What is the difference between using a class method and a static method when implementing a singleton?

In iOS, when implementing the Singleton pattern, you can use either a class method or a static method (for example, a static property in Swift).

Class method (class func) can be overridden in subclasses, allowing the behavior of the Singleton to be changed in inheriting classes.

Static method/property (static func/var) cannot be overridden, providing a more strict guarantee of a single instance.

Example of a Singleton using a static property in Swift:

class MySingleton {
    static let shared = MySingleton()
    private init() {}
}

Here, shared is a static property initialized once and accessible globally.

Using a class method to get the Singleton might look like this:

class MySingleton {
    private static var instance: MySingleton?
    class func shared() -> MySingleton {
        if instance == nil {
            instance = MySingleton()
        }
        return instance!
    }
    private init() {}
}

The main difference is the ability to override and flexibility when using class methods, whereas static methods/properties are simpler and safer.