What is the Singleton pattern and what are its downsides?

URLCache stores responses from network requests in memory and/or on disk.
A Singleton ensures only one instance of a class exists globally.
The Factory pattern creates objects without exposing the creation logic.
multipart/form-data is a content type used to send mixed data — like a file and text fields together — in a single request.

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