Read once and then set to nil

Question

I'd like to implement such property, that it's value is available for reading only one time, and then the property should be set to nil.

I've implemented it in such way:

private var _readOnce: String?

var readOnce: String? {
    get {
        let value = _readOnce
        _readOnce = nil
        return value
        }
    set {
        _readOnce = newValue
    }
}

readOnce = "Andrej"

print("read once = \(readOnce)")  // prints read once = Optional("Andrej")\n"
print("read once = \(readOnce)")  // prints read once = nil\n"

But I'feel like using a separate property _readOnce is not the "swifty" / "most elegant" way to do it.

Does anyone know of a different way, that wouldn't require to use a separate property?

I can confirm that the above code works, it's only that I feel it could be more elegant with less lines to achieve the same behaviour.

I thought I could use such approach to react to push notification and make sure I react only once. So I've been exploring this approach. But as it looks right now I'll have to take a different approach as some other things don't behave exactly as I was expecting. — Andrej, Jul 07 '17 at 20:53
I'm don't know the exact issue of your problem. But I'm thinking of multiple other ways you could solve this 1. Just have boolean property *somewhere* 2. in iOS 10 the different delegate methods have made it easier to manage what you intend. That is if your notification has `content-available : 1` then your `application(_:didReceiveRemoteNotification:fetchCompletionHandler:)` will get called. If user taps on the notification `didreceivenotificationresponse` will get called. If user is in the app the `willPresentNotification` will get called. — mfaani, Jul 07 '17 at 21:33
Tricky part is if you have a push notification that sends `alert`/`sound` but also has `content-available:1` meaning you will get 2 callbacks. For that should understand and separate your concerns. For **iOS9** managing the callbacks is much more difficult. See [here](https://stackoverflow.com/a/22085855/5175709). As for managing app state in regards to push notifications. See [here](https://stackoverflow.com/questions/26302770/ios-apns-best-effort-fallback) and [here](https://stackoverflow.com/questions/44442192/should-i-update-my-app-upon-receiving-payload-or-i-should-always-update-it-by-a) — mfaani, Jul 08 '17 at 12:38

score 2 · Accepted Answer · answered Jul 07 '17 at 16:25

I don't know that there's a way to avoid having a backing property, but what I'd probably do is to make a helper type to wrap up the behavior. Something like this:

struct OneTimeValue<T>
{
    private var isUnread = true

    private let value : T

    init(_ value: T)
    {
        self.value = value
    }

    func get() -> T?
    {
        guard isUnread else {
            return nil
        }
        self.isUnread = false
        return self.value
    }
}

You could also write this a little differently if you prefer, by nilling out value inside of get(), for example, but the general plan holds.

Then your class becomes:

class Miser
{
    var readOnce : String?
    {
        return self._readOnce.get()
    }
    private let _readOnce = OneTimeValue("Can't touch this (twice)")
}

I've also used this pattern for a UserDefaultsValue (storage to/from user defaults) and a SynchronizedValue (read-write lock on a property) and I think it works well.

kaz boddington · Answer 2 · 2017-07-07T18:23:54.457

1

As far as I know it is not possible without a second variable. This is because computed properties do not store any data for the variable they represent:

In addition to stored properties, classes, structures, and enumerations can define computed properties, which do not actually store a value.

For non-computed properties, the only observers you can have are based upon the setting of the variable, not the getting (i.e. willSet and didSet)

Hope that helps!

EDIT: It can be done with closures and property observers if you're careful: This requires no other variables (instead the value is captured by the closure), but it is rather unclear — I wouldn't recommend it.

var readOnce: () -> String? = {nil} {
    didSet{
        readOnce = { [weak self, readOnce] in
            self?.readOnce = {nil}
            return readOnce()
        }
    }
}

readOnce()        // returns nil
readOnce = {"Hi"} 
readOnce()        // returns optional wrapped "Hi"
readOnce()        // returns nil

A more 'Swifty' answer for you :D

edited Jul 07 '17 at 18:23

answered Jul 07 '17 at 16:36

kaz boddington

61
5

`didSet` is after your first *set* not after your first *get* /read – mfaani Jul 07 '17 at 18:35
@Honey Yes, agreed — however this solution is based on the idea of altering the value to be a closure after `readOnce` is set. **This closure is then executed when the variable is read**, and readOnce reset to {nil} so that it cannot be read twice. The code above works, but again, due to the hackery involved, I wouldn't recommend. – kaz boddington Jul 07 '17 at 18:45
Sure, very interesting approach, learned something new. :) Also agree with you that it's kind of hackey and wouldn't use in project with other developers as it's not instantly clear what's going on. – Andrej Jul 07 '17 at 20:57

score 0 · Answer 3 · answered Sep 26 '22 at 03:33

After Swift 5.1, We can use Property Wrapper

@propertyWrapper
struct ReturnAndFree<T> {
  private var value: T?
  init(wrappedValue: T?) {
    value = wrappedValue
  }
  
  var wrappedValue: T? {
    mutating get {
      defer { value = nil }
      return value
    }
    set {
      value = newValue
    }
  }
  
}

Read once and then set to nil

3 Answers3