If a singleton must be used (and effort should be taken to avoid this where possible), then they should always be implemented as a class with a shared reference to an instance of itself, with instance properties, rather than static properties.
Following this convention makes it clear that this class is a singleton and should be used that way. More importantly, it makes testing significantly simpler as new versions can be instatiated and passed around.
// good: The singleton has no static properties
class MyManager {
static let shared = MyManager()
var someVar: String = "Hello"
}
// bad: The singleton only has static properties
class MyManager {
static var someVar: String = "Hello"
}
// A more complete example
public class OurDefaults {
// Called throughout the application
public static let shared = OurDefaults(userDefaults: .standard)
private let userDefaults: UserDefaults
// Called during test case set up
// Initializer also makes it clear how many dependencies this class has.
init(userDefaults: UserDefaults) {
self.userDefaults = userDefaults
}
}