What are the problems of using the Singleton pattern in system design?

The Singleton pattern guarantees that a class has only one instance and provides a global point of access to it. However, its use can lead to several problems:

• Testing difficulties: Singleton creates a global state, which makes isolation and mocking in unit tests difficult.
• Hidden dependencies: Components using Singleton implicitly depend on it, which worsens code readability and maintainability.
• Multithreading issues: If Singleton is not implemented correctly, race conditions may occur.
• Reduced flexibility: Global access makes it difficult to replace the implementation or use multiple instances if needed.

In iOS development, it is better to use Dependency Injection or other state management patterns to avoid these problems.