The Singleton pattern and its concurrency issues in Swift

The Singleton pattern is used to ensure there is only one instance of a type. If not implemented well, may bring concurrency issues: data-corruption and crashes.

Let’s start with a read-only singleton:
- safe even if we read the shared instance from different threads running in parallel

final public class AppSettings {
    
    public static let shared = AppSettings()
    private var settings: [String: Any] = ["Theme": "Light"]
    private init() {}…