Protocol defining singleton with properties

Question

I have the following protocol that defines a singleton with property:

protocol SingletonProtocol {
    static var shared: SingletonProtocol { get }
    var variable : Int { get set }
}

And the following class that implements this protocol:

class Singleton : SingletonProtocol{
    static let shared : SingletonProtocol = Singleton()
    var variable = 4
}

If I call:

Singleton.shared.variable = 5

I get the following error:

 change 'let' to 'var' to make it mutable

If I implement this class without the protocol I don't get the error and the variable can be changed. I can solve this by adding setVariable: method but I want to access and modify the variable directly.

How can I write a protocol that defines a singleton with variables that can be modified?

You can't really ensure that a conforming type only has a singleton instance, since for that you'd need to declare a private initializer in your class, which you can do, but you can't declare it in `SingletonProtocol`, so a class can conform to your protocol without having only a singleton instance. With your current implementation, it is possible to instantiate any number of `Singleton` instances. — Dávid Pásztor, Jun 13 '18 at 09:11
Thank you @DávidPásztor for this comment, is there a way for a protocol to "enforce" private init in its comforting protocols. — amir, Jun 13 '18 at 09:16
You can set it like `var shared = Singleton.shared shared.variable = 5` — Mukesh, Jun 13 '18 at 10:12
I'm curious, why do you need a protocol here? What other types would conform to `SingletonProtocol`? — Hamish, Jun 13 '18 at 10:40
I want to unitest a singleton class using dependency injection and for that I need a variable in the protocol with the type of an other protocol to set as a mock in the testing. — amir, Jun 13 '18 at 11:27

Rico Crescenzio · Accepted Answer · 2018-06-13T11:54:23.600

7

Make the protocol available only for classes (struct won't be able to conform to this protocol):

protocol SingletonProtocol: AnyObject {
    static var shared: SingletonProtocol { get }
    var variable: Int { get set }
}

Now you can set the shared property as a let

class Singleton: SingletonProtocol {
    static let shared: SingletonProtocol = Singleton()
    var variable: Int = 0
}

edited Jun 13 '18 at 11:54

answered Jun 13 '18 at 09:20

Rico Crescenzio

3,952
1
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28

In your class, don't forget to make your init private – Rico Crescenzio Jun 13 '18 at 09:21
1) with private set I get the error "cannot assign to property: 'shared' setter is inaccessible" 2) how does making it only for classes help me? – amir Jun 13 '18 at 09:26
Sorry, my mistake; that problem is about struct, which are value-types; I changed answer. If a protocol is only for classes, it means that only reference-types can conform to it; a reference type can be immutable even if its members aren't. So now you can use let instead of var – Rico Crescenzio Jun 13 '18 at 09:30
If I do, the class will not be a singleton anymore, shared must be immutable. – amir Jun 13 '18 at 09:41
If you do this, in the class you can declare shared as let, so it’s immutable – Rico Crescenzio Jun 13 '18 at 09:43
And back to the original problem, if I try to do Singleton.shared.variable = 2 I get the error '"change 'let' to 'var' to make it mutable" – amir Jun 13 '18 at 10:03
1

I'm testing in Playground and it works good for me, swift 4.1 – Rico Crescenzio Jun 13 '18 at 10:08

Tom E · Answer 2 · 2018-06-13T09:20:50.807

0

Change

static let shared : SingletonProtocol = Singleton()

to

static var shared : SingletonProtocol = Singleton()
private init() {}

edited Jun 13 '18 at 09:20

answered Jun 13 '18 at 09:09

Tom E

1,530
9
17

If I do, the class will not be a singleton anymore, shared must be immutable. – amir Jun 13 '18 at 09:12
1

If you add `private init() {}` there shouldn’t be much potential left for more than a single instance. – Tom E Jun 13 '18 at 09:19
How can a var remains immutable? – Kamran Jun 13 '18 at 09:59
Good point. However, how would you change it in the case above once it has been initially set? – Tom E Jun 13 '18 at 10:01

Van · Answer 3 · 2018-07-05T12:09:27.387

-2

In your singleton class change the variable declaration from let to var. so that will be same as in protocol's declaration.

class Singleton : SingletonProtocol{
    static var shared : SingletonProtocol = Singleton()
    var variable = 4
}

EDIT:

also add only getter for sharedInstance in protocol declaration to serve purpose of singleton class as:

protocol SingletonProtocol: AnyObject {
    static var shared: SingletonProtocol { get }
    var variable: Int { get set }
}

edited Jul 05 '18 at 12:09

answered Jun 13 '18 at 09:14

Van

1,225
10
18

If I do, the class will not be a singleton anymore, shared must be immutable. – amir Jun 13 '18 at 09:14
@amir check the tom's comment above, same can be applicable here. – Van Jun 14 '18 at 06:17
yes but I still want shared to be let because it is the correct way of writing singleton, so the correct solution is Rico Crescenzio's – amir Jun 14 '18 at 10:13

Protocol defining singleton with properties

3 Answers3