I'm trying to understand what happens at a lower-level when we create the first instance of a Singleton object within the Singleton's own class declaration.
As I understand it, the static keyword allows the marked property or method to be shared across all instances of a class, which I'm sure has a role here that I'm not seeing fully.
Additionally, how does this work when we consider the instance's creation during compilation/runtime?
Here's an example of a Singleton class declaration:
class Person {
static let details = Person()
var name = "Alan Turing"
let age = "42"
}
I understand that the class and its properties will only be created once and that any reference to the class object will point back to that same point in memory. My confusion is specifically about why we create the Singleton's first instance within itself.
Basically static variables are class variables that are always accessible via the class itself.
I suggest reading a more in depth explanation of static here
By having a static variable inside the class that contains an object of the class itself you ensure that at runtime there already is an object of that class (or at least at the time of accessing Person.instance for the first time).
For a true singleton inside Swift it is mandatory to make the init of that class private though, like so:
public class Person {
static let instance = Person()
var name: String
let age: Int
private init() {
self.name = "Alan Turing"
self.age = 42
}
}
That way you ensure that there really is only one object present at any give time (hence the name singleton).
Related
A class variable that contains the class itself, for example, the main property of the class DispatchQueue
Question 1
What kind of design pattern it is? Is this related to Singleton?
Question 2
How does the object initialized when referring this property?
Yes, it is the singleton pattern. It is all over swift like UserDefaults.standard, NotificationCenter.default, FileManager.default, ... For instance, URLSession.shared is defined in the apple docs as:
The shared singleton session object.
The singleton pattern makes sure that only one instance of the class is created. The class lazily creates its sole instance the first time it is requested and thereafter ensures that no other instance can be created.
Here is some sample code:
class MyClass {
static let shared = MyClass()
//Make the initializer private
private init(){}
//Here is a sample method
func doSomething() {
print("Doing something")
}
}
Making the initializer prevents the possibility of creating many instances, and thus the following line would yield an error:
let instance = MyClass() //MyClass initializer is inaccessible due to private protection level
To access the singleton use MyClass.shared:
let instance = MyClass.shared
instance.doSomething() //Prints: Doing something
For more details on the singleton pattern in Swift, you may look here and here.
To create a singleton class, I wrote something like this:
class SingletonEx{
var name = ""
private init(){}
static let sharedInstance = SingletonEx()
func instanceMethod(){
}
static func classMethod(){
}
}
Some tutorials say final is necessary while others just ignore final keyword. After I tried subclassing SingletonEx, I got the following results.
It seems I can't write an initializer for subclass, which means I can't use an override instance method in a subclass.
As far as I know, singleton definition is all about single instantiation and accessing instance methods through the only instance. So I don't think it is necessary to use final in the singleton definition. But both my teachers and some online tutorials say it is necessary.
I got confused, since you can't create a subclass instance anyway, even you override the instance methods, you can't use it or access it, what's the point to say final is necessary for a singleton class?
If I am wrong, please point out.
Super Class
First of all you need to know the properties and methods that are marked with private are just known to the Super class and Sub classes won't access them!
A class can inherit methods, properties, and other characteristics from another class. When one class inherits from another, the inheriting class is known as a subclass, and the class it inherits from is known as its superclass. Inheritance is a fundamental behavior that differentiates classes from other types in Swift.
Classes in Swift can call and access methods, properties, and subscripts belonging to their superclass and can provide their own overriding versions of those methods, properties, and subscripts to refine or modify their behavior. Swift helps to ensure your overrides are correct by checking that the override definition has a matching superclass definition.
In your case in SingletonEx class you market init with private which means that you can create object just in the body of the class! that means no one, no where, can't create an object of SingletonEx!
If you want to a method end up in Super Class and you don't want to Sub classes overide that method you need to mark that method with final which means it's not private but its available only from Super class!
Sub Class
When class Y Inheritance from SingletonEx which means that cant create an object outside of the class ! because Super class initializer is unavailable during init() method from class Y ! While you need to call the super.init() if you want to initialize an object from Y class !
if you remove private from private init() {} from SingletonEx class you be able to create object from SingletonEx class and also from Y class !
your code should looks like this :
Swift 4 :
class SingletonEx{
var name = ""
init(){}
static let sharedInstance = SingletonEx()
func instanceMethod(){
}
static func classMethod(){
}
}
class Y : SingletonEx {
private var yName = "Y name is : "
init(name:String) {
super.init()
self.name = self.yName + name
}
}
Usage :
override func viewDidLoad() {
super.viewDidLoad()
let yObject = Y.init(name: "badGirl :D ")
print(yObject)
// --> Output : Y name is : badGirl :D
}
As the question may seem duplicated, I point out first that this is not asking for how to get a class type from an instance ( answer is using type(of:) ).
protocol Owner {
static func name() -> String
}
extension Owner {
static func name() -> String {
return "\(self)"
}
}
class Foo {
var ownerName: String
init(with owner: Owner.Type) {
ownerName = owner.name()
}
}
class Bar: Owner {
var foo = Foo(with: Bar.self)
}
The code above simply works, but imagine that I want to rename the class Bar to BarBar then I need to change the initialization of foo to Foo(with: BarBar.self) manually. Is there any keyword that can be used instead of ClassName.self (e.g. Bar.self BarBar.self) to get the class type? Something like Self or Class (they don't compile actually)
My guess is that your whole question is a red herring and that type(of:self) is exactly what you want to say. In that case, the problem is merely that var foo is declared as an instance property. There are special rules for when you are allowed to say self while initializing an instance property (rightly, because self is exactly what does not yet exist during initialization). However, there are ways around that, as I have explained elsewhere; you can make this a computed instance property, or a lazy instance property, which is initialized by a function to be executed later, and then you are allowed to say type(of:self).
Really basic question. I'm currently trying to code in a more Object Oriented way but there is something that I don't fully understand. I believe this applies to multiple programming languages not only Swift because I have seen this in some PHP code too.
I was under the impression that in order for you to access methods in a class you would need to create an instance but I'm not sure how this happens when you pass objects as parameters.
In the code below the moveDog(move:Animal) method gets an Animal object as a parameter, what I don't know understand is how can you access methods form the Animal object without instantiating the class first (print(move.run()))?
Does instantiation happens when declaring local parameters moveDog(move:Animal)? Can we say that it is the same as if we would do let move = Animal()?
class Animal{
func run(){
print("Running...")
}
}
class Dog{
func moveDog(move:Animal){
//how can the run method be called without
//having to instantiate Animal?
print(move.run())
}
}
let dog = Animal()
let buddy = Dog()
buddy.moveDog(dog)
The initialisation happens when you actually create it, e.g. let dog = Animal() or let buddy = Dog().
A quick note, you can make this code better by using the same run() function by using inheritance like so:
class Animal{
func run(){
print("Running...")
}
}
class Dog: Animal{
override func run() {
super.run()
}
}
let dog = Animal() //You don't need to instantiate this to use the Dog class
let buddy = Dog()
buddy.run()
In swift parameters are normally passed by reference (except structs) rather than being copied. So there's no initialisation for passing parameters.
I am trying out to create a singleton in SWIFT and this is what I have done so far
class Global {
class var sharedInstance:Global {
struct singleton {
static let instance:Global = Global()
}
return singleton.instance
}
}
var a = Global.sharedInstance
var b = Global()
if a === b {
println("Pointing to Same Instance")
}
else {
println("Pointing to different instance")
}
I have used computed type property to create a singleton (learnt that from another stackoverflow question).As of now the output is "Pointing to different instance".
What I am looking for is "a" and "b" in above example points to different instance of GLOBAL class and this breaks the point of singleton. How to make "a" and "b" in above example to point to the same instance of the class.
Thank you
This pattern does not guarantee there will only ever be one instance of the Global class. It just allows for anyone to access a single common instance of Global via its sharedinstance property.
So Global() declares a new instance of the Global class. But Global.sharedinstance does not create a new instance of Global, just fetches a pre-created one (that is created the first time anyone accesses it).
(If you alter your declaration of b to read var b = Global.sharedinstance you’ll see it confirms that a and b are pointing to the same instance.)
If you want to ban the creation of further instances of Global, make its init private:
private init() { }
But bear in mind you’ll still be able to create other Globals from within the file in which it’s declared, so if you’re doing the above in a playground or single-file test project, you won’t see any effect.
Class instance once in App life cycle.
class AccountManager {
static var sharedInstance = AccountManager()
var userInfo = (ID:"Arjun",Password:"123")
private init(){
print("allocate AccountManager")
}
}
here we set Private because :
Private access restricts the use of an entity to the enclosing declaration, and to extensions of that declaration that are in the same file. Use private access to hide the implementation details of a specific piece of functionality when those details are used only within a single declaration.
also set static property of sharedInstance
because if you need to access class property without instance of class you must have to declare "Static".