Struct with an 'expiry date' - how to create elegantly?

Question

Created Jun ’23

Replies 4

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Participants 3

I am trying to create a struct which contains a value along with an 'expiry time'. I want to use a struct rather than a class so it can be set as a @Published property in an Observable object. Here is the code:

struct MarineDatum {
	
	var value: Double? {
		didSet {
			isNew = true
			timer?.invalidate()
			timer = Timer.scheduledTimer(withTimeInterval: 5, repeats: false, block: { timer in self.makeOld()})
		}
	}
	var isNew: Bool
	var timer: Timer?

	init(value: Double) {
		self.value = value
		self.isNew = true
	}
	
	mutating func makeOld() {
		self.isNew = false
	}
}

This code does exactly what I want, were it to actually compile, but I get an 'Escaping closure captures mutating 'self' parameter' error in the timer block.

Can anyone think of a way around this without creating an external variable to represent the timer object?

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Answer 1

Claude31 OP

Jun ’23

This code does exactly what I want, were it to actually compile

So, it does not do it…

There are several issues:

first, as the error says, you capture a value of self. It cannot be changed anymore (in older versions of Swift, you could, but the changes were ignored…).
second, take care: when instance is created, didSet is not called. It will be called when you further update the value
What is the problem using a class ? You may just have to make it hashable.

This works:

class MarineDatum {
    
    var value: Double? {
        didSet {
            isNew = true
            timer?.invalidate()
            print("Start")
            timer = Timer.scheduledTimer(withTimeInterval: 5, repeats: false, block: { timer in self.isNew = false ; print("Done")})
        }
    }
    var isNew: Bool
    var timer: Timer?

    init(value: Double) {
        print("init")
        self.value = value
        self.isNew = true
    }
    
//    mutating func makeOld() {
//        self.isNew = false
//    }
}

let marineDatum = MarineDatum(value: 100)
marineDatum.value = 200

Done is printed 5 s after Start…

0

Answer 2

endecotp OP

Jun ’23

I am trying to create a struct which contains a value along with an 'expiry time'

That's straightforward. But it doesn't seem to be what you are actually doing. What you actually seem to have implemented is a struct (or class) containing a timer. Don't do that, just store the expiry time.

Then in your is_new getter, compare the expiry time with the current time.

(Do you have some other reason for wanting to store the timer in the object?)

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Answer 3

Baylward OP

Jun ’23

I've read much more about Combine now, and I think it's not possible to put this functionality into the struct itself. However, it CAN be put into a view model like this:

class MarineDatumViewModel: ObservableObject {
	@Published var marineDatum: MarineDatum = MarineDatum(value: 0.0) {
		didSet {
			if marineDatum.isNew {
				DispatchQueue.main.asyncAfter(deadline: .now() + 2) { self.marineDatum.makeOld() }
			}
		}
	}
}

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Answer 4

endecotp OP

Jun ’23

I still need a timer somewhere to trigger the 'is_new' getter (do you mean the didSet?).

No, I mean getter. Don't store the isNew boolean. Compute it in the getter by comparing the expiry time in the struct with the current time. No timer required.

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