swift-actor-persistence

name	swift-actor-persistence
description	Thread-safe data persistence in Swift using actors — in-memory cache with file-backed storage, eliminating data races by design.
source_path	skills/swift-actor-persistence/SKILL.md
origin	ECC

Swift Actors for Thread-Safe Persistence

Patterns for building thread-safe data persistence layers using Swift actors. Combines in-memory caching with file-backed storage, leveraging the actor model to eliminate data races at compile time.

When to Activate

• Building a data persistence layer in Swift 5.5+
• Need thread-safe access to shared mutable state
• Want to eliminate manual synchronization (locks, DispatchQueues)
• Building offline-first apps with local storage

Core Pattern

Actor-Based Repository

The actor model guarantees serialized access — no data races, enforced by the compiler.

public actor LocalRepository<T: Codable & Identifiable> where T.ID == String {
    private  cache: [: ]  [:]
      fileURL: 

     (:   .documentsDirectory, :   ) {
        .fileURL  directory.appendingPathComponent(filename)
        
        .cache  .loadSynchronously(from: fileURL)
    }

    

      ( : )  {
        cache[item.id]  item
         persistToFile()
    }

      ( : )  {
        cache[id]  
         persistToFile()
    }

      ( : ) -> ? {
        cache[id]
    }

      () -> [] {
        (cache.values)
    }

    

      ()  {
         data   ().encode((cache.values))
         data.write(to: fileURL, options: .atomic)
    }

       ( : ) -> [: ] {
          data   (contentsOf: url),
               items   ().decode([]., from: data)  {
             [:]
        }
         (uniqueKeysWithValues: items.map { (.id, ) })
    }
}