What reasons make the Singleton pattern undesirable in development?

The Singleton pattern is often considered undesirable for several reasons:

• Global state: Singleton essentially creates a global object, which complicates state control and can lead to hard-to-debug bugs.
• Testing difficulties: due to the global state, it is difficult to isolate tests, mock, or replace the Singleton.
• Violation of OOP principles: Singleton violates the single responsibility principle and can create hidden dependencies.
• Multithreading issues: if the Singleton is not implemented with thread safety in mind, race conditions and errors can occur.
• Tight coupling: code that uses Singleton becomes tightly coupled to a specific implementation, reducing flexibility.

In iOS development, it is often recommended to use other approaches, such as dependency injection or factories, to avoid the drawbacks of Singleton.