EDIT: The previous answers alluded to in the comments don't answer the question, which was how to determine if any given Type was a reference type and how to safely conform said type to AnyObject.
Testing against the passed type doesn't work, as the underlying type could be optional, or it could be a protocol, in which case one needs to know the passed instance is a class type or value type.
The solution I came up with is similar to the revised answer provided below.
So I have a new dependency injection framework, Factory.
Factory allows for scoped instances, basically allowing you to cache services once they're created. And one of those scopes is shared. Any instance shared will be cached and returned just as long as someone in the outside world maintains a strong reference to it. After the last reference releases the object the cache releases the object and a new instance will be created on the next resolution.
This is implemented, obviously, as simply maintaining a weak reference to the created object. If the weak reference is nil it's time to create a new object.
And therein lies the problem
Weak references can only apply to reference types.
Factory uses generics internally to manage type information. But I can create Factories of any type: Classes, structs, strings, whatever.)
Scopes use dictionaries of boxed types internally. If an instance exists in the cache and in the box it's returned. So what I'd like to do is create this...
private struct WeakBox<T:AnyObject>: AnyBox {
weak var boxed: T
}
The AnyObject conformance is need in order to allow weak. You get a compiler error otherwise. Now I want to box and cache an object in my shared scope with something like this...
func cache<T>(id: Int, instance: T) {
cache[id] = WeakBox(boxed: instance)
}
But this also gives a compiler error. (Generic struct WeakBox requires T to be a class type.)
So how to bridge from on to the other? Doing the following doesn't work. Swift shows a warning that "Conditional cast from 'T' to 'AnyObject' always succeeds" and then converts the type anyway.
func cache<T>(id: Int, instance: T) {
if let instance = instance as? AnyObject {
cache[id] = WeakBox(boxed: instance)
}
}
I'd be happy with the following, but again, same problem. You can't test for class conformance and you can't conditionally cast to AnyObject. Again, it always succeeds.
private struct WeakBox: AnyBox {
weak var boxed: AnyObject?
}
func cache<T>(id: Int, instance: T) {
if let instance = instance as? AnyObject {
cache[id] = WeakBox(boxed: instance)
}
}
What I'm doing at the moment is something like...
private struct WeakBox: AnyBox {
weak var boxed: AnyObject?
}
func cache<T>(id: Int, instance: T) {
cache[id] = WeakBox(boxed: instance as AnyObject)
}
Which works, but that instance as AnyObject cast depends on some very weird Swift to Objective-C bridging behavior.
Not being able to test for class conformance at runtime is driving me bonkers, and seems like a semi-major loophole in the language.
You can't test for conformance, and you can't cast for conformance.
So what can you do?
As Martin notes in a comment, any value can be cast to AnyObject in Swift, because Swift will wrap value types in an opaque _SwiftValue class, and the cast will always succeed. There is a way around this, though.
The way to check whether a value is a reference type without this implicit casting is to check whether its type is AnyObject.Type, like so:
func printIsObject(_ value: Any) {
if type(of: value) is AnyObject.Type {
print("Object")
} else {
print("Other")
}
}
class Foo {}
struct Bar {}
enum Quux { case q }
printIsObject(Foo()) // => Object
printIsObject(Bar()) // => Other
printIsObject(Quux.q) // => Other
Note that it's crucial that you check whether the type is AnyObject.Type not is AnyObject. T.self, the object representing the type of the value, is itself an object, so is AnyObject will always succeed. Instead, is AnyObject.Type asks "does this inherit from the metatype of all objects", i.e., "does this object which represents a type inherit from an object that represents all object types?"
Edit: Evidently, I'd forgotten that Swift includes AnyClass as a synonym for AnyObject.Type, so the check can be simplified to be is AnyClass. However, leaving the above as a marginally-expanded explanation for how this works.
If you want this method to also be able to handle Optional values, you're going to have to do a bit of special-casing to add support. Specifically, because Optional<T> is an enum regardless of the type of T, you're going to need to reach in to figure out what T is.
There are a few ways to do this, but because Optional is a generic type, and it's not possible to ask "is this value an Optional<T>?" without knowing what T is up-front, one of the easier and more robust ways to do this is to introduce a protocol which Optional adopts that erases the type of the underlying value while still giving you access to it:
protocol OptionalProto {
var wrappedValue: Any? { get }
}
extension Optional: OptionalProto {
var wrappedValue: Any? {
switch self {
case .none: return nil
case let .some(value):
// Recursively reach in to grab nested optionals as needed.
if let innerOptional = value as? OptionalProto {
return innerOptional.wrappedValue
} else {
return value
}
}
}
}
We can then use this protocol to our advantage in cache:
func cache(id: Int, instance: Any) {
if let opt = instance as? OptionalProto {
if let wrappedValue = opt.wrappedValue {
cache(id: id, instance: wrappedValue)
}
return
}
// In production:
// cache[id] = WeakBox(boxed: instance as AnyObject)
if type(of: instance) is AnyClass {
print("\(type(of: instance)) is AnyClass")
} else {
print("\(type(of: instance)) is something else")
}
}
This approach handles all of the previous cases, but also infinitely-deeply-nested Optionals, and protocol types inside of Optionals:
class Foo {}
struct Bar {}
enum Quux { case q }
cache(id: 1, instance: Foo()) // => Foo is AnyClass
cache(id: 2, instance: Bar()) // => Bar is something else
cache(id: 3, instance: Quux.q) // => Quux is something else
let f: Optional<Foo> = Foo()
cache(id: 4, instance: f) // => Foo is AnyClass
protocol SomeProto {}
extension Foo: SomeProto {}
let p: Optional<SomeProto> = Foo()
cache(id: 5, instance: p) // => Foo is AnyClass
So this took a while to figure out and even longer to track down the clues needed for a solution, so I'm providing my own code and answer to the problem
Given the following protocol...
private protocol OptionalProtocol {
var hasWrappedValue: Bool { get }
var wrappedValue: Any? { get }
}
extension Optional : OptionalProtocol {
var hasWrappedValue: Bool {
switch self {
case .none:
return false
case .some:
return true
}
}
var wrappedValue: Any? {
switch self {
case .none:
return nil
case .some(let value):
return value
}
}
}
And a box type to hold a weak reference...
private protocol AnyBox {
var instance: Any { get }
}
private struct WeakBox: AnyBox {
weak var boxed: AnyObject?
var instance: Any {
boxed as Any
}
}
Then the code to test and box a give type looks like...
func box<T>(_ instance: T) -> AnyBox? {
if let optional = instance as? OptionalProtocol {
if let unwrapped = optional.wrappedValue, type(of: unwrapped) is AnyObject.Type {
return WeakBox(boxed: unwrapped as AnyObject)
}
} else if type(of: instance) is AnyObject.Type {
return WeakBox(boxed: instance as AnyObject)
}
return nil
}
Note that the type passed in could be a class, or a struct or some other value, or it could be a protocol. And it could be an optional version of any of those things.
As such, if it's optional we need to unwrap it and test the actual wrapped type to see if it's a class. If it is, then it's safe to perform our AnyObject cast.
If the passed value isn't optional, then we still need to check to see if it's a class.
There's also a StrongBox type used for non-shared type caching.
struct StrongBox<T>: AnyBox {
let boxed: T
var instance: Any {
boxed as Any
}
}
And the final cache routine looks like this.
func resolve<T>(id: UUID, factory: () -> T) -> T {
defer { lock.unlock() }
lock.lock()
if let box = cache[id], let instance = box.instance as? T {
if let optional = instance as? OptionalProtocol {
if optional.hasWrappedValue {
return instance
}
} else {
return instance
}
}
let instance: T = factory()
if let box = box(instance) {
cache[id] = box
}
return instance
}
Source for the entire project is in the Factory repository.
I have created a Realm object that needs to store an enum value. To do that I use a method outlined in this question which involves declaring a private property of type String, and then declaring another property of type Enum that sets/reads the private property using getters and setters.
For ease of reference here is the code for that:
#objcMembers
class PlaylistRealmObject: Object {
dynamic var id: String = UUID().uuidString
dynamic var created: Date = Date()
dynamic var title: String = ""
private dynamic var revisionTypeRaw: String = RevisionType.noReminder.rawValue
var revisionType: RevisionType {
get { return RevisionType(rawValue: revisionTypeRaw)! }
set { revisionTypeRaw = newValue.rawValue }
}
let reminders = List<ReminderRealmObject>()
let cardsInPlaylist = List<CardRealmObject>()
override static func primaryKey() -> String? {
return "id"
}
}
I have noticed though that if I add a convenience init to the class declaration (to make it a bit easier to initialise the object) the revisionType properties on the objects I end up with adopt the default value declared in the class, and NOT the revision type value passed to the class using the convenience init.
Here is the class declaration with a convenience init
#objcMembers
class PlaylistRealmObject: Object {
dynamic var id: String = UUID().uuidString
dynamic var created: Date = Date()
dynamic var title: String = ""
private dynamic var revisionTypeRaw: String = RevisionType.noReminder.rawValue
var revisionType: RevisionType {
get { return RevisionType(rawValue: revisionTypeRaw)! }
set { revisionTypeRaw = newValue.rawValue }
}
let reminders = List<ReminderRealmObject>()
let cardsInPlaylist = List<CardRealmObject>()
convenience init(title: String, revisionType: RevisionType) {
self.init()
self.title = title
self.revisionType = revisionType
}
override static func primaryKey() -> String? {
return "id"
}
}
And - to make things even more perplexing - if I simply remove the word 'private' from the revisionTypeRaw property, everything works fine!
I am confused. 1) Why does adding a convenience init have this effect? 2) Why does making the property 'public' resolve the issue?
I have created a demo Xcode project to illustrate the issue and can share it if anyone needs it.
Update:
I found the problem. It has nothing to do with the convenience init. I am using #objcMembers at the top of the class as per the Realm docs: https://realm.io/docs/swift/latest/#property-attributes
If you remove this and place #objc in front of the private keyword, everything works as would be expected. I guess the question then is: what explains this behaviour?
This is a good question but I think the issue is elsewhere in the code. Let's test it.
I created a TestClass that has a Realm managed publicly visible var, name, as well as a non-managed public var visibleVar which is backed by a Realm managed private var, privateVar. I also included a convenience init per the question. The important part is the privateVar is being set to the string "placeholder" so we need to see if that is overwritten.
class TestClass: Object {
#objc dynamic var name = ""
#objc private dynamic var privateVar = "placeholder"
var visibleVar: String {
get {
return self.privateVar
}
set {
self.privateVar = newValue
}
}
convenience init(aName: String, aString: String) {
self.init()
self.name = aName
self.visibleVar = aString
}
}
We then create two instances and save them in Realm
let a = TestClass(aName: "some name", aString: "some string")
let b = TestClass(aName: "another name", aString: "another string")
realm.add(a)
realm.add(b)
Then a button action to load the two objects from Realm and print them.
let testResults = realm.objects(TestClass.self)
for test in testResults {
print(test.name, test.visibleVar)
}
and the output:
some name some string
another name another string
So in this case, the default value of "placeholder" is being overwritten correctly when the instances are being created.
Edit:
A bit more info.
By defining your entire class with #objMembers, it exposes your propertites to Objective-C, but then private hides them again. So that property is not exposed to ObjC. To reverse that hiding, you have to say #objc explicitly. So, better practice is to define the managed Realm properties per line with #objc dynamic.
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.
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.
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
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.
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() {
// ...
}
}
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.
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().
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.
Swift 4+
protocol Singleton: class {
static var sharedInstance: Self { get }
}
final class Kraken: Singleton {
static let sharedInstance = Kraken()
private init() {}
}
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
}
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.
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.
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()
}
final class MySingleton {
private init() {}
static let shared = MySingleton()
}
Then call it;
let shared = MySingleton.shared
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)")
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.
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
}()
}
I would suggest an enum, as you would use in Java, e.g.
enum SharedTPScopeManager: TPScopeManager {
case Singleton
}
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()
}
}
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()
}
}
I prefer this implementation:
class APIClient {
}
var sharedAPIClient: APIClient = {
return APIClient()
}()
extension APIClient {
class func sharedClient() -> APIClient {
return sharedAPIClient
}
}
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
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.
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.
}
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!
}
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
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()
}
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!
}
}
}
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).
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
}
}
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