How to initialize a variable, but once done, its value is final

Question

I have a model object variable in Swift View Controller. The thing I'd like to do is that at initialisation of the VC, i don't have a value for it. But after an async network call I have the parsed model object which is what this variable should hold, but from then on I don't want anything to change the value of the model variable.

Is it possible to do so in Swift? if yes, how?

Answer 1

Based on Babul Prabhakar answer, but a bit more clean.

Approach 1:

var test: String? { didSet { test = oldValue ?? test } }

test = "Initial string"
test = "Some other string"
test = "Lets try one last time"

print(test) // "Initial string"

Edit: made it shorter

Approach 2:

let value: String
value = "Initial string"

I guess the second approach is the "built-in" option of Swift to declare a variable once. This is only possible when the variable is assigned right after the declaration though.

Answer 2

What you can do is

private var myPrivateVar:String?;

var publicGetter:String {
    set {
        if myPrivateVar == nil {
            myPrivateVar = newValue;

        }

    }
    get {
        return myPrivateVar!;
    }
}

Answer 3

I'd personally go with @Eendje or @Babul Prabhakar's approach if myPrivateVar was somehow set to nil the setter could still write twice. I wanted to share another option that is guaranteed to never get set twice (as long as the token is never written to!):

    private var token: dispatch_once_t = 0

    private var setOnce : String?

    var publicGetter: String {

        set (newValue) {
            dispatch_once(&token) {
                self.setOnce = newValue
            }
        }

        get {
            guard let setOnce = setOnce else {
                return ""
            }
            return setOnce
        }
    }

The dispatch_once() function provides a simple and efficient mechanism to run an initializer exactly once, similar to pthread_once(3). Well designed code hides the use of lazy initialization. For exam-ple: example:

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