv');
```

### 10. Adapter Pattern for RDF Formats

**Problem**: Working with different RDF serialization formats uniformly.

**Solution**: Create adapters that provide consistent interface.

```typescript
interface RDFAdapter {
  parse(content: string): Promise<Triple[]>;
  serialize(triples: Triple[]): Promise<string>;
  validate(content: string): boolean;
}

class TurtleAdapter implements RDFAdapter {
  async parse(content: string): Promise<Triple[]> {
    const parser = new N3.Parser({ format: 'turtle' });
    return new Promise((resolve, reject) => {
      const triples: Triple[] = [];
      parser.parse(content, (error, quad) => {
        if (error) reject(error);
        if (quad) triples.push(quadToTriple(quad));
        else resolve(triples);
      });
    });
  }
  
  async serialize(triples: Triple[]): Promise<string> {
    const writer = new N3.Writer({ format: 'turtle' });
    triples.forEach(t => writer.addQuad(tripleToQuad(t)));
    return new Promise((resolve, reject) => {
      writer.end((error, result) => {
        if (error) reject(error);
        else resolve(result);
      });
    });
  }
  
  validate(content: string): boolean {
    try {
      const parser = new N3.Parser({ format: 'turtle' });
      parser.parse(content);
      return true;
    } catch {
      return false;
    }
  }
}

class JSONLDAdapter implements RDFAdapter {
  async parse(content: string): Promise<Triple[]> {
    const doc = JSON.parse(content);
    const nquads = await jsonld.toRDF(doc, { format: 'application/n-quads' });
    return parseNQuads(nquads);
  }
  
  async serialize(triples: Triple[]): Promise<string> {
    const nquads = serializeToNQuads(triples);
    const doc = await jsonld.fromRDF(nquads, { format: 'application/n-quads' });
    return JSON.stringify(doc, null, 2);
  }
  
  validate(content: string): boolean {
    try {
      const doc = JSON.parse(content);
      return doc['@context'] !== undefined;
    } catch {
      return false;
    }
  }
}

class RDFAdapterFactory {
  private adapters: Map<RDFFormat, RDFAdapter> = new Map([
    ['turtle', new TurtleAdapter()],
    ['jsonld', new JSONLDAdapter()],
    ['ntriples', new NTriplesAdapter()],
    ['rdfxml', new RDFXMLAdapter()],
  ]);
  
  getAdapter(format: RDFFormat): RDFAdapter {
    const adapter = this.adapters.get(format);
    if (!adapter) {
      throw new Error(`No adapter found for format: ${format}`);
    }
    return adapter;
  }
}

// Usage
const factory = new RDFAdapterFactory();
const adapter = factory.getAdapter('turtle');
const triples = await adapter.parse(rdfContent);
```

## UI Component Patterns

### 11. Compound Components Pattern

**Problem**: Creating components with implicit parent-child relationships.

**Solution**: Use React Context for compound components.

```typescript
interface TabsContextValue {
  activeTab: string;
  setActiveTab: (tab: string) => void;
}

const TabsContext = createContext<TabsContextValue | null>(null);

function Tabs({ children, defaultTab }: { children: React.ReactNode; defaultTab: string }) {
  const [activeTab, setActiveTab] = useState(defaultTab);
  
  return (
    <TabsContext.Provider value={{ activeTab, setActiveTab }}>
      <div className="tabs">{children}</div>
    </TabsContext.Provider>
  );
}

function TabList({ children }: { children: React.ReactNode }) {
  return <div className="tab-list flex border-b">{children}</div>;
}

function Tab({ value, children }: { value: string; children: React.ReactNode }) {
  const context = useContext(TabsContext);
  if (!context) throw new Error('Tab must be used within Tabs');
  
  const isActive = context.activeTab === value;
  
  return (
    <button
      className={`px-4 py-2 ${isActive ? 'border-b-2 border-blue-500' : ''}`}
      onClick={() => context.setActiveTab(value)}
    >
      {children}
    </button>
  );
}

function TabPanel({ value, children }: { value: string; children: React.ReactNode }) {
  const context = useContext(TabsContext);
  if (!context) throw new Error('TabPanel must be used within Tabs');
  
  if (context.activeTab !== value) return null;
  
  return <div className="tab-panel p-4">{children}</div>;
}

// Export compound component
export const TabsComponent = Object.assign(Tabs, {
  List: TabList,
  Tab: Tab,
  Panel: TabPanel,
});

// Usage
<TabsComponent defaultTab="overview">
  <TabsComponent.List>
    <TabsComponent.Tab value="overview">Overview</TabsComponent.Tab>
    <TabsComponent.Tab value="visualizations">Visualizations</TabsComponent.Tab>
    <TabsComponent.Tab value="query">Query</TabsComponent.Tab>
  </TabsComponent.List>
  
  <TabsComponent.Panel value="overview">
    <OverviewContent />
  </TabsComponent.Panel>
  <TabsComponent.Panel value="visualizations">
    <VisualizationsContent />
  </TabsComponent.Panel>
  <TabsComponent.Panel value="query">
    <QueryContent />
  </TabsComponent.Panel>
</TabsComponent>
```

### 12. Controlled vs Uncontrolled Components

**Problem**: Balancing component flexibility and control.

**Solution**: Support both controlled and uncontrolled modes.

```typescript
interface SearchBoxProps {
  // Controlled mode
  value?: string;
  onChange?: (value: string) => void;
  
  // Uncontrolled mode
  defaultValue?: string;
  
  // Common props
  placeholder?: string;
  onSearch?: (value: string) => void;
}

function SearchBox({
  value: controlledValue,
  onChange,
  defaultValue = '',
  placeholder = 'Search...',
  onSearch,
}: SearchBoxProps) {
  const [internalValue, setInternalValue] = useState(defaultValue);
  
  // Determine if controlled or uncontrolled
  const isControlled = controlledValue !== undefined;
  const value = isControlled ? controlledValue : internalValue;
  
  const handleChange = (newValue: string) => {
    if (!isControlled) {
      setInternalValue(newValue);
    }
    onChange?.(newValue);
  };
  
  const handleSubmit = (e: React.FormEvent) => {
    e.preventDefault();
    onSearch?.(value);
  };
  
  return (
    <form onSubmit={handleSubmit}>
      <input
        type="text"
        value={value}
        onChange={(e) => handleChange(e.target.value)}
        placeholder={placeholder}
      />
      <button type="submit">Search</button>
    </form>
  );
}

// Uncontrolled usage
<SearchBox
  defaultValue="Rijksmuseum"
  onSearch={(query) => performSearch(query)}
/>

// Controlled usage
const [searchQuery, setSearchQuery] = useState('');
<SearchBox
  value={searchQuery}
  onChange={setSearchQuery}
  onSearch={(query) => performSearch(query)}
/>
```

## State Management Patterns

### 13. Zustand Store Slices

**Problem**: Organizing large state stores.

**Solution**: Split store into logical slices.

```typescript
// stores/slices/rdfSlice.ts
export interface RDFSlice {
  triples: Triple[];
  format: RDFFormat;
  isLoading: boolean;
  error: Error | null;
  loadRDF: (content: string, format: RDFFormat) => Promise<void>;
  clearRDF: () => void;
}

export const createRDFSlice: StateCreator<RDFSlice> = (set, get) => ({
  triples: [],
  format: 'turtle',
  isLoading: false,
  error: null,
  
  loadRDF: async (content, format) => {
    set({ isLoading: true, error: null });
    try {
      const adapter = new RDFAdapterFactory().getAdapter(format);
      const triples = await adapter.parse(content);
      set({ triples, format, isLoading: false });
    } catch (error) {
      set({ error: error as Error, isLoading: false });
    }
  },
  
  clearRDF: () => set({ triples: [], error: null }),
});

// stores/slices/querySlice.ts
export interface QuerySlice {
  currentQuery: string;
  results: QueryResult[];
  isExecuting: boolean;
  setQuery: (query: string) => void;
  executeQuery: () => Promise<void>;
}

export const createQuerySlice: StateCreator<QuerySlice> = (set, get) => ({
  currentQuery: '',
  results: [],
  isExecuting: false,
  
  setQuery: (query) => set({ currentQuery: query }),
  
  executeQuery: async () => {
    set({ isExecuting: true });
    try {
      const { triples } = get() as any; // Access RDF slice
      const engine = new SPARQLEngine(triples);
      const results = await engine.execute(get().currentQuery);
      set({ results, isExecuting: false });
    } catch (error) {
      console.error('Query execution failed:', error);
      set({ isExecuting: false });
    }
  },
});

// stores/index.ts
import { create } from 'zustand';
import { devtools } from 'zustand/middleware';

type Store = RDFSlice & QuerySlice & UISlice;

export const useStore = create<Store>()(
  devtools(
    (...a) => ({
      ...createRDFSlice(...a),
      ...createQuerySlice(...a),
      ...createUISlice(...a),
    })
  )
);
```

## Performance Patterns

### 14. Memoization Pattern

**Problem**: Expensive computations running on every render.

**Solution**: Use useMemo and useCallback appropriately.

```typescript
function CustodianNetwork({ custodians }: { custodians: CustodianObservation[] }) {
  // Memoize expensive graph computation
  const graphData = useMemo(() => {
    const nodes = custodians.map(c => ({ id: c.id, label: c.name }));
    const edges = computeRelationships(custodians);
    return { nodes, edges };
  }, [custodians]); // Only recompute if custodians change
  
  // Memoize callback to prevent child re-renders
  const handleNodeClick = useCallback((nodeId: string) => {
    const custodian = custodians.find(c => c.id === nodeId);
    if (custodian) {
      eventBus.emit('custodian:selected', custodian);
    }
  }, [custodians]);
  
  return <NetworkGraph data={graphData} onNodeClick={handleNodeClick} />;
}

// Memoized component (React.memo)
export const CustodianCard = React.memo<CustodianCardProps>(
  ({ custodian, onClick }) => {
    return (
      <div onClick={() => onClick(custodian)}>
        <h3>{custodian.name}</h3>
        <p>{custodian.institution_type}</p>
      </div>
    );
  },
  (prevProps, nextProps) => {
    // Custom comparison function
    return prevProps.custodian.id === nextProps.custodian.id;
  }
);
```

### 15. Debouncing and Throttling

**Problem**: Handling high-frequency events efficiently.

**Solution**: Debounce user input, throttle scroll/resize events.

```typescript
// Custom hook for debouncing
function useDebounce<T>(value: T, delay: number): T {
  const [debouncedValue, setDebouncedValue] = useState<T>(value);
  
  useEffect(() => {
    const handler = setTimeout(() => {
      setDebouncedValue(value);
    }, delay);
    
    return () => {
      clearTimeout(handler);
    };
  }, [value, delay]);
  
  return debouncedValue;
}

// Usage in search
function SearchComponent() {
  const [searchTerm, setSearchTerm] = useState('');
  const debouncedSearchTerm = useDebounce(searchTerm, 300);
  
  const { data } = useQuery(
    ['search', debouncedSearchTerm],
    () => searchCustodians(debouncedSearchTerm),
    { enabled: debouncedSearchTerm.length > 0 }
  );
  
  return (
    <input
      value={searchTerm}
      onChange={(e) => setSearchTerm(e.target.value)}
      placeholder="Search custodians..."
    />
  );
}

// Custom hook for throttling
function useThrottle<T extends (...args: any[]) => any>(
  callback: T,
  delay: number
): T {
  const lastRun = useRef(Date.now());
  
  return useCallback((...args: Parameters<T>) => {
    const now = Date.now();
    if (now - lastRun.current >= delay) {
      callback(...args);
      lastRun.current = now;
    }
  }, [callback, delay]) as T;
}

// Usage for scroll events
function InfiniteScrollList() {
  const loadMore = useThrottle(() => {
    // Load more items
  }, 200);
  
  useEffect(() => {
    window.addEventListener('scroll', loadMore);
    return () => window.removeEventListener('scroll', loadMore);
  }, [loadMore]);
  
  return <div>...</div>;
}
```

## Error Handling Patterns

### 16. Error Boundary Pattern

**Problem**: Gracefully handling component errors.

**Solution**: Implement error boundaries.

```typescript
interface ErrorBoundaryProps {
  children: React.ReactNode;
  fallback?: React.ComponentType<{ error: Error; reset: () => void }>;
}

interface ErrorBoundaryState {
  hasError: boolean;
  error?: Error;
}

export class ErrorBoundary extends React.Component<ErrorBoundaryProps, ErrorBoundaryState> {
  constructor(props: ErrorBoundaryProps) {
    super(props);
    this.state = { hasError: false };
  }
  
  static getDerivedStateFromError(error: Error): ErrorBoundaryState {
    return { hasError: true, error };
  }
  
  componentDidCatch(error: Error, errorInfo: React.ErrorInfo) {
    console.error('Error boundary caught error:', error, errorInfo);
    // Send to error tracking service
    reportError(error, errorInfo);
  }
  
  reset = () => {
    this.setState({ hasError: false, error: undefined });
  };
  
  render() {
    if (this.state.hasError && this.state.error) {
      const Fallback = this.props.fallback || DefaultErrorFallback;
      return <Fallback error={this.state.error} reset={this.reset} />;
    }
    
    return this.props.children;
  }
}

// Default fallback component
function DefaultErrorFallback({ error, reset }: { error: Error; reset: () => void }) {
  return (
    <div className="error-boundary">
      <h2>Something went wrong</h2>
      <pre>{error.message}</pre>
      <button onClick={reset}>Try again</button>
    </div>
  );
}

// Usage
<ErrorBoundary fallback={CustomErrorFallback}>
  <CustodianVisualization />
</ErrorBoundary>
```

## Testing Patterns

### 17. Test Data Builders

**Problem**: Creating test data is repetitive and brittle.

**Solution**: Use builder pattern for test data.

```typescript
class CustodianBuilder {
  private custodian: Partial<CustodianObservation> = {
    id: 'test-id',
    name: 'Test Museum',
    institution_type: 'MUSEUM',
  };
  
  withId(id: string): this {
    this.custodian.id = id;
    return this;
  }
  
  withName(name: string): this {
    this.custodian.name = name;
    return this;
  }
  
  withType(type: InstitutionType): this {
    this.custodian.institution_type = type;
    return this;
  }
  
  withLocation(city: string, country: string): this {
    this.custodian.locations = [{
      city,
      country,
      latitude: 0,
      longitude: 0,
    }];
    return this;
  }
  
  build(): CustodianObservation {
    return this.custodian as CustodianObservation;
  }
}

// Usage in tests
describe('CustodianCard', () => {
  it('displays museum information', () => {
    const museum = new CustodianBuilder()
      .withName('Rijksmuseum')
      .withType('MUSEUM')
      .withLocation('Amsterdam', 'NL')
      .build();
    
    render(<CustodianCard custodian={museum} />);
    
    expect(screen.getByText('Rijksmuseum')).toBeInTheDocument();
    expect(screen.getByText('MUSEUM')).toBeInTheDocument();
  });
});
```

## Related Documents

- [01-architecture.md](01-architecture.md) - System architecture
- [03-tdd-strategy.md](03-tdd-strategy.md) - Testing strategy
- [04-example-ld-mapping.md](04-example-ld-mapping.md) - Example LD reuse patterns
- [05-d3-visualization.md](05-d3-visualization.md) - D3.js patterns