feat(storage-kit): implement SharedPreferencesStore for persistent key-value storage

[husamettinarabaci]

Is your feature request related to a problem?

The SharedPreferencesStore skeleton class exists in packages/storage_kit/lib/key_value/shared_preferences_store.dart but needs full implementation to provide a concrete KeyValueStore implementation using SharedPreferences for persistent, simple data storage.

Describe the solution you'd like

Implement a fully functional SharedPreferencesStore class that implements the KeyValueStore interface using Flutter's shared_preferences package, providing persistent storage for simple key-value data across app sessions.

What is SharedPreferencesStore and why do we need it?

SharedPreferencesStore is a concrete implementation of KeyValueStore using SharedPreferences, providing simple persistent storage for app preferences and settings.

Why it's important:

• Persistence: Data survives app restarts
• Performance: Fast synchronous access after initialization
• Platform Support: Works on iOS, Android, web, desktop
• Simple Data: Perfect for preferences and settings
• Standard Solution: Industry-standard for simple storage

Common use cases:

• User app preferences
• Theme selection (dark/light mode)
• Language preference
• Onboarding completion flag
• Feature flags
• Simple caching

Required Deliverables

1. SharedPreferencesStore implementation with:

   • Initialize SharedPreferences
   • Implement all KeyValueStore methods
   • Type-safe getters/setters
   • Error handling
   • Null safety
   • Integration with shared_preferences package
   • Singleton pattern
   • Clear documentation

2. Widgetbook stories (6-8 use cases):

   • Store string data
   • Store int data
   • Store bool data
   • Store list data
   • Retrieve stored data
   • Remove key demo
   • Clear all demo
   • Storage inspector UI

3. SPDX headers in all source files

Key Implementation Specs

• Implements KeyValueStore interface
• Use shared_preferences package
• Singleton instance: SharedPreferencesStore._()
• Factory constructor: SharedPreferencesStore.instance
• initialize() returns Future
• All methods delegate to SharedPreferences
• Error handling for storage failures
• Null safety with nullable returns
• Type conversions handled internally

Acceptance Criteria

• SharedPreferencesStore implements KeyValueStore
• initialize() loads SharedPreferences
• getString() retrieves string values
• setString() stores string values
• getInt() retrieves int values
• setInt() stores int values
• getBool() retrieves bool values
• setBool() stores bool values
• getDouble() retrieves double values
• setDouble() stores double values
• getStringList() retrieves lists
• setStringList() stores lists
• remove() deletes keys
• clear() removes all data
• containsKey() checks existence
• Singleton pattern implemented
• Error handling works
• All 6-8 Widgetbook stories created and functional
• SPDX headers present in all files
• Code follows STYLE_GUIDE.md standards
• Unit tests cover storage operations

Related Files

• packages/storage_kit/lib/key_value/shared_preferences_store.dart (skeleton)
• packages/storage_kit/lib/key_value/key_value_store.dart (interface)
• widgetbook_kit/lib/stories/storage_kit/shared_preferences_store_story.dart (Widgetbook)

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Impact: Primary simple storage solution; used throughout the app

Priority: High