For me this is the best way, make init private. **Swift 3 \ 4 \ 5 syntax**


    // MARK: - Singleton

    final class Singleton {
        
        // Can&#39;t init is singleton
        private init() { }
        
        // MARK: Shared Instance
        
        static let shared = Singleton()
        
        // MARK: Local Variable
        
        var emptyStringArray = [String]()
        
    }



You can initialize an empty array like this.

    class Singleton {
        
        //MARK: Shared Instance
        
        static let sharedInstance : Singleton = {
            let instance = Singleton(array: [])
            return instance
        }()
        
        //MARK: Local Variable
    
        var emptyStringArray : [String]
        
        //MARK: Init
    
        init( array : [String]) {
            emptyStringArray = array
        }
    }

Or if you prefer a different approach, this one will do fine.

    class Singleton {
        
        //MARK: Shared Instance
        
        static let sharedInstance : Singleton = {
            let instance = Singleton()
            return instance
        }()
        
        //MARK: Local Variable
    
        var emptyStringArray : [String]? = nil
        
        //MARK: Init
    
        convenience init() {
            self.init(array : [])
        }
        
        //MARK: Init Array
    
        init( array : [String]) {
            emptyStringArray = array
        }
    }



As per the apple&#39;s documentation: [In Swift, you can simply use a static type property, which is guaranteed to be lazily initialized only once, even when accessed across multiple threads simultaneously](https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html).

    class Singleton {

        // MARK: - Shared

        static let shared = Singleton()
    }

With initialization method:

    class Singleton {
        
        // MARK: - Shared
        
        static let shared = Singleton()
        
        // MARK: - Initializer
        
        private init() {
        }
        
    }

Any initialisation would be done in an init method. No difference here between a singleton and a non-singleton. 

I have a long string, which I build using ES6 template strings, but I want it to be without line breaks:

    var string = `As all string substitutions in Template Strings are JavaScript
                  expressions, we can substitute a lot more than variable names.
                  For example, below we can use expression interpolation to 
                  embed for some readable inline math:`

    console.log(string);
Result:

    As all string substitutions in Template Strings are JavaScript
    expressions, we can substitute a lot more than variable names.
    For example, below we can use expression interpolation to
    embed for some readable inline math:

My expectations:

    As all string substitutions in Template Strings are JavaScript expressions, we can substitute a lot more than variable names. For example, below we can use expression interpolation to embed for some readable inline math:

Template Strings ES6 prevent line breaks

When should I declare my function as:

`void foo(Widget w);`

as opposed to

`void foo(Widget&amp;&amp; w);`?

Assume this is the only overload (as in, I pick one or the other, not both, and no other overloads). No templates involved. Assume that the function `foo` requires ownership of the `Widget` (e.g. `const Widget&amp;` is not part of this discussion). I&#39;m not interested in any answer outside the scope of these circumstances. See addendum at end of post for *why* these constraints are part of the question.

The primary difference that my colleagues and I can come up with is that the rvalue reference parameter forces you to be explicit about copies. The caller is responsible for making an explicit copy and then passing it in with `std::move` when you want a copy. In the pass by value case, the cost of the copy is hidden:

        //If foo is a pass by value function, calling + making a copy:
        Widget x{};
        foo(x); //Implicit copy
        //Not shown: continues to use x locally
        
        //If foo is a pass by rvalue reference function, calling + making a copy:
        Widget x{};
        //foo(x); //This would be a compiler error
        auto copy = x; //Explicit copy
        foo(std::move(copy));
        //Not shown: continues to use x locally

Other than that difference. Other than forcing people to be explicit about copying and changing how much syntactic sugar you get when calling the function, how else are these different? What do they say differently about the interface? Are they more or less efficient than one another?

Other things that my colleagues and I have already thought of:

 * The rvalue reference parameter means that you *may* move the argument, but does not mandate it. It is possible that the argument you passed in at the call site will be in its original state afterwards. It&#39;s also possible the function would eat/change the argument without ever calling a move constructor but assume that because it was an rvalue reference, the caller relinquished control. Pass by value, if you move into it, you must assume that a move happened; there&#39;s no choice.
 * Assuming no elisions, a single move constructor call is eliminated with pass by rvalue.
 * The compiler has better opportunity to elide copies/moves with pass by value. Can anyone substantiate this claim? Preferably with a link to gcc.godbolt.org showing optimized generated code from gcc/clang rather than a line in the standard. My attempt at showing this was probably not able to successfully isolate the behavior: https://godbolt.org/g/4yomtt


**Addendum:** *why* am I constraining this problem so much?

 * No overloads - if there were other overloads, this would devolve into a discussion of pass by value vs a set of overloads that include both const reference and rvalue reference, at which point the set of overloads is obviously more efficient and wins. This is well known, and therefore not interesting.
 * No templates - I&#39;m not interested in how forwarding references fit into the picture. If you have a forwarding reference, you call std::forward anyway. The goal with a forwarding reference is to pass things as you received them. Copies aren&#39;t relevant because you just pass an lvalue instead. It&#39;s well known, and not interesting.
 * `foo` requires ownership of `Widget` (aka no `const Widget&amp;`) - We&#39;re not talking about read-only functions. If the function was read-only or didn&#39;t need to own or extend the lifetime of the `Widget`, then the answer trivially becomes `const Widget&amp;`, which again, is well known, and not interesting. I also refer you to why we don&#39;t want to talk about overloads.

Pass by value vs pass by rvalue reference

In Apple&#39;s [Using Swift with Cocoa and Objective-C document](https://developer.apple.com/library/prerelease/content/documentation/Swift/Conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html#//apple_ref/doc/uid/TP40014216-CH7-ID6) (updated for Swift 3) they give the following example of the Singleton pattern:

    class Singleton {
        static let sharedInstance: Singleton = {
            let instance = Singleton()

            // setup code

            return instance
        }()
    }

Let&#39;s imagine that this singleton needs to manage a variable array of Strings. How/where would I declare that property and ensure it gets initialized properly to an empty `[String]` array?

Singleton with properties in Swift 3

In Apple's <a href="https://developer.apple.com/library/prerelease/content/documentation/Swift/Conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html#//apple_ref/doc/uid/TP40014216-CH7-ID6" target="_blank" rel="noopener noreferrer">Using Swift with Cocoa and Objective-C document</a> (updated for Swift 3) they give the following example of the Singleton pattern:
<pre><code class="hljs language-csharp">class Singleton {
 static let sharedInstance: Singleton = {
 let instance = Singleton()

 // setup code

 return instance
 }()
}
</code></pre>
Let's imagine that this singleton needs to manage a variable array of Strings. How/where would I declare that property and ensure it gets initialized properly to an empty <code>[String]</code> array?

So let&#39;s say I have a singleton class instance that I register in the DI like this:

    services.AddSingleton&lt;IFoo, Foo&gt;();

And let&#39;s say the `Foo` class has a number of other dependencies (mostly repository classes that allow it to load data). 

With my current understanding, the Foo instance is not created until it&#39;s first used (asked). Is there a way to initialize this class other than the constructor? Like right after `ConfigureServices()` completes? Or should the initialization code (loading data from db) be done in Foo&#39;s constructor?

(It would be nice if this class could load its data before the first use to speed up first time access)

ASP.NET Core initialize singleton after configuring DI

I&#39;m trying to get my app working in **Xcode 8.0**, and am running into an error. I know this code worked fine in previous versions of swift, but I&#39;m assuming the code for this is changed in the new version. Here&#39;s the code I&#39;m trying to run:

    let settings = UIUserNotificationSettings(forTypes: [.Sound, .Alert, .Badge], categories: nil)     
    UIApplication.sharedApplication().registerUserNotificationSettings(settings)
    UIApplication.shared().registerForRemoteNotifications()

The error that I&#39;m getting is &quot;Argument labels &#39;(forTypes:, categories:)&#39; do not match any available overloads&quot;

Is there a different command that I could try to get this working?

Registering for Push Notifications in Xcode 8/Swift 3.0?

I have this in Playground using Swift 3, Xcode 8.0:

    import Foundation
    class Person: NSObject, NSCoding {
        var name: String
        var age: Int
        init(name: String, age: Int) {
            self.name = name
            self.age = age
        }
        required convenience init(coder aDecoder: NSCoder) {
            let name = aDecoder.decodeObject(forKey: &quot;name&quot;) as! String
            let age = aDecoder.decodeObject(forKey: &quot;age&quot;) as! Int
            self.init(
                name: name,
                age: age
            )
        }
        func encode(with aCoder: NSCoder) {
            aCoder.encode(name, forKey: &quot;name&quot;)
            aCoder.encode(age, forKey: &quot;age&quot;)
        }
    }

***
    
create array of Person

    let newPerson = Person(name: &quot;Joe&quot;, age: 10)
    var people = [Person]()
    people.append(newPerson)

***    
encode the array

    let encodedData = NSKeyedArchiver.archivedData(withRootObject: people)
    print(&quot;encodedData: \(encodedData))&quot;)

***    
save to userDefaults

    let userDefaults: UserDefaults = UserDefaults.standard()
    userDefaults.set(encodedData, forKey: &quot;people&quot;)
    userDefaults.synchronize()

***    
check

    print(&quot;saved object: \(userDefaults.object(forKey: &quot;people&quot;))&quot;)
    
***
retreive from userDefaults

    if let data = userDefaults.object(forKey: &quot;people&quot;) {
        let myPeopleList = NSKeyedUnarchiver.unarchiveObject(with: data as! Data)
        print(&quot;myPeopleList: \(myPeopleList)&quot;)
    }else{
        print(&quot;There is an issue&quot;)
    }

***    
just check the archived data

    if let myPeopleList = NSKeyedUnarchiver.unarchiveObject(with: encodedData){
       print(&quot;myPeopleList: \(myPeopleList)&quot;)
    }else{
       print(&quot;There is an issue&quot;)
    }

I&#39;m not able to correctly save the data object to userDefaults, and in addition, the check at the bottom creates the error:

  &gt; Fatal error: Unexpectedly found nil while unwrapping an Optional value

The &quot;check&quot; line also shows the saved object is nil. Is this an error in my object&#39;s NSCoder?

Swift saving and retrieving custom object from UserDefaults

Unfortunately the new Core Data semantics make me crazy. My previous question had a clean code that didn&#39;t work because of incorrect auto generation of header files. Now I continue my work with deleting objects. 
My code seems to be very simple: 

    func deleteProfile(withID: Int) {
        let fetchRequest: NSFetchRequest&lt;Profile&gt; = Profile.fetchRequest()
        fetchRequest.predicate = Predicate.init(format: &quot;profileID==\(withID)&quot;)
        let object = try! context.fetch(fetchRequest)
        context.delete(object)
    } 
I did a &quot;hard&quot; debug with `print(object)` instead of `context.delete(object)` and it showed me the right object.
So I need just to delete it.

P.S. there is no `deleteObject`. Now NSManagedContext has only `public func delete(_ sender: AnyObject?)`

Swift 3 Core Data Delete Object

With Swift 3 leaning towards `Data` instead of `[UInt8]`, I&#39;m trying to ferret out what the most efficient/idiomatic way to encode/decode swifts various number types (UInt8, Double, Float, Int64, etc) as Data objects.

There&#39;s [this answer for using \[UInt8\]][1], but it seems to be using various pointer APIs that I can&#39;t find on Data.

I&#39;d like to basically some custom extensions that look something like:

    let input = 42.13 // implicit Double
    let bytes = input.data
    let roundtrip = bytes.to(Double) // --&gt; 42.13

The part that really eludes me, I&#39;ve looked through a bunch of the docs, is how I can get some sort of pointer thing (OpaquePointer or BufferPointer or UnsafePointer?) from any basic struct (which all of the numbers are). In C, I would just slap an ampersand in front of it, and there ya go.

  [1]: https://stackoverflow.com/questions/26953591/how-to-convert-a-double-into-a-byte-array-in-swift

round trip Swift number types to/from Data

In earlier versions of Swift, one could create a delay with the following code:

    let time = dispatch_time(dispatch_time_t(DISPATCH_TIME_NOW), 4 * Int64(NSEC_PER_SEC))
    dispatch_after(time, dispatch_get_main_queue()) {
        //put your code which should be executed with a delay here
    }

But now, in Swift 3, Xcode automatically changes 6 different things but then the following error appears: &quot;Cannot convert `DispatchTime.now` to expected value `dispatch_time_t` aka `UInt64`.&quot;

How can one create a delay before running a sequence of code in Swift 3?

Content Type	Original Author	Original Content on Stackoverflow
Question	RobertJoseph	View Question on Stackoverflow
Solution 1 - Singleton	YannSteph	View Answer on Stackoverflow
Solution 2 - Singleton	user6375148	View Answer on Stackoverflow
Solution 3 - Singleton	Mehul Sojitra	View Answer on Stackoverflow
Solution 4 - Singleton	gnasher729	View Answer on Stackoverflow

Singleton with properties in Swift 3

Singleton Problem Overview

Singleton Solutions

Solution 1 - Singleton

Solution 2 - Singleton

Solution 3 - Singleton

Solution 4 - Singleton

Pass by value vs pass by rvalue reference

Template Strings ES6 prevent line breaks

Attributions