Answer: A Singleton ensures only one instance of a class exists globally. It's convenient but widely overused in iOS.

Code Example:

// Standard Swift singleton
class AnalyticsManager {
    static let shared = AnalyticsManager()
    private init() {}  // prevent external instantiation

    func track(_ event: String) { ... }
}

// Usage
AnalyticsManager.shared.track("button_tapped")

Downsides:

• Hard to test — can't inject a mock; tests share global state
• Hidden dependencies — callers depend on it without declaring it
• Threading issues — shared mutable state needs synchronization
• Tight coupling — callers are coupled to the concrete type

When it's acceptable:

• Logging, analytics, app-level config (read-only)
• Avoid for anything that manages data or has side effects