Singleton_Pattern
Singleton
Pros
- Easy understanding
- One source of truth
- Object remains in the memory throughout the app lifecycle.
- Supports lazy initialization.
- Used by lot of analytics SDKs
- iOS UIKit, CocoaTouch relies a lot on Singleton and MVC pattern.
- App Delegate is important singleton instance.
Cons
- Data inconsistency - Not multi thread safe. (Not good for CRUD operations)
- More dependencies - Highly coupled
- Less testable
- Breaks SOLID - Single responsibility (Does too many things, Concrete implementation)
- Gets overuse quickly.
- If you have lots of singletons for App Delegate or Scene Delegate.
- Could be time dependent with some callback.
Code
Important to privatize the init() in order to not let invocation rights to anyone else.
class SingletonClass {
static let shared: SingletonClass = SingletonClass()
private init() { }
func foo() { }
}
Also defining the class final and having just Singleton pattern for invocation only once is kind of similar but there is definitely need for each in
their own right.
Examples
iOS examples of singleton pattern.
Migrating away from Singleton
Switch to Dependency injection and protocol driven interface
https://theswiftdev.com/swift-singleton-design-pattern/
References
I know Java has few differences & I haven't taken my time to research these things in iOS Swift world.
SO | difference-between-static-class-and-singleton-pattern
SO | is-final-necessary-for-a-singleton-class-in-swift