Thread Safe Singleton

A robust, thread-safe singleton in Swift that centralizes:

• Configuration Storage
  Store and retrieve key/value settings safely across threads using NSLock.
• Event Bus
  Publish and subscribe to application-wide events with a concurrent queue + barrier writes.
• Lazy-Initialized Services
  Initialize heavyweight services (e.g. networking) only when needed, with fallback to default configuration.

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Features

1. Singleton Guarantee

   • static let shared ensures a single, lazily-initialized instance.
   • private init() prevents external instantiation.

2. Thread-Safe Configuration

   • Config values held in a [String: Any] store.
   • Protected by NSLock for safe, atomic reads and writes.

3. Event Bus (Publish/Subscribe)

   • Subscribers register closures and receive an opaque UUID token.
   • Backed by a .concurrent DispatchQueue with .barrier for writes.
   • Publishing creates a snapshot of handlers and dispatches them asynchronously.

4. Lazy Service Initialization

   • Example network service (NetworkService) is created on first use.
   • Reads optional custom APIConfig from the config store or falls back to APIConfig.default.

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Usage

/// 1. Set a custom API endpoint
guard let url = URL(string: "https://api.custom.com") else {
  return
}
AppManager.shared.setConfig(value: APIConfig(baseURL: url), 
                            forKey: "apiConfig")

/// 2. Subscribe to events
let token = AppManager.shared.subscribe { event in
    switch event {
    case .userLoggedIn(let id):
        print("Logged in:", id)
    case .dataRefreshed:
        print("Data was refreshed")
    default:
        break
    }
}

/// 3. Publish an event
AppManager.shared.publish(event: .userLoggedIn(userID: "42"))

/// 4. Perform a network request
AppManager.shared.performRequest(to: "users/42") { result in
    switch result {
    case .success(let data):
        /// handle data
    case .failure(let error):
        /// handle error
    }
}

/// 5. Unsubscribe when no longer needed
AppManager.shared.unsubscribe(token)