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Using a dispatch_once singleton model in Swift

I'm trying to work out an appropriate singleton model for usage in Swift. So far, I've been able to get a non-thread safe model working as:

class var sharedInstance: TPScopeManager {
    get {
        struct Static {
            static var instance: TPScopeManager? = nil
        }

        if !Static.instance {
            Static.instance = TPScopeManager()
        }

        return Static.instance!
    }
}

Wrapping the singleton instance in the Static struct should allow a single instance that doesn't collide with singleton instances without complex naming schemings, and it should make things fairly private. Obviously though, this model isn't thread-safe. So I tried to add dispatch_once to the whole thing:

class var sharedInstance: TPScopeManager {
    get {
        struct Static {
            static var instance: TPScopeManager? = nil
            static var token: dispatch_once_t = 0
        }

        dispatch_once(Static.token) { Static.instance = TPScopeManager() }

        return Static.instance!
    }
}

But I get a compiler error on the dispatch_once line:

Cannot convert the expression's type 'Void' to type '()'

I've tried several different variants of the syntax, but they all seem to have the same results:

dispatch_once(Static.token, { Static.instance = TPScopeManager() })

What is the proper usage of dispatch_once using Swift? I initially thought the problem was with the block due to the () in the error message, but the more I look at it, the more I think it may be a matter of getting the dispatch_once_t correctly defined.

I would remove all that static code and use a readonly property with a @lazy initializer.
That's what I meant. Unfortunately we still don't have enough information about the internals. However, IMHO any implementation of @lazy should be thread safe.
And this way also has the advantage of not exposing the implementation to the predations of callers.
It also doesn't seem like you can have @lazy class variables.
Be careful! Two things to note with this approach. First, any classes that inherit from this will have to override the sharedInstance property. Static.instance = TPScopeManager() forces the instance type. If you use something like Static.instance = self() with a required initializer, the appropriate type class will be generated. Even so, and this is the important thing to note, only once for all instances in the hierarchy! First type to initialize is the type set for all instances. I don't think objective-c behaved the same.

A
Alexander Vasenin

tl;dr: Use the class constant approach if you are using Swift 1.2 or above and the nested struct approach if you need to support earlier versions.

From my experience with Swift there are three approaches to implement the Singleton pattern that support lazy initialization and thread safety.

Class constant

class Singleton  {
   static let sharedInstance = Singleton()
}

This approach supports lazy initialization because Swift lazily initializes class constants (and variables), and is thread safe by the definition of let. This is now officially recommended way to instantiate a singleton.

Class constants were introduced in Swift 1.2. If you need to support an earlier version of Swift, use the nested struct approach below or a global constant.

Nested struct

class Singleton {
    class var sharedInstance: Singleton {
        struct Static {
            static let instance: Singleton = Singleton()
        }
        return Static.instance
    }
}

Here we are using the static constant of a nested struct as a class constant. This is a workaround for the lack of static class constants in Swift 1.1 and earlier, and still works as a workaround for the lack of static constants and variables in functions.

dispatch_once

The traditional Objective-C approach ported to Swift. I'm fairly certain there's no advantage over the nested struct approach but I'm putting it here anyway as I find the differences in syntax interesting.

class Singleton {
    class var sharedInstance: Singleton {
        struct Static {
            static var onceToken: dispatch_once_t = 0
            static var instance: Singleton? = nil
        }
        dispatch_once(&Static.onceToken) {
            Static.instance = Singleton()
        }
        return Static.instance!
    }
}

See this GitHub project for unit tests.


"thread safe by virtue of let" — has this been stated anywhere? I can't find mention of it in the documentation.
@jtbandes Constants are thread safe in all the languages I know.
@DaveWood I assume you're talking about the last approach. I'll quote myself: "I'd say it's no longer necessary to use this approach but I'm putting it here anyway as I find the differences in syntax interesting."
Should init be also be declared private to guarantee one and only one instance of the object will ever exist throughout the app's lifetime?
In the "Class constant" approach, I'd suggest (a) declaring the class to be final so you don't subclass it; and (b) declaring the init method to be private so that you can't accidentally instantiate another instance somewhere.
6
6 revs, 2 users 99%

Since Apple has now clarified that static struct variables are initialized both lazy and wrapped in dispatch_once (see the note at the end of the post), I think my final solution is going to be:

class WithSingleton {
    class var sharedInstance: WithSingleton {
        struct Singleton {
            static let instance = WithSingleton()
        }

        return Singleton.instance
    }
}

This takes advantage of the automatic lazy, thread-safe initialization of static struct elements, safely hides the actual implementation from the consumer, keeps everything compactly compartmentalized for legibility, and eliminates a visible global variable.

Apple has clarified that lazy initializer are thread-safe, so there's no need for dispatch_once or similar protections

The lazy initializer for a global variable (also for static members of structs and enums) is run the first time that global is accessed, and is launched as dispatch_once to make sure that the initialization is atomic. This enables a cool way to use dispatch_once in your code: just declare a global variable with an initializer and mark it private.

From here


To confirm: global variables have lazy, thread-safe initialization, but class variables don't. Right?
I would add that a good practice would be to declare the initializer as private: private init() {}, to further enforce the fact that this class is not meant to be externally instantiated.
so static struct var initialization is lazy and thread safe, what if that static struct var is a dictionary for multitons, then we have to manually synchronize/queue calls to it for each access, right?
If I understand your question correctly, dictionary and array accesses are not inherently thread-safe, so you will need to use some form of thread synchronization.
@DavidBerry How should I call a function inside this singleton class? I need a function to be called on the first call of myClass.sharedInstance.
J
Jack

For Swift 1.2 and beyond:

class Singleton  {
   static let sharedInstance = Singleton()
}

With a proof of correctness (all credit goes here), there is little to no reason now to use any of the previous methods for singletons.

Update: This is now the official way to define singletons as described in the official docs!

As for concerns on using static vs class. static should be the one to use even when class variables become available. Singletons are not meant to be subclassed since that would result in multiple instances of the base singleton. Using static enforces this in a beautiful, Swifty way.

For Swift 1.0 and 1.1:

With the recent changes in Swift, mostly new access control methods, I am now leaning towards the cleaner way of using a global variable for singletons.

private let _singletonInstance = SingletonClass()
class SingletonClass {
  class var sharedInstance: SingletonClass {
    return _singletonInstance
  }
}

As mentioned in the Swift blog article here:

The lazy initializer for a global variable (also for static members of structs and enums) is run the first time that global is accessed, and is launched as dispatch_once to make sure that the initialization is atomic. This enables a cool way to use dispatch_once in your code: just declare a global variable with an initializer and mark it private.

This way of creating a singleton is thread safe, fast, lazy, and also bridged to ObjC for free.


Anyone reading only this answer: Remember to make the token static, otherwise the behavior is undefined. See David's edited question for the complete code.
@nschum otherwise, the behaviour is not undefined, it is just broken in a well-defined way: the block will always execute.
@Michael: The documentation states it is undefined. The current behavior is therefore coincidental.
That's an odd thing to say. If the documentation calls it "undefined" that just means whoever wrote the code doesn't make any promises to what it does. It has nothing to do with the code knowing if the variable is static. It just means that the current (or apparent) behavior cannot not be relied upon.
You might want to add private init() {} as initialiser of SingletonClass. to prevent instantiate from outside.
S
Sahil

Swift 1.2 or later now supports static variables/constants in classes. So you can just use a static constant:

class MySingleton {

    static let sharedMySingleton = MySingleton()

    private init() {
        // ...
    }
}

N
Nilanshu Jaiswal

There is a better way to do it. You can declare a global variable in your class above the class declaration like this:

var tpScopeManagerSharedInstance = TPScopeManager()

This just calls your default init or whichever init and global variables are dispatch_once by default in Swift. Then in whichever class you want to get a reference, you just do this:

var refrence = tpScopeManagerSharedInstance
// or you can just access properties and call methods directly
tpScopeManagerSharedInstance.someMethod()

So basically you can get rid of the entire block of shared instance code.


Why a "var" and lot a "let"?
maybe could be a let, I only tested it out with a var.
I like this answer, however I need to access this (Singleton) from Interface Builder. Any idea on how could I access this tpScopeManagerSharedInstance from within IB?. Thanks.-
This is my preferred way of having a singleton. It has all the usual features (thread-safety & lazy instantiation) and it supports a very lightweight syntax: no need to write TPScopeManager.sharedInstance.doIt() all the time, just name your class TPScopeManagerClass, have this declaration next to the class public let TPScopeManager = TPScopeManagerClass(), and when using just write TPScopeManager.doIt(). Very clean!
There's nothing here to prevent creation of additional instances of TPScopeManager, and it is therefore not a singleton by definition.
N
Nilanshu Jaiswal

Swift singletons are exposed in the Cocoa frameworks as class functions, e.g. NSFileManager.defaultManager(), NSNotificationCenter.defaultCenter(). So it makes more sense as a class function to mirror this behavior, rather than a class variable as some other solutions. e.g:

class MyClass {

    private static let _sharedInstance = MyClass()

    class func sharedInstance() -> MyClass {
        return _sharedInstance
    }
}

Retrieve the singleton via MyClass.sharedInstance().


upvoted for the comment of LearnCocos2D :) , also for the style.
the global variable should be changed to a class variable via a static inside the class.
@malhal when a variable is marked private but outside a class, it's not global - but scoped only to the file it is in. A static inside the class would work pretty much the same, but i've updated the answer to use the static as you suggested, as it better groups the variable to the class if you happen to use multiple classes within the file.
"Swift Singletons are exposed in the cocoa frameworks as class functions" ... Not in Swift 3. They're now usually static properties.
A
Adrian Macneil

Per the Apple documentation, it has been repeated many times that the easiest way to do this in Swift is with a static type property:

class Singleton {
    static let sharedInstance = Singleton()
}

However, if you're looking for a way to perform additional setup beyond a simple constructor call, the secret is to use an immediately invoked closure:

class Singleton {
    static let sharedInstance: Singleton = {
        let instance = Singleton()
        // setup code
        return instance
    }()
}

This is guaranteed to be thread-safe and lazily initialized only once.


how can you set the static let instance back to nil?
@user1463853 - You can't, and generally shouldn't.
A
Adam Smaka

Swift 4+

protocol Singleton: class {
    static var sharedInstance: Self { get }
}

final class Kraken: Singleton {
    static let sharedInstance = Kraken()
    private init() {}
}

this needs final class, can you explain more the difference, coz I have issue with the other solution of singleton with struct
should that be private override init() {}
P
Peter Mortensen

Looking at Apple's sample code I came across this pattern. I'm not sure how Swift deals with statics, but this would be thread safe in C#. I include both the property and method for Objective-C interop.

struct StaticRank {
    static let shared = RankMapping()
}

class func sharedInstance() -> RankMapping {
    return StaticRank.shared
}

class var shared:RankMapping {
    return StaticRank.shared
}

I am pretty sure that just using this default static syntax will do the all annoying jobs.
unfortunately statics only work inside of structs, so that's why this pattern.
My intention was that we don't have to use dispatch_once stuffs. I am betting on your style. :)
Isn't class within a class declaration the equivalent of static in a struct declaration?
@Sam Yes it is. See the Apple blog entry on Files and Initialization which makes it clear that both globals and static members of structs and enums benefit from this dispatch_once capability.
o
onmyway133

In brief,

class Manager {
    static let sharedInstance = Manager()
    private init() {}
}

You may want to read Files and Initialization

The lazy initializer for a global variable (also for static members of structs and enums) is run the first time that global is accessed, and is launched as dispatch_once to make sure that the initialization is atomic.


M
Michael

If you are planning on using your Swift singleton class in Objective-C, this setup will have the compiler generate appropriate Objective-C-like header(s):

class func sharedStore() -> ImageStore {
struct Static {
    static let instance : ImageStore = ImageStore()
    }
    return Static.instance
}

Then in Objective-C class you can call your singleton the way you did it in pre-Swift days:

[ImageStore sharedStore];

This is just my simple implementation.


This is actually more concise & correct than the other example because it is implemented the same way as other Swift singletons are. i.e: as class functions like NSFileManager.defaultManager(), but still uses the lazy thread-safe static member mechanisms of Swift.
Cocoa generally implements these as static properties, nowadays, not as class functions.
I am aware of that, my comment is over 2 years old. Thanks for mentioning.
P
Peter Mortensen

First solution

let SocketManager = SocketManagerSingleton();

class SocketManagerSingleton {

}

Later in your code:

func someFunction() {        
    var socketManager = SocketManager        
}

Second solution

func SocketManager() -> SocketManagerSingleton {
    return _SocketManager
}
let _SocketManager = SocketManagerSingleton();

class SocketManagerSingleton {

}

And later in your code you will be able to keep braces for less confusion:

func someFunction() {        
    var socketManager = SocketManager()        
}

K
Kemal Can Kaynak
final class MySingleton {
     private init() {}
     static let shared = MySingleton()
}

Then call it;

let shared = MySingleton.shared

Well done for not only marking init as private, but also for making the sharedMyModel as final! For the sake of future readers, in Swift 3, we might be inclined to rename sharedMyModel to be simply shared.
This is the only correct answer, except that the override and call to super.init are erroneous and will not even compile.
P
Peter Mortensen

Use:

class UtilSingleton: NSObject {

    var iVal: Int = 0

    class var shareInstance: UtilSingleton {
        get {
            struct Static {
                static var instance: UtilSingleton? = nil
                static var token: dispatch_once_t = 0
            }
            dispatch_once(&Static.token, {
                Static.instance = UtilSingleton()
            })
            return Static.instance!
        }
    }
}

How to use:

UtilSingleton.shareInstance.iVal++
println("singleton new iVal = \(UtilSingleton.shareInstance.iVal)")

This is exactly the same as one of the answers I went through on the way to the current answer. Since global variables are initialized both lazy and thread-safe, there's no reason for the additional complexity.
@David Other than not having a global variable. :)
@hpique no, exactly like one of my earlier attempts. Look at the edit history.
P
Peter Mortensen

The best approach in Swift above 1.2 is a one-line singleton, as -

class Shared: NSObject {

    static let sharedInstance = Shared()

    private override init() { }
}

To know more detail about this approach you can visit this link.


Why a NSObject subclass?. Apart from that, this seems to be essentially the same as stackoverflow.com/a/28436202/1187415.
R
Rukshan

From Apple Docs (Swift 3.0.1),

You can simply use a static type property, which is guaranteed to be lazily initialized only once, even when accessed across multiple threads simultaneously:

class Singleton {
    static let sharedInstance = Singleton()
}

If you need to perform additional setup beyond initialization, you can assign the result of the invocation of a closure to the global constant:

class Singleton {
    static let sharedInstance: Singleton = {
        let instance = Singleton()
        // setup code
        return instance
    }()
}

N
Nilanshu Jaiswal

I would suggest an enum, as you would use in Java, e.g.

enum SharedTPScopeManager: TPScopeManager {
    case Singleton
}

IMO, this is the only correct Swift way to implement Singleton. other answers are ObjC/C/C++ way
Could you elaborate on this answer? It's not clear to me where Singleton is instantiated from this snippet
@KennyWinker I don't have an Apple developer login, therefore no swift and so I can't answer when the initialisation occurs. In Java, it is on first use. Perhaps you could try it with a print on initialization and see if the print occurs at launch or after access. It will depend on how enum is implemented by the compiler.
@KennyWinkler: Apple have just clarified how this works, see developer.apple.com/swift/blog/?id=7. In it they say "run the initializer for a global the first time it is referenced, similar to Java" and in particular. They also say that under the covers they are using "dispatch_once to make sure that the initialization is atomic". Therefore enum is almost certainly the way to go unless you have some fancy init to do, then a private static let is the solution.
S
SchoonSauce

Just for reference, here is an example Singleton implementation of Jack Wu/hpique's Nested Struct implementation. The implementation also shows how archiving could work, as well as some accompanying functions. I couldn't find this complete of an example, so hopefully this helps somebody!

import Foundation

class ItemStore: NSObject {

    class var sharedStore : ItemStore {
        struct Singleton {
            // lazily initiated, thread-safe from "let"
            static let instance = ItemStore()
        }
        return Singleton.instance
    }

    var _privateItems = Item[]()
    // The allItems property can't be changed by other objects
    var allItems: Item[] {
        return _privateItems
    }

    init() {
        super.init()
        let path = itemArchivePath
        // Returns "nil" if there is no file at the path
        let unarchivedItems : AnyObject! = NSKeyedUnarchiver.unarchiveObjectWithFile(path)

        // If there were archived items saved, set _privateItems for the shared store equal to that
        if unarchivedItems {
            _privateItems = unarchivedItems as Array<Item>
        } 

        delayOnMainQueueFor(numberOfSeconds: 0.1, action: {
            assert(self === ItemStore.sharedStore, "Only one instance of ItemStore allowed!")
        })
    }

    func createItem() -> Item {
        let item = Item.randomItem()
        _privateItems.append(item)
        return item
    }

    func removeItem(item: Item) {
        for (index, element) in enumerate(_privateItems) {
            if element === item {
                _privateItems.removeAtIndex(index)
                // Delete an items image from the image store when the item is 
                // getting deleted
                ImageStore.sharedStore.deleteImageForKey(item.itemKey)
            }
        }
    }

    func moveItemAtIndex(fromIndex: Int, toIndex: Int) {
        _privateItems.moveObjectAtIndex(fromIndex, toIndex: toIndex)
    }

    var itemArchivePath: String {
        // Create a filepath for archiving
        let documentDirectories = NSSearchPathForDirectoriesInDomains(NSSearchPathDirectory.DocumentDirectory, NSSearchPathDomainMask.UserDomainMask, true)
        // Get the one document directory from that list
        let documentDirectory = documentDirectories[0] as String
        // append with the items.archive file name, then return
        return documentDirectory.stringByAppendingPathComponent("items.archive")
    }

    func saveChanges() -> Bool {
        let path = itemArchivePath
        // Return "true" on success
        return NSKeyedArchiver.archiveRootObject(_privateItems, toFile: path)
    }
}

And if you didn't recognize some of those functions, here is a little living Swift utility file I've been using:

import Foundation
import UIKit

typealias completionBlock = () -> ()

extension Array {
    func contains(#object:AnyObject) -> Bool {
        return self.bridgeToObjectiveC().containsObject(object)
    }

    func indexOf(#object:AnyObject) -> Int {
        return self.bridgeToObjectiveC().indexOfObject(object)
    }

    mutating func moveObjectAtIndex(fromIndex: Int, toIndex: Int) {
        if ((fromIndex == toIndex) || (fromIndex > self.count) ||
            (toIndex > self.count)) {
                return
        }
        // Get object being moved so it can be re-inserted
        let object = self[fromIndex]

        // Remove object from array
        self.removeAtIndex(fromIndex)

        // Insert object in array at new location
        self.insert(object, atIndex: toIndex)
    }
}

func delayOnMainQueueFor(numberOfSeconds delay:Double, action closure:()->()) {
    dispatch_after(
        dispatch_time(
            DISPATCH_TIME_NOW,
            Int64(delay * Double(NSEC_PER_SEC))
        ),
        dispatch_get_main_queue()) {
            closure()
    }
}

V
Vicky Prajapati

In swift, you can create a singleton class following way:

class AppSingleton: NSObject {

    //Shared instance of class
    static let sharedInstance = AppSingleton()

    override init() {
        super.init()
    }
}

D
Dark Matter

I prefer this implementation:

class APIClient {

}

var sharedAPIClient: APIClient = {
    return APIClient()
}()

extension APIClient {
    class func sharedClient() -> APIClient {
        return sharedAPIClient
    }
}

u
user2737730

My way of implementation in Swift...

ConfigurationManager.swift

import Foundation

    let ConfigurationManagerSharedInstance = ConfigurationManager()
 class ConfigurationManager : NSObject {
    var globalDic: NSMutableDictionary = NSMutableDictionary()

class var sharedInstance:ConfigurationManager {
    return ConfigurationManagerSharedInstance

}

init() {

    super.init()

    println ("Config Init been Initiated, this will be called only onece irrespective of many calls")   

}

Access the globalDic from any screen of the application by the below.

Read:

 println(ConfigurationManager.sharedInstance.globalDic)  

Write:

 ConfigurationManager.sharedInstance.globalDic = tmpDic // tmpDict is any value that to be shared among the application

E
Ely

The only right approach is below.

final class Singleton {
    static let sharedInstance: Singleton = {
        let instance = Singleton()
        // setup code if anything
        return instance
    }()

    private init() {}
}

To Access

let signleton = Singleton.sharedInstance

Reasons:

static type property is guaranteed to be lazily initialized only once, even when accessed across multiple threads simultaneously, so no need of using dispatch_once

Privatising the init method so instance can't be created by other classes.

final class as you do not want other classes to inherit Singleton class.


Why did you use the closure initialization while you can directly use static let sharedInstance = Singleton()
if you do not want to do any additional setup then what you saying is right.
N
Nilanshu Jaiswal

After seeing David's implementation, it seems like there is no need to have a singleton class function instanceMethod since let is doing pretty much the same thing as a sharedInstance class method. All you need to do is declare it as a global constant and that would be it.

let gScopeManagerSharedInstance = ScopeManager()

class ScopeManager {
   // No need for a class method to return the shared instance. Use the gScopeManagerSharedInstance directly. 
}

As I say in my comments, the only reason to do it is that at some point in the future you can move/hide the global variable and get more singleton-like behavior. At that point, if everything is using a consistent pattern, you can just change the singleton classes themselves without having to change the usage.
D
DD.amor
   func init() -> ClassA {
    struct Static {
        static var onceToken : dispatch_once_t = 0
        static var instance : ClassA? = nil
    }

    dispatch_once(&Static.onceToken) {
        Static.instance = ClassA()
    }

    return Static.instance!
}

As has been discussed at great length here, it's not necessary in swift to wrap initialization in dispatch_once since static variable initialization is lazy and automatically protected via dispatch_once Apple actually recommends using statics instead of dispatch_once for that reason.
T
Tim

Swift to realize singleton in the past, is nothing more than the three ways: global variables, internal variables and dispatch_once ways.

Here are two good singleton.(note: no matter what kind of writing will must pay attention to the init () method of privatisation.Because in Swift, all the object's constructor default is public, needs to be rewritten init can be turned into private, prevent other objects of this class '()' by default initialization method to create the object.)

Method 1:

class AppManager {
    private static let _sharedInstance = AppManager()

    class func getSharedInstance() -> AppManager {
       return _sharedInstance
    }

    private init() {} // Privatizing the init method
}

// How to use?
AppManager.getSharedInstance()

Method 2:

class AppManager {
    static let sharedInstance = AppManager()

    private init() {} // Privatizing the init method
}

// How to use?
AppManager.sharedInstance

M
Mojtaba Hosseini

Swift 5.2

You can point to the type with Self. So:

static let shared = Self()

And should be inside a type, like:

class SomeTypeWithASingletonInstance {
   static let shared = Self()
}

A
Abhishek Biswas

This is the simplest one with thread safe capabilities. No other thread can access the same singleton object even if they want. Swift 3/4

struct DataService {

    private static var _instance : DataService?

    private init() {}   //cannot initialise from outer class

    public static var instance : DataService {
        get {
            if _instance == nil {
                DispatchQueue.global().sync(flags: .barrier) {
                    if _instance == nil {
                        _instance = DataService()
                    }
                }
            }
            return _instance!
        }
    }
}

What is the advantage over a static type property (which is guaranteed to be lazily initialized only once, even when accessed across multiple threads simultaneously) ?
N
Nilanshu Jaiswal

I required my singleton to allow inheritance, and none of these solutions actually allowed it. So I came up with this:

public class Singleton {
    private static var sharedInstanceVar = Singleton()

    public class func sharedInstance() -> Singleton {
        return sharedInstanceVar
    }
}


public class SubSingleton: Singleton {

    private static var sharedInstanceToken: dispatch_once_t = 0

    public class override func sharedInstance() -> SubSingleton {
        dispatch_once(&sharedInstanceToken) {
            sharedInstanceVar = SubSingleton()
        }
    return sharedInstanceVar as! SubSingleton
    }
}

This way when doing Singleton.sharedInstance() first it will return the instance of Singleton

When doing SubSingleton.sharedInstance() first it will return the instance of SubSingleton created.

If the above is done, then SubSingleton.sharedInstance() is Singleton is true and the same instance is used.

The issue with this first dirty approach is that I cannot guarantee that subclasses would implement the dispatch_once_t and make sure that sharedInstanceVar is only modified once per class.

I will try to refine this further, but it would be interesting to see if anyone has strong feelings against this (besides the fact that it is verbose and requires to manually update it).


P
Peter Mortensen

This is my implementation. It also prevents the programmer from creating a new instance:

let TEST = Test()

class Test {

    private init() {
        // This is a private (!) constructor
    }
}

private init was already suggested here: stackoverflow.com/a/28436202/1187415.
N
Nilanshu Jaiswal

I use the following syntax:

public final class Singleton {    
    private class func sharedInstance() -> Singleton {
        struct Static {
            //Singleton instance.
            static let sharedInstance = Singleton()
        }
        return Static.sharedInstance
    }

    private init() { }

    class var instance: Singleton {
        return sharedInstance()
    }
}

This works from Swift 1.2 up to 4, and has several advantages:

Reminds the user not to subclass implementation Prevents creation of additional instances Ensures lazy creation and unique instantiation Shortens syntax (avoids ()) by allowing to access instance as Singleton.instance